Using SELinux

Chapter 1. Getting started with SELinux

Security Enhanced Linux (SELinux) provides an additional layer of system security. SELinux fundamentally answers the question: May <subject> do <action> to <object>?, for example: May a web server access files in users' home directories?

1.1. Introduction to SELinux

The standard access policy based on the user, group, and other permissions, known as Discretionary Access Control (DAC), does not enable system administrators to create comprehensive and fine-grained security policies, such as restricting specific applications to only viewing log files, while allowing other applications to append new data to the log files.

Security Enhanced Linux (SELinux) implements Mandatory Access Control (MAC). Every process and system resource has a special security label called an SELinux context. A SELinux context, sometimes referred to as an SELinux label, is an identifier which abstracts away the system-level details and focuses on the security properties of the entity. Not only does this provide a consistent way of referencing objects in the SELinux policy, but it also removes any ambiguity that can be found in other identification methods. For example, a file can have multiple valid path names on a system that makes use of bind mounts.

The SELinux policy uses these contexts in a series of rules which define how processes can interact with each other and the various system resources. By default, the policy does not allow any interaction unless a rule explicitly grants access.

Note

Remember that SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny access first, which means that no SELinux denial is logged if the traditional DAC rules prevent the access.

SELinux contexts have several fields: user, role, type, and security level. The SELinux type information is perhaps the most important when it comes to the SELinux policy, as the most common policy rule which defines the allowed interactions between processes and system resources uses SELinux types and not the full SELinux context. SELinux types end with _t. For example, the type name for the web server is httpd_t. The type context for files and directories normally found in /var/www/html/ is httpd_sys_content_t. The type contexts for files and directories normally found in /tmp and /var/tmp/ is tmp_t. The type context for web server ports is http_port_t.

There is a policy rule that permits Apache (the web server process running as httpd_t) to access files and directories with a context normally found in /var/www/html/ and other web server directories (httpd_sys_content_t). There is no allow rule in the policy for files normally found in /tmp and /var/tmp/, so access is not permitted. With SELinux, even if Apache is compromised, and a malicious script gains access, it is still not able to access the /tmp directory.

Figure 1.1. An example how can SELinux help to run Apache and MariaDB in a secure way.

As the previous scheme shows, SELinux allows the Apache process running as httpd_t to access the /var/www/html/ directory and it denies the same process to access the /data/mysql/ directory because there is no allow rule for the httpd_t and mysqld_db_t type contexts. On the other hand, the MariaDB process running as mysqld_t is able to access the /data/mysql/ directory and SELinux also correctly denies the process with the mysqld_t type to access the /var/www/html/ directory labeled as httpd_sys_content_t.

Additional resources

selinux(8) man page and man pages listed by the apropos selinux command.
Man pages listed by the man -k _selinux command when the selinux-policy-doc package is installed.
The SELinux Coloring Book helps you to better understand SELinux basic concepts.
SELinux Wiki FAQ

1.2. Benefits of running SELinux

SELinux provides the following benefits:

All processes and files are labeled. SELinux policy rules define how processes interact with files, as well as how processes interact with each other. Access is only allowed if an SELinux policy rule exists that specifically allows it.
SELinux provides fine-grained access control. Stepping beyond traditional UNIX permissions that are controlled at user discretion and based on Linux user and group IDs, SELinux access decisions are based on all available information, such as an SELinux user, role, type, and, optionally, a security level.
SELinux policy is administratively-defined and enforced system-wide.
SELinux can mitigate privilege escalation attacks. Processes run in domains, and are therefore separated from each other. SELinux policy rules define how processes access files and other processes. If a process is compromised, the attacker only has access to the normal functions of that process, and to files the process has been configured to have access to. For example, if the Apache HTTP Server is compromised, an attacker cannot use that process to read files in user home directories, unless a specific SELinux policy rule was added or configured to allow such access.
SELinux can enforce data confidentiality and integrity, and can protect processes from untrusted inputs.

SELinux is designed to enhance existing security solutions, not replace antivirus software, secure passwords, firewalls, or other security systems. Even when running SELinux, it is important to continue to follow good security practices, such as keeping software up-to-date, using hard-to-guess passwords, and firewalls.

1.3. SELinux examples

The following examples demonstrate how SELinux increases security:

The default action is deny. If an SELinux policy rule does not exist to allow access, such as for a process opening a file, access is denied.
SELinux can confine Linux users. A number of confined SELinux users exist in the SELinux policy. Linux users can be mapped to confined SELinux users to take advantage of the security rules and mechanisms applied to them. For example, mapping a Linux user to the SELinux user_u user, results in a Linux user that is not able to run unless configured otherwise set user ID (setuid) applications, such as sudo and su.
Increased process and data separation. The concept of SELinux domains allows defining which processes can access certain files and directories. For example, when running SELinux, unless otherwise configured, an attacker cannot compromise a Samba server, and then use that Samba server as an attack vector to read and write to files used by other processes, such as MariaDB databases.
SELinux helps mitigate the damage made by configuration mistakes. Domain Name System (DNS) servers often replicate information between each other in a zone transfer. Attackers can use zone transfers to update DNS servers with false information. When running the Berkeley Internet Name Domain (BIND) as a DNS server in RHEL, even if an administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy prevent updates for zone files ^[1] that use zone transfers, by the BIND named daemon itself, and by other processes.
Without SELinux, an attacker can misuse a vulnerability to path traversal on an Apache web server and access files and directories stored on the file system by using special elements such as ../. If an attacker attempts an attack on a server running with SELinux in enforcing mode, SELinux denies access to files that the httpd process must not access. SELinux cannot block this type of attack completely but it effectively mitigates it.
SELinux in enforcing mode successfully prevents exploitation of kernel NULL pointer dereference operators on non-SMAP platforms (CVE-2019-9213). Attackers use a vulnerability in the mmap function, which does not check mapping of a null page, for placing arbitrary code on this page.
The deny_ptrace SELinux boolean and SELinux in enforcing mode protect systems from the PTRACE_TRACEME vulnerability (CVE-2019-13272). Such configuration prevents scenarios when an attacker can get root privileges.
The nfs_export_all_rw and nfs_export_all_ro SELinux booleans provide an easy-to-use tool to prevent misconfigurations of Network File System (NFS) such as accidental sharing /home directories.

Additional resources

SELinux as a security pillar of an operating system - Real-world benefits and examples Knowledgebase article
SELinux hardening with Ansible Knowledgebase article
selinux-playbooks Github repository with Ansible playbooks for SELinux hardening

^[1] Text files that include DNS information, such as hostname to IP address mappings.

1.4. SELinux architecture and packages

SELinux is a Linux Security Module (LSM) that is built into the Linux kernel. The SELinux subsystem in the kernel is driven by a security policy which is controlled by the administrator and loaded at boot. All security-relevant, kernel-level access operations on the system are intercepted by SELinux and examined in the context of the loaded security policy. If the loaded policy allows the operation, it continues. Otherwise, the operation is blocked and the process receives an error.

SELinux decisions, such as allowing or disallowing access, are cached. This cache is known as the Access Vector Cache (AVC). When using these cached decisions, SELinux policy rules need to be checked less, which increases performance. Remember that SELinux policy rules have no effect if DAC rules deny access first. Raw audit messages are logged to the /var/log/audit/audit.log and they start with the type=AVC string.

In RHEL 8, system services are controlled by the systemd daemon; systemd starts and stops all services, and users and processes communicate with systemd using the systemctl utility. The systemd daemon can consult the SELinux policy and check the label of the calling process and the label of the unit file that the caller tries to manage, and then ask SELinux whether or not the caller is allowed the access. This approach strengthens access control to critical system capabilities, which include starting and stopping system services.

The systemd daemon also works as an SELinux Access Manager. It retrieves the label of the process running systemctl or the process that sent a D-Bus message to systemd. The daemon then looks up the label of the unit file that the process wanted to configure. Finally, systemd can retrieve information from the kernel if the SELinux policy allows the specific access between the process label and the unit file label. This means a compromised application that needs to interact with systemd for a specific service can now be confined by SELinux. Policy writers can also use these fine-grained controls to confine administrators.

If a process is sending a D-Bus message to another process and if the SELinux policy does not allow the D-Bus communication of these two processes, then the system prints a USER_AVC denial message, and the D-Bus communication times out. Note that the D-Bus communication between two processes works bidirectionally.

Important

To avoid incorrect SELinux labeling and subsequent problems, ensure that you start services using a systemctl start command.

RHEL 8 provides the following packages for working with SELinux:

policies: selinux-policy-targeted, selinux-policy-mls
tools: policycoreutils, policycoreutils-gui, libselinux-utils, policycoreutils-python-utils, setools-console, checkpolicy

1.5. SELinux states and modes

SELinux can run in one of three modes: enforcing, permissive, or disabled.

Enforcing mode is the default, and recommended, mode of operation; in enforcing mode SELinux operates normally, enforcing the loaded security policy on the entire system.
In permissive mode, the system acts as if SELinux is enforcing the loaded security policy, including labeling objects and emitting access denial entries in the logs, but it does not actually deny any operations. While not recommended for production systems, permissive mode can be helpful for SELinux policy development and debugging.
Disabled mode is strongly discouraged; not only does the system avoid enforcing the SELinux policy, it also avoids labeling any persistent objects such as files, making it difficult to enable SELinux in the future.

Use the setenforce utility to change between enforcing and permissive mode. Changes made with setenforce do not persist across reboots. To change to enforcing mode, enter the setenforce 1 command as the Linux root user. To change to permissive mode, enter the setenforce 0 command. Use the getenforce utility to view the current SELinux mode:

# getenforce
Enforcing

# setenforce 0
# getenforce
Permissive

# setenforce 1
# getenforce
Enforcing

In Red Hat Enterprise Linux, you can set individual domains to permissive mode while the system runs in enforcing mode. For example, to make the httpd_t domain permissive:

# semanage permissive -a httpd_t

Note that permissive domains are a powerful tool that can compromise security of your system. Red Hat recommends to use permissive domains with caution, for example, when debugging a specific scenario.

Chapter 2. Changing SELinux states and modes

When enabled, SELinux can run in one of two modes: enforcing or permissive. The following sections show how to permanently change into these modes.

2.1. Permanent changes in SELinux states and modes

As discussed in SELinux states and modes, SELinux can be enabled or disabled. When enabled, SELinux has two modes: enforcing and permissive.

Use the getenforce or sestatus commands to check in which mode SELinux is running. The getenforce command returns Enforcing, Permissive, or Disabled.

The sestatus command returns the SELinux status and the SELinux policy being used:

$ sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     actual (secure)
Max kernel policy version:      31

Warning

When systems run SELinux in permissive mode, users and processes might label various file-system objects incorrectly. File-system objects created while SELinux is disabled are not labeled at all. This behavior causes problems when changing to enforcing mode because SELinux relies on correct labels of file-system objects.

To prevent incorrectly labeled and unlabeled files from causing problems, SELinux automatically relabels file systems when changing from the disabled state to permissive or enforcing mode. Use the fixfiles -F onboot command as root to create the /.autorelabel file containing the -F option to ensure that files are relabeled upon next reboot.

Before rebooting the system for relabeling, make sure the system will boot in permissive mode, for example by using the enforcing=0 kernel option. This prevents the system from failing to boot in case the system contains unlabeled files required by systemd before launching the selinux-autorelabel service. For more information, see RHBZ#2021835.

2.2. Changing SELinux to permissive mode

When SELinux is running in permissive mode, SELinux policy is not enforced. The system remains operational and SELinux does not deny any operations but only logs AVC messages, which can be then used for troubleshooting, debugging, and SELinux policy improvements. Each AVC is logged only once in this case.

Prerequisites

The selinux-policy-targeted, libselinux-utils, and policycoreutils packages are installed on your system.
The selinux=0 or enforcing=0 kernel parameters are not used.

Procedure

Open the /etc/selinux/config file in a text editor of your choice, for example:
```
# vi /etc/selinux/config
```

Configure the SELINUX=permissive option:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these two values:
#       targeted - Targeted processes are protected,
#       mls - Multi Level Security protection.
SELINUXTYPE=targeted

Restart the system:
```
# reboot
```

Verification

After the system restarts, confirm that the getenforce command returns Permissive:
```
$ getenforce
Permissive
```

2.3. Changing SELinux to enforcing mode

When SELinux is running in enforcing mode, it enforces the SELinux policy and denies access based on SELinux policy rules. In RHEL, enforcing mode is enabled by default when the system was initially installed with SELinux.

Prerequisites

The selinux-policy-targeted, libselinux-utils, and policycoreutils packages are installed on your system.
The selinux=0 or enforcing=0 kernel parameters are not used.

Procedure

Open the /etc/selinux/config file in a text editor of your choice, for example:
```
# vi /etc/selinux/config
```

Configure the SELINUX=enforcing option:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
#       targeted - Targeted processes are protected,
#       mls - Multi Level Security protection.
SELINUXTYPE=targeted

Save the change, and restart the system:
```
# reboot
```
On the next boot, SELinux relabels all the files and directories within the system and adds SELinux context for files and directories that were created when SELinux was disabled.

Verification

After the system restarts, confirm that the getenforce command returns Enforcing:
```
$ getenforce
Enforcing
```

Troubleshooting

After changing to enforcing mode, SELinux may deny some actions because of incorrect or missing SELinux policy rules.

To view what actions SELinux denies, enter the following command as root:
```
# ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR -ts today
```
Alternatively, with the setroubleshoot-server package installed, enter:
```
# grep "SELinux is preventing" /var/log/messages
```
If SELinux is active and the Audit daemon (auditd) is not running on your system, then search for certain SELinux messages in the output of the dmesg command:
```
# dmesg | grep -i -e type=1300 -e type=1400
```

See Troubleshooting problems related to SELinux for more information.

2.4. Enabling SELinux on systems that previously had it disabled

To avoid problems, such as systems unable to boot or process failures, when enabling SELinux on systems that previously had it disabled, resolve Access Vector Cache (AVC) messages in permissive mode first.

When systems run SELinux in permissive mode, users and processes might label various file-system objects incorrectly. File-system objects created while SELinux is disabled are not labeled at all. This behavior causes problems when changing to enforcing mode because SELinux relies on correct labels of file-system objects.

To prevent incorrectly labeled and unlabeled files from causing problems, SELinux automatically relabels file systems when changing from the disabled state to permissive or enforcing mode.

Warning

Before rebooting the system for relabeling, make sure the system will boot in permissive mode, for example by using the enforcing=0 kernel option. This prevents the system from failing to boot in case the system contains unlabeled files required by systemd before launching the selinux-autorelabel service. For more information, see RHBZ#2021835.

Procedure

Enable SELinux in permissive mode. For more information, see Changing to permissive mode.
Restart your system:
```
# reboot
```
Check for SELinux denial messages. For more information, see Identifying SELinux denials.
Ensure that files are relabeled upon the next reboot:
```
# fixfiles -F onboot
```
This creates the /.autorelabel file containing the -F option.
Warning
Always switch to permissive mode before entering the fixfiles -F onboot command.
By default, autorelabel uses as many threads in parallel as the system has available CPU cores. To use only a single thread during automatic relabeling, use the fixfiles -T 1 onboot command.
If there are no denials, switch to enforcing mode. For more information, see Changing SELinux modes at boot time.

Verification

After the system restarts, confirm that the getenforce command returns Enforcing:
```
$ getenforce
Enforcing
```

Next steps

To run custom applications with SELinux in enforcing mode, choose one of the following scenarios:

Run your application in the unconfined_service_t domain.
Write a new policy for your application. See the Writing a custom SELinux policy section for more information.

Additional resources

SELinux states and modes section covers temporary changes in modes.

2.5. Disabling SELinux

When you disable SELinux, your system does not load your SELinux policy. As a result, the system does not enforce the SELinux policy and does not log Access Vector Cache (AVC) messages. Therefore, all benefits of running SELinux are lost.

Do not disable SELinux except in specific scenarios, such as performance-sensitive systems where the weakened security does not impose significant risks.

Important

If your scenario requires to perform debugging in a production environment, temporarily use permissive mode instead of permanently disabling SELinux. See Changing to permissive mode for more information about permissive mode.

Prerequisites

The grubby package is installed:
```
$ rpm -q grubby
grubby-<version>
```

Procedure

Configure your boot loader to add selinux=0 to the kernel command line:
```
$ sudo grubby --update-kernel ALL --args selinux=0
```
Restart your system:
```
$ reboot
```

Verification

After the reboot, confirm that the getenforce command returns Disabled:
```
$ getenforce
Disabled
```

Alternative method

In RHEL 8, you can still use the deprecated method for disabling SELinux by using the SELINUX=disabled option in the /etc/selinux/config file. This results the kernel booting with SELinux enabled and switching to disabled mode later in the boot process. Consequently, memory leaks and race conditions might occur that cause kernel panics. To use this method:

Open the /etc/selinux/config file in a text editor of your choice, for example:
```
# vi /etc/selinux/config
```

Configure the SELINUX=disabled option:

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#       enforcing - SELinux security policy is enforced.
#       permissive - SELinux prints warnings instead of enforcing.
#       disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
#       targeted - Targeted processes are protected,
#       mls - Multi Level Security protection.
SELINUXTYPE=targeted

Save the change, and restart your system:
```
# reboot
```

2.6. Changing SELinux modes at boot time

On boot, you can set the following kernel parameters to change the way SELinux runs:

enforcing=0

Setting this parameter causes the system to start in permissive mode, which is useful when troubleshooting issues. Using permissive mode might be the only option to detect a problem if your file system is too corrupted. Moreover, in permissive mode, the system continues to create the labels correctly. The AVC messages that are created in this mode can be different than in enforcing mode.

In permissive mode, only the first denial from a series of the same denials is reported. However, in enforcing mode, you might get a denial related to reading a directory, and an application stops. In permissive mode, you get the same AVC message, but the application continues reading files in the directory and you get an AVC for each denial in addition.

selinux=0

This parameter causes the kernel to not load any part of the SELinux infrastructure. The init scripts notice that the system booted with the selinux=0 parameter and touch the /.autorelabel file. This causes the system to automatically relabel the next time you boot with SELinux enabled.

Important

Do not use the selinux=0 parameter in a production environment. To debug your system, temporarily use permissive mode instead of disabling SELinux.

autorelabel=1

This parameter forces the system to relabel similarly to the following commands:

# touch /.autorelabel
# reboot

If a file system contains a large amount of mislabeled objects, start the system in permissive mode to make the autorelabel process successful.

Additional resources

For additional SELinux-related kernel boot parameters, such as checkreqprot, see the /usr/share/doc/kernel-doc-<KERNEL_VER>/Documentation/admin-guide/kernel-parameters.txt file installed with the kernel-doc package. Replace the <KERNEL_VER> string with the version number of the installed kernel, for example:
```
# yum install kernel-doc
$ less /usr/share/doc/kernel-doc-4.18.0/Documentation/admin-guide/kernel-parameters.txt
```

Chapter 3. Managing confined and unconfined users

Each Linux user is mapped to an SELinux user according to the rules in the SELinux policy. Administrators can modify these rules by using the semanage login utility or by assigning Linux users directly to specific SELinux users. Therefore, a Linux user has the restrictions of the SELinux user to which it is assigned. When a Linux user that is assigned to an SELinux user launches a process, this process inherits the SELinux user’s restrictions, unless other rules specify a different role or type.

3.1. Confined and unconfined users in SELinux

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users.

The security context for a Linux user consists of the SELinux user, the SELinux role, and the SELinux type. For example:

user_u:user_r:user_t

Where:

user_u: Is the SELinux user.
user_r: Is the SELinux role.
user_t: Is the SELinux type.

After a Linux user logs in, its SELinux user cannot change. However, its type and role can change, for example, during transitions.

To see the SELinux user mapping on your system, use the semanage login -l command as root:

# semanage login -l
Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *

In Red Hat Enterprise Linux, Linux users are mapped to the SELinux __default__ login by default, which is mapped to the SELinux unconfined_u user. The following line defines the default mapping:

default          unconfined_u         s0-s0:c0.c1023       *

Confined users are restricted by SELinux rules explicitly defined in the current SELinux policy. Unconfined users are subject to only minimal restrictions by SELinux.

Confined and unconfined Linux users are subject to executable and writable memory checks, and are also restricted by MCS or MLS.

To list the available SELinux users, enter the following command:

$ seinfo -u
Users: 8
   guest_u
   root
   staff_u
   sysadm_u
   system_u
   unconfined_u
   user_u
   xguest_u

Note that the seinfo command is provided by the setools-console package, which is not installed by default.

If an unconfined Linux user executes an application that SELinux policy defines as one that can transition from the unconfined_t domain to its own confined domain, the unconfined Linux user is still subject to the restrictions of that confined domain. The security benefit of this is that, even though a Linux user is running unconfined, the application remains confined. Therefore, the exploitation of a flaw in the application can be limited by the policy.

Similarly, we can apply these checks to confined users. Each confined user is restricted by a confined user domain. The SELinux policy can also define a transition from a confined user domain to its own target confined domain. In such a case, confined users are subject to the restrictions of that target confined domain. The main point is that special privileges are associated with the confined users according to their role.

3.2. Roles and access rights of SELinux users

The SELinux policy maps each Linux user to an SELinux user. This allows Linux users to inherit the restrictions of SELinux users.

You can customize the permissions for confined users in your SELinux policy according to specific needs by adjusting booleans in the policy. You can determine the current state of these booleans by using the semanage boolean -l command. To list all SELinux users, their SELinux roles, and levels and ranges for MLS and MCS, use the semanage user -l command as root.

Table 3.1. Roles of SELinux users
User	Default role	Additional roles
`unconfined_u`	`unconfined_r`	`system_r`
`guest_u`	`guest_r`
`xguest_u`	`xguest_r`
`user_u`	`user_r`
`staff_u`	`staff_r`	`sysadm_r`
		`unconfined_r`
		`system_r`
`sysadm_u`	`sysadm_r`
`root`	`staff_r`	`sysadm_r`
		`unconfined_r`
		`system_r`
`system_u`	`system_r`

Note that system_u is a special user identity for system processes and objects, and system_r is the associated role. Administrators must never associate this system_u user and the system_r role to a Linux user. Also, unconfined_u and root are unconfined users. For these reasons, the roles associated to these SELinux users are not included in the following table Types and access rights of SELinux roles.

Each SELinux role corresponds to an SELinux type and provides specific access rights.

Table 3.2. Types and access rights of SELinux roles
Role	Type	Log in using X Window System	`su` and `sudo`	Execute in home directory and `/tmp` (default)	Networking
`unconfined_r`	`unconfined_t`	yes	yes	yes	yes
`guest_r`	`guest_t`	no	no	yes	no
`xguest_r`	`xguest_t`	yes	no	yes	web browsers only (Mozilla Firefox, GNOME Web)
`user_r`	`user_t`	yes	no	yes	yes
`staff_r`	`staff_t`	yes	only `sudo`	yes	yes
`auditadm_r`	`auditadm_t`		yes	yes	yes
`dbadm_r`	`dbadm_r`		yes	yes	yes
`logadm_r`	`logadm_t`		yes	yes	yes
`webadm_r`	`webadm_r`		yes	yes	yes
`secadm_r`	`secadm_t`		yes	yes	yes
`sysadm_r`	`sysadm_t`	only when the `xdm_sysadm_login` boolean is `on`	yes	yes	yes

For more detailed descriptions of the non-administrator roles, see Confined non-administrator roles in SELinux.

For more detailed descriptions of the administrator roles, see Confined administrator roles in SELinux.

To list all available roles, enter the seinfo -r command:

$ seinfo -r
Roles: 14
   auditadm_r
   dbadm_r
   guest_r
   logadm_r
   nx_server_r
   object_r
   secadm_r
   staff_r
   sysadm_r
   system_r
   unconfined_r
   user_r
   webadm_r
   xguest_r

Note that the seinfo command is provided by the setools-console package, which is not installed by default.

Additional resources

seinfo(1), semanage-login(8), and xguest_selinux(8) man pages installed with the selinux-policy-doc package
How to modify SELinux settings with booleans

3.3. Confined non-administrator roles in SELinux

In SELinux, confined non-administrator roles grant specific sets of privileges and permissions for performing specific tasks to the Linux users assigned to them. By assigning separate confined non-administrator roles, you can assign specific privileges to individual users. This is useful in scenarios with multiple users who each have a different level of authorizations.

You can also customize the permissions of SELinux roles by changing the related SELinux booleans on your system. To see the SELinux booleans and their current state, use the semanage boolean -l command as root. You can get more detailed descriptions if you install the selinux-policy-devel package.

# semanage boolean -l
SELinux boolean                State  Default Description
…
xguest_connect_network         (on   ,   on)  Allow xguest users to configure Network Manager and connect to apache ports
xguest_exec_content            (on   ,   on)  Allow xguest to exec content
…

Linux users in the user_t, guest_t, and xguest_t domains can only run set user ID (setuid) applications if SELinux policy permits it (for example, passwd). These users cannot run the setuid applications su and sudo, and therefore cannot use these applications to become root.

By default, Linux users in the staff_t, user_t, guest_t, and xguest_t domains can execute applications in their home directories and /tmp. Applications inherit the permissions of the user that executed them.

To prevent guest_t, and xguest_t users from executing applications in directories in which they have write access, set the guest_exec_content and xguest_exec_content booleans to off.

SELinux has the following confined non-administrator roles, each with specific privileges and limitations:

guest_r

Has very limited permissions. Users assigned to this role cannot access the network, but can execute files in the /tmp and /home directories.

Related boolean:

SELinux boolean                State  Default Description
guest_exec_content             (on   ,   on)  Allow guest to exec content

xguest_r

Has limited permissions. Users assigned to this role can log into X Window, access web pages by using network browsers, and access media. They can also execute files in the /tmp and /home directories.

Related booleans:

SELinux boolean                State  Default Description
xguest_connect_network         (on   ,   on)  Allow xguest users to configure Network Manager and connect to apache ports
xguest_exec_content            (on   ,   on)  Allow xguest to exec content
xguest_mount_media             (on   ,   on)  Allow xguest users to mount removable media
xguest_use_bluetooth           (on   ,   on)  Allow xguest to use blue tooth devices

user_r

Has non-privileged access with full user permissions. Users assigned to this role can perform most actions that do not require administrative privileges.

Related booleans:

SELinux boolean                State  Default Description
unprivuser_use_svirt           (off  ,  off)  Allow unprivileged user to create and transition to svirt domains.

staff_r

Has permissions similar to user_r and additional privileges. In particular, users assigned to this role are allowed to run sudo to execute administrative commands that are normally reserved for the root user. This changes roles and the effective user ID (EUID) but does not change the SELinux user.

Related booleans:

SELinux boolean                State  Default Description
staff_exec_content             (on   ,   on)  Allow staff to exec content
staff_use_svirt                (on   ,   on)  allow staff user to create and transition to svirt domains.

Additional resources

To map a Linux user to staff_u and configure sudo, see Confining an administrator using sudo and the sysadm_r role.
For additional information about each role and the associated types, see the relevant man pages installed with the selinux-policy-doc package:
guest_selinux(8), xguest_selinux(8), user_selinux(8), and staff_selinux(8)

3.4. Confined administrator roles in SELinux

In SELinux, confined administrator roles grant specific sets of privileges and permissions for performing specific tasks to the Linux users assigned to them. By assigning separate confined administrator roles, you can divide the privileges over various domains of system administration to individual users. This is useful in scenarios with multiple administrators, each with a separate domain.

You can assign these roles to SELinux users by using the semanage user command.

SELinux has the following confined administrator roles:

auditadm_r

The audit administrator role allows managing processes related to the Audit subsystem.

Related boolean:

SELinux boolean                State  Default Description
auditadm_exec_content          (on   ,   on)  Allow auditadm to exec content

dbadm_r

The database administrator role allows managing MariaDB and PostgreSQL databases.

Related booleans:

SELinux boolean                State  Default Description
dbadm_exec_content             (on   ,   on)  Allow dbadm to exec content
dbadm_manage_user_files        (off  ,  off)  Determine whether dbadm can manage generic user files.
dbadm_read_user_files          (off  ,  off)  Determine whether dbadm can read generic user files.

logadm_r

The log administrator role allows managing logs, specifically, SELinux types related to the Rsyslog logging service and the Audit subsystem.

Related boolean:

SELinux boolean                State  Default Description
logadm_exec_content            (on   ,   on)  Allow logadm to exec content

webadm_r

The web administrator allows managing the Apache HTTP Server.

Related booleans:

SELinux boolean                State  Default Description
webadm_manage_user_files       (off  ,  off)  Determine whether webadm can manage generic user files.
webadm_read_user_files         (off  ,  off)  Determine whether webadm can read generic user files.

secadm_r

The security administrator role allows managing the SELinux database.

Related booleans:

SELinux boolean                State  Default Description
secadm_exec_content            (on   ,   on)  Allow secadm to exec content

sysadm_r

The system administrator role allows doing everything of the previously listed roles and has additional privileges. In non-default configurations, security administration can be separated from system administration by disabling the sysadm_secadm module in the SELinux policy. For detailed instructions, see Separating system administration from security administration in MLS.

The sysadm_u user cannot log in directly using SSH. To enable SSH logins for sysadm_u, set the ssh_sysadm_login boolean to on:

# setsebool -P ssh_sysadm_login on

Related booleans:

SELinux boolean                State  Default Description
ssh_sysadm_login               (on   ,   on)  Allow ssh logins as sysadm_r:sysadm_t
sysadm_exec_content            (on   ,   on)  Allow sysadm to exec content
xdm_sysadm_login               (on   ,   on)  Allow the graphical login program to login directly as sysadm_r:sysadm_t

Additional resources

To assign a Linux user to a confined administrator role, see Confining an administrator by mapping to sysadm_u.
For additional information about each role, and the associated types, see the relevant man pages installed with the selinux-policy-doc package:
auditadm_selinux(8), dbadm_selinux (8), logadm_selinux(8), webadm_selinux(8), secadm_selinux(8), and sysadm_selinux(8)

3.5. Adding a new user automatically mapped to the SELinux unconfined_u user

The following procedure demonstrates how to add a new Linux user to the system. The user is automatically mapped to the SELinux unconfined_u user.

Prerequisites

The root user is running unconfined, as it does by default in Red Hat Enterprise Linux.

Procedure

Enter the following command to create a new Linux user named <example_user>:
```
# useradd <example_user>
```

To assign a password to the Linux <example_user> user:

# passwd <example_user>
Changing password for user <example_user>.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

Log out of your current session.
Log in as the Linux <example_user> user. When you log in, the pam_selinux PAM module automatically maps the Linux user to an SELinux user (in this case, unconfined_u), and sets up the resulting SELinux context. The Linux user’s shell is then launched with this context.

Verification

When logged in as the <example_user> user, check the context of a Linux user:
```
$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
```

Additional resources

pam_selinux(8) man page on your system

3.6. Adding a new user as an SELinux-confined user

Use the following steps to add a new SELinux-confined user to the system. This example procedure maps the user to the SELinux staff_u user right with the command for creating the user account.

Prerequisites

The root user is running unconfined, as it does by default in Red Hat Enterprise Linux.

Procedure

Enter the following command to create a new Linux user named <example_user> and map it to the SELinux staff_u user:
```
# useradd -Z staff_u <example_user>
```

To assign a password to the Linux <example_user> user:

# passwd <example_user>
Changing password for user <example_user>.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

Log out of your current session.
Log in as the Linux <example_user> user. The user’s shell launches with the staff_u context.

Verification

When logged in as the <example_user> user, check the context of a Linux user:

$ id -Z
uid=1000(<example_user>) gid=1000(<example_user>) groups=1000(<example_user>) context=staff_u:staff_r:staff_t:s0-s0:c0.c1023

Additional resources

pam_selinux(8) man page on your system

3.7. Confining regular users in SELinux

You can confine all regular users on your system by mapping them to the user_u SELinux user.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Procedure

Display the list of SELinux login records. The list displays the mappings of Linux users to SELinux users:

# semanage login -l

Login Name    SELinux User  MLS/MCS Range   Service

__default__   unconfined_u  s0-s0:c0.c1023       *
root          unconfined_u  s0-s0:c0.c1023       *

Map the __default__ user, which represents all users without an explicit mapping, to the user_u SELinux user:
```
# semanage login -m -s user_u -r s0 __default__
```

Verification

Check that the __default__ user is mapped to the user_u SELinux user:

# semanage login -l

Login Name    SELinux User   MLS/MCS Range    Service

__default__   user_u         s0               *
root          unconfined_u   s0-s0:c0.c1023   *

Verify that the processes of a new user run in the user_u:user_r:user_t:s0 SELinux context.

Create a new user:
```
# adduser <example_user>
```
Define a password for <example_user>:
```
# passwd <example_user>
```
Log out as root and log in as the new user.

Show the security context for the user’s ID:

[<example_user>@localhost ~]$ id -Z
user_u:user_r:user_t:s0

Show the security context of the user’s current processes:

[<example_user>@localhost ~]$ ps axZ
LABEL                           PID TTY      STAT   TIME COMMAND
-                                 1 ?        Ss     0:05 /usr/lib/systemd/systemd --switched-root --system --deserialize 18
-                              3729 ?        S      0:00 (sd-pam)
user_u:user_r:user_t:s0        3907 ?        Ss     0:00 /usr/lib/systemd/systemd --user
-                              3911 ?        S      0:00 (sd-pam)
user_u:user_r:user_t:s0        3918 ?        S      0:00 sshd: <example_user>@pts/0
user_u:user_r:user_t:s0        3922 pts/0    Ss     0:00 -bash
user_u:user_r:user_dbusd_t:s0  3969 ?        Ssl    0:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
user_u:user_r:user_t:s0        3971 pts/0    R+     0:00 ps axZ

3.8. Confining an administrator by mapping to `sysadm_u`

You can confine a user with administrative privileges by mapping the user directly to the sysadm_u SELinux user. When the user logs in, the session runs in the sysadm_u:sysadm_r:sysadm_t SELinux context.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Prerequisites

The root user runs unconfined. This is the Red Hat Enterprise Linux default.

Procedure

Optional: To allow sysadm_u users to connect to the system by using SSH:
```
# setsebool -P ssh_sysadm_login on
```
Map a new or existing user to the sysadm_u SELinux user:
- To map a new user, add a new user to the wheel user group and map the user to the sysadm_u SELinux user:
```
# adduser -G wheel -Z sysadm_u <example_user>
```
- To map an existing user, add the user to the wheel user group and map the user to the sysadm_u SELinux user:
```
# usermod -G wheel -Z sysadm_u <example_user>
```
Restore the context of the user’s home directory:
```
# restorecon -R -F -v /home/<example_user>
```

Verification

Check that <example_user> is mapped to the sysadm_u SELinux user:

# semanage login -l | grep <example_user>
<example_user>     sysadm_u    s0-s0:c0.c1023   *

Log in as <example_user>, for example, by using SSH, and show the user’s security context:
```
[<example_user>@localhost ~]$ id -Z
sysadm_u:sysadm_r:sysadm_t:s0-s0:c0.c1023
```

Switch to the root user:

$ sudo -i
[sudo] password for <example_user>:

Verify that the security context remains unchanged:

# id -Z
sysadm_u:sysadm_r:sysadm_t:s0-s0:c0.c1023

Try an administrative task, for example, restarting the sshd service:
```
# systemctl restart sshd
```
If there is no output, the command finished successfully.
If the command does not finish successfully, it prints the following message:
```
Failed to restart sshd.service: Access denied
See system logs and 'systemctl status sshd.service' for details.
```

3.9. Confining an administrator by using sudo and the sysadm_r role

You can map a specific user with administrative privileges to the staff_u SELinux user, and configure sudo so that the user can gain the sysadm_r SELinux administrator role. This role allows the user to perform administrative tasks without SELinux denials. When the user logs in, the session runs in the staff_u:staff_r:staff_t SELinux context, but when the user enters a command by using sudo, the session changes to the staff_u:sysadm_r:sysadm_t context.

By default, all Linux users in Red Hat Enterprise Linux, including users with administrative privileges, are mapped to the unconfined SELinux user unconfined_u. You can improve the security of the system by assigning users to SELinux confined users. This is useful to conform with the V-71971 Security Technical Implementation Guide.

Prerequisites

The root user runs unconfined. This is the Red Hat Enterprise Linux default.

Procedure

Map a new or existing user to the staff_u SELinux user:
1. To map a new user, add a new user to the wheel user group and map the user to the staff_u SELinux user:
```
# adduser -G wheel -Z staff_u <example_user>
```
2. To map an existing user, add the user to the wheel user group and map the user to the staff_u SELinux user:
```
# usermod -G wheel -Z staff_u <example_user>
```
Restore the context of the user’s home directory:
```
# restorecon -R -F -v /home/<example_user>
```
To allow <example_user> to gain the SELinux administrator role, create a new file in the /etc/sudoers.d/ directory, for example:
```
# visudo -f /etc/sudoers.d/<example_user>
```

Add the following line to the new file:

<example_user> ALL=(ALL) TYPE=sysadm_t ROLE=sysadm_r ALL

Verification

Check that <example_user> is mapped to the staff_u SELinux user:

# semanage login -l | grep <example_user>
<example_user>     staff_u    s0-s0:c0.c1023   *

Log in as <example_user>, for example, using SSH, and switch to the root user:

[<example_user>@localhost ~]$ sudo -i
[sudo] password for <example_user>:

Show the root security context:

# id -Z
staff_u:sysadm_r:sysadm_t:s0-s0:c0.c1023

Try an administrative task, for example, restarting the sshd service:
```
# systemctl restart sshd
```
If there is no output, the command finished successfully.
If the command does not finish successfully, it prints the following message:
```
Failed to restart sshd.service: Access denied
See system logs and 'systemctl status sshd.service' for details.
```

3.10. Additional resources

unconfined_selinux(8), user_selinux(8), staff_selinux(8), and sysadm_selinux(8) man pages installed with the selinux-policy-doc package.
How to set up a system with SELinux confined users
How to modify SELinux settings with booleans

Chapter 4. Configuring SELinux for applications and services with non-standard configurations

When SELinux is in enforcing mode, the default policy is the targeted policy. The following sections provide information about setting up and configuring the SELinux policy for various services after you change configuration defaults, such as ports, database locations, or file-system permissions for processes.

You learn to change SELinux types for non-standard ports, to identify and fix incorrect labels for changes of default directories, and to adjust the policy using SELinux booleans.

4.1. Customizing the SELinux policy for the Apache HTTP server in a non-standard configuration

You can configure the Apache HTTP server to listen on a different port and to provide content in a non-default directory. To prevent consequent SELinux denials, follow the steps in this procedure to adjust your system’s SELinux policy.

Prerequisites

The httpd package is installed and the Apache HTTP server is configured to listen on TCP port 3131 and to use the /var/test_www/ directory instead of the default /var/www/ directory.
The policycoreutils-python-utils and setroubleshoot-server packages are installed on your system.

Procedure

Start the httpd service and check the status:

# systemctl start httpd
# systemctl status httpd
...
httpd[14523]: (13)Permission denied: AH00072: make_sock: could not bind to address [::]:3131
...
systemd[1]: Failed to start The Apache HTTP Server.
...

The SELinux policy assumes that httpd runs on port 80:

# semanage port -l | grep http
http_cache_port_t              tcp      8080, 8118, 8123, 10001-10010
http_cache_port_t              udp      3130
http_port_t                    tcp      80, 81, 443, 488, 8008, 8009, 8443, 9000
pegasus_http_port_t            tcp      5988
pegasus_https_port_t           tcp      5989

Change the SELinux type of port 3131 to match port 80:
```
# semanage port -a -t http_port_t -p tcp 3131
```
Start httpd again:
```
# systemctl start httpd
```

However, the content remains inaccessible:

# wget localhost:3131/index.html
...
HTTP request sent, awaiting response... 403 Forbidden
...

Find the reason with the sealert tool:

# sealert -l "*"
...
SELinux is preventing httpd from getattr access on the file /var/test_www/html/index.html.
...

Compare SELinux types for the standard and the new path using the matchpathcon tool:

# matchpathcon /var/www/html /var/test_www/html
/var/www/html       system_u:object_r:httpd_sys_content_t:s0
/var/test_www/html  system_u:object_r:var_t:s0

Change the SELinux type of the new /var/test_www/html/ content directory to the type of the default /var/www/html directory:
```
# semanage fcontext -a -e /var/www /var/test_www
```

Relabel the /var directory recursively:

# restorecon -Rv /var/
...
Relabeled /var/test_www/html from unconfined_u:object_r:var_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0
Relabeled /var/test_www/html/index.html from unconfined_u:object_r:var_t:s0 to unconfined_u:object_r:httpd_sys_content_t:s0

Verification

Check that the httpd service is running:

# systemctl status httpd
...
Active: active (running)
...
systemd[1]: Started The Apache HTTP Server.
httpd[14888]: Server configured, listening on: port 3131
...

Verify that the content provided by the Apache HTTP server is accessible:

# wget localhost:3131/index.html
...
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/html]
Saving to: ‘index.html’
...

Additional resources

semanage(8), matchpathcon(8), and sealert(8) man pages.

4.3. Finding the correct SELinux type for managing access to non-standard directories

If you need to set access-control rules that the default SELinux policy does not cover, start by searching for a boolean that matches your use case. If you cannot find a suitable boolean, you can use a matching SELinux type or even create a local policy module.

Prerequisites

The selinux-policy-doc and setools-console packages are installed on your system.

Procedure

List all SELinux-related topics and limit the results to a component you want to configure. For example:
```
# man -k selinux | grep samba
samba_net_selinux (8) - Security Enhanced Linux Policy for the samba_net processes
samba_selinux (8)    - Security Enhanced Linux Policy for the smbd processes
…
```
In the man page that corresponds to your scenario, find the related SELinux booleans, port types, and file types.
Note that the man -k selinux or apropos selinux commands are available only after you install the selinux-policy-doc package.

Optional: You can display the default mapping of processes on default locations by using the semanage fcontext -l command, for example:

# semanage fcontext -l | grep samba
…
/var/cache/samba(/.*)?                             all files          system_u:object_r:samba_var_t:s0
…
/var/spool/samba(/.*)?                             all files          system_u:object_r:samba_spool_t:s0
…

Use the sesearch command to display rules in the default SELinux policy. You can find the type and boolean to use by listing the corresponding rule, for example:
```
$ sesearch -A | grep samba | grep httpd
…
allow httpd_t cifs_t:dir { getattr open search }; [ use_samba_home_dirs && httpd_enable_homedirs ]:True
…
```

An SELinux boolean might be the most straightforward solution for your configuration problem. You can display all available booleans and their values by using the getsebool -a command, for example:

$ getsebool -a | grep homedirs
git_cgi_enable_homedirs --> off
git_system_enable_homedirs --> off
httpd_enable_homedirs --> off
mock_enable_homedirs --> off
mpd_enable_homedirs --> off
openvpn_enable_homedirs --> on
ssh_chroot_rw_homedirs --> off

You can verify that the selected boolean does exactly what you want by using the sesearch command, for example:

$ sesearch -A | grep httpd_enable_homedirs
…
allow httpd_suexec_t autofs_t:dir { getattr open search }; [ use_nfs_home_dirs && httpd_enable_homedirs ]:True
allow httpd_suexec_t autofs_t:dir { getattr open search }; [ use_samba_home_dirs && httpd_enable_homedirs ]:True
…

If no boolean matches your scenario, find an SELinux type that suits your case. You can find a type for your files by querying a corresponding rule from the default policy by using sesearch, for example:

$ sesearch -A -s httpd_t -c file -p read
…
allow httpd_t httpd_t:file { append getattr ioctl lock open read write };
allow httpd_t httpd_tmp_t:file { append create getattr ioctl link lock map open read rename setattr unlink write };
…

If none of the previous solutions cover your scenario, you can add a custom rule to the SELinux policy. See the Creating a local SELinux policy module section for more information.

Additional resources

SELinux-related man pages provided by the man -k selinux command
sesearch(1), semanage-fcontext(8), semanage-boolean(8), and getsebool(8) man pages on your system

4.4. Managing access to non-standard shared directories for unprivileged SELinux users

You can configure access to a non-standard shared directory for the generic unprivileged SELinux user user_u by finding and mapping the corresponding SELinux file type. The user_u user has the default role user_r and the default domain user_t.

Prerequisites

The selinux-policy-doc and setools-console packages are installed on your system.

Procedure

Open the user_selinux(8) man page in your terminal:
```
$ man user_selinux
```
In the MANAGED FILES section, find an attribute or a type that corresponds with your scenario. For example, the user_home_type attribute.

Optional: To list all types assigned to an attribute, use the seinfo command with the -x and -a options, for example:

$ seinfo -x -a user_home_type

Type Attributes: 1
   attribute user_home_type;
…
	chrome_sandbox_home_t
	config_home_t
	cvs_home_t
	data_home_t
	dbus_home_t
	fetchmail_home_t
	gconf_home_t
	git_user_content_t
…

After you identify a candidate for the corresponding type, the data_home_t type in this example, check its SELinux mapping:

$ semanage fcontext -l | grep data_home_t
…
/root/\.local/share(/.*)?                          all files          system_u:object_r:data_home_t:s0
…

Map the corresponding type to a directory that you want to make accessible for user_u, for example, /shared-data:
```
$ semanage fcontext -a -t data_home_t '/shared-data(/.*)?'
```

Verification

Check the mapping of the directory you configured:

# semanage fcontext -l | grep "shared-data"
/shared-data(/.*)?                             	all files      	system_u:object_r:data_home_t:s0

Log in as a Linux user mapped to the user_u SELinux user, and verify you can access the directory.

Additional resources

Managing confined and unconfined users
seinfo(1), semanage-fcontext(8), and user_selinux(8) man pages on your system

Chapter 5. Troubleshooting problems related to SELinux

If you plan to enable SELinux on systems where it has been previously disabled or if you run a service in a non-standard configuration, you might need to troubleshoot situations potentially blocked by SELinux. Note that in most cases, SELinux denials are signs of misconfiguration.

5.3. Fixing analyzed SELinux denials

In most cases, suggestions provided by the sealert tool give you the right guidance about how to fix problems related to the SELinux policy. See Analyzing SELinux denial messages for information how to use sealert to analyze SELinux denials.

Be careful when the tool suggests using the audit2allow tool for configuration changes. You should not use audit2allow to generate a local policy module as your first option when you see an SELinux denial. Troubleshooting should start with a check if there is a labeling problem. The second most often case is that you have changed a process configuration, and you forgot to tell SELinux about it.

Labeling problems

A common cause of labeling problems is when a non-standard directory is used for a service. For example, instead of using /var/www/html/ for a website, an administrator might want to use /srv/myweb/. On Red Hat Enterprise Linux, the /srv directory is labeled with the var_t type. Files and directories created in /srv inherit this type. Also, newly-created objects in top-level directories, such as /myserver, can be labeled with the default_t type. SELinux prevents the Apache HTTP Server (httpd) from accessing both of these types. To allow access, SELinux must know that the files in /srv/myweb/ are to be accessible by httpd:

# semanage fcontext -a -t httpd_sys_content_t "/srv/myweb(/.*)?"

This semanage command adds the context for the /srv/myweb/ directory and all files and directories under it to the SELinux file-context configuration. The semanage utility does not change the context. As root, use the restorecon utility to apply the changes:

# restorecon -R -v /srv/myweb

Incorrect context

The matchpathcon utility checks the context of a file path and compares it to the default label for that path. The following example demonstrates the use of matchpathcon on a directory that contains incorrectly labeled files:

$ matchpathcon -V /var/www/html/*
/var/www/html/index.html has context unconfined_u:object_r:user_home_t:s0, should be system_u:object_r:httpd_sys_content_t:s0
/var/www/html/page1.html has context unconfined_u:object_r:user_home_t:s0, should be system_u:object_r:httpd_sys_content_t:s0

In this example, the index.html and page1.html files are labeled with the user_home_t type. This type is used for files in user home directories. Using the mv command to move files from your home directory may result in files being labeled with the user_home_t type. This type should not exist outside of home directories. Use the restorecon utility to restore such files to their correct type:

# restorecon -v /var/www/html/index.html
restorecon reset /var/www/html/index.html context unconfined_u:object_r:user_home_t:s0->system_u:object_r:httpd_sys_content_t:s0

To restore the context for all files under a directory, use the -R option:

# restorecon -R -v /var/www/html/
restorecon reset /var/www/html/page1.html context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:httpd_sys_content_t:s0
restorecon reset /var/www/html/index.html context unconfined_u:object_r:samba_share_t:s0->system_u:object_r:httpd_sys_content_t:s0

Confined applications configured in non-standard ways

Services can be run in a variety of ways. To account for that, you need to specify how you run your services. You can achieve this through SELinux booleans that allow parts of SELinux policy to be changed at runtime. This enables changes, such as allowing services access to NFS volumes, without reloading or recompiling SELinux policy. Also, running services on non-default port numbers requires policy configuration to be updated using the semanage command.

For example, to allow the Apache HTTP Server to communicate with MariaDB, enable the httpd_can_network_connect_db boolean:

# setsebool -P httpd_can_network_connect_db on

Note that the -P option makes the setting persistent across reboots of the system.

If access is denied for a particular service, use the getsebool and grep utilities to see if any booleans are available to allow access. For example, use the getsebool -a | grep ftp command to search for FTP related booleans:

$ getsebool -a | grep ftp
ftpd_anon_write --> off
ftpd_full_access --> off
ftpd_use_cifs --> off
ftpd_use_nfs --> off

ftpd_connect_db --> off
httpd_enable_ftp_server --> off
tftp_anon_write --> off

To get a list of booleans and to find out if they are enabled or disabled, use the getsebool -a command. To get a list of booleans including their meaning, and to find out if they are enabled or disabled, install the selinux-policy-devel package and use the semanage boolean -l command as root.

Port numbers

Depending on policy configuration, services can only be allowed to run on certain port numbers. Attempting to change the port a service runs on without changing policy may result in the service failing to start. For example, run the semanage port -l | grep http command as root to list http related ports:

# semanage port -l | grep http
http_cache_port_t              tcp      3128, 8080, 8118
http_cache_port_t              udp      3130
http_port_t                    tcp      80, 443, 488, 8008, 8009, 8443
pegasus_http_port_t            tcp      5988
pegasus_https_port_t           tcp      5989

The http_port_t port type defines the ports Apache HTTP Server can listen on, which in this case, are TCP ports 80, 443, 488, 8008, 8009, and 8443. If an administrator configures httpd.conf so that httpd listens on port 9876 (Listen 9876), but policy is not updated to reflect this, the following command fails:

# systemctl start httpd.service
Job for httpd.service failed. See 'systemctl status httpd.service' and 'journalctl -xn' for details.

# systemctl status httpd.service
httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
   Active: failed (Result: exit-code) since Thu 2013-08-15 09:57:05 CEST; 59s ago
  Process: 16874 ExecStop=/usr/sbin/httpd $OPTIONS -k graceful-stop (code=exited, status=0/SUCCESS)
  Process: 16870 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)

An SELinux denial message similar to the following is logged to /var/log/audit/audit.log:

type=AVC msg=audit(1225948455.061:294): avc:  denied  { name_bind } for  pid=4997 comm="httpd" src=9876 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=system_u:object_r:port_t:s0 tclass=tcp_socket

To allow httpd to listen on a port that is not listed for the http_port_t port type, use the semanage port command to assign a different label to the port:

# semanage port -a -t http_port_t -p tcp 9876

The -a option adds a new record; the -t option defines a type; and the -p option defines a protocol. The last argument is the port number to add.

Corner cases, evolving or broken applications, and compromised systems

Applications may contain bugs, causing SELinux to deny access. Also, SELinux rules are evolving – SELinux may not have seen an application running in a certain way, possibly causing it to deny access, even though the application is working as expected. For example, if a new version of PostgreSQL is released, it may perform actions the current policy does not account for, causing access to be denied, even though access should be allowed.

For these situations, after access is denied, use the audit2allow utility to create a custom policy module to allow access. You can report missing rules in the SELinux policy in Red Hat Bugzilla. For Red Hat Enterprise Linux 8, create bugs against the Red Hat Enterprise Linux 8 product, and select the selinux-policy component. Include the output of the audit2allow -w -a and audit2allow -a commands in such bug reports.

If an application asks for major security privileges, it could be a signal that the application is compromised. Use intrusion detection tools to inspect such suspicious behavior.

The Solution Engine on the Red Hat Customer Portal can also provide guidance in the form of an article containing a possible solution for the same or very similar problem you have. Select the relevant product and version and use SELinux-related keywords, such as selinux or avc, together with the name of your blocked service or application, for example: selinux samba.

5.5. SELinux denials in the Audit log

The Linux Audit system stores log entries in the /var/log/audit/audit.log file by default.

To list only SELinux-related records, use the ausearch command with the message type parameter set to AVC and AVC_USER at a minimum, for example:

# ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR

An SELinux denial entry in the Audit log file can look as follows:

type=AVC msg=audit(1395177286.929:1638): avc:  denied  { read } for  pid=6591 comm="httpd" name="webpages" dev="0:37" ino=2112 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:nfs_t:s0 tclass=dir

The most important parts of this entry are:

avc: denied - the action performed by SELinux and recorded in Access Vector Cache (AVC)
{ read } - the denied action
pid=6591 - the process identifier of the subject that tried to perform the denied action
comm="httpd" - the name of the command that was used to invoke the analyzed process
httpd_t - the SELinux type of the process
nfs_t - the SELinux type of the object affected by the process action
tclass=dir - the target object class

The previous log entry can be translated to:

SELinux denied the httpd process with PID 6591 and the httpd_t type to read from a directory with the nfs_t type.

The following SELinux denial message occurs when the Apache HTTP Server attempts to access a directory labeled with a type for the Samba suite:

type=AVC msg=audit(1226874073.147:96): avc:  denied  { getattr } for  pid=2465 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file

{ getattr } - the getattr entry indicates the source process was trying to read the target file’s status information. This occurs before reading files. SELinux denies this action because the process accesses the file and it does not have an appropriate label. Commonly seen permissions include getattr, read, and write.
path="/var/www/html/file1" - the path to the object (target) the process attempted to access.
scontext="unconfined_u:system_r:httpd_t:s0" - the SELinux context of the process (source) that attempted the denied action. In this case, it is the SELinux context of the Apache HTTP Server, which is running with the httpd_t type.
tcontext="unconfined_u:object_r:samba_share_t:s0" - the SELinux context of the object (target) the process attempted to access. In this case, it is the SELinux context of file1.

This SELinux denial can be translated to:

SELinux denied the httpd process with PID 2465 to access the /var/www/html/file1 file with the samba_share_t type, which is not accessible to processes running in the httpd_t domain unless configured otherwise.

Additional resources

auditd(8) and ausearch(8) man pages on your system

Chapter 6. Using Multi-Level Security (MLS)

The Multi-Level Security (MLS) policy uses levels of clearance as originally designed by the US defense community. MLS meets a very narrow set of security requirements based on information management in rigidly controlled environments such as the military.

Using MLS is complex and does not map well to general use-case scenarios.

6.1. Multi-Level Security (MLS)

The Multi-Level Security (MLS) technology classifies data in a hierarchical classification using information security levels, for example:

[lowest] Unclassified
[low] Confidential
[high] Secret
[highest] Top secret

By default, the MLS SELinux policy uses 16 sensitivity levels:

s0 is the least sensitive.
s15 is the most sensitive.

MLS uses specific terminology to address sensitivity levels:

Users and processes are called subjects, whose sensitivity level is called clearance.
Files, devices, and other passive components of the system are called objects, whose sensitivity level is called classification.

To implement MLS, SELinux uses the Bell-La Padula Model (BLP) model. This model specifies how information can flow within the system based on labels attached to each subject and object.

The basic principle of BLP is “No read up, no write down.” This means that users can only read files at their own sensitivity level and lower, and data can flow only from lower levels to higher levels, and never the reverse.

The MLS SELinux policy, which is the implementation of MLS on RHEL, applies a modified principle called Bell-La Padula with write equality. This means that users can read files at their own sensitivity level and lower, but can write only at exactly their own level. This prevents, for example, low-clearance users from writing content into top-secret files.

For example, by default, a user with clearance level s2:

Can read files with sensitivity levels s0, s1, and s2.
Cannot read files with sensitivity level s3 and higher.
Can modify files with sensitivity level of exactly s2.
Cannot modify files with sensitivity level other than s2.

Note

Security administrators can adjust this behavior by modifying the system’s SELinux policy. For example, they can allow users to modify files at lower levels, which increases the file’s sensitivity level to the user’s clearance level.

In practice, users are typically assigned to a range of clearance levels, for example s1-s2. A user can read files with sensitivity levels lower than the user’s maximum level, and write to any files within that range.

For example, by default, a user with a clearance range s1-s2:

Can read files with sensitivity levels s0 and s1.
Cannot read files with sensitivity level s2 and higher.
Can modify files with sensitivity level s1.
Cannot modify files with sensitivity level other than s1.
Can change own clearance level to s2.

The security context for a non-privileged user in an MLS environment is, for example:

user_u:user_r:user_t:s1

Where:

user_u: Is the SELinux user.
user_r: Is the SELinux role.
user_t: Is the SELinux type.
s1: Is the range of MLS sensitivity levels.

The system always combines MLS access rules with conventional file access permissions. For example, if a user with a security level of "Secret" uses Discretionary Access Control (DAC) to block access to a file by other users, even “Top Secret” users cannot access that file. A high security clearance does not automatically permit a user to browse the entire file system.

Users with top-level clearances do not automatically acquire administrative rights on multi-level systems. While they might have access to all sensitive information about the system, this is different from having administrative rights.

In addition, administrative rights do not provide access to sensitive information. For example, even when someone logs in as root, they still cannot read top-secret information.

You can further adjust access within an MLS system by using categories. With Multi-Category Security (MCS), you can define categories such as projects or departments, and users will only be allowed to access files in the categories to which they are assigned. For additional information, see Using Multi-Category Security (MCS) for data confidentiality .

6.2. SELinux roles in MLS

The SELinux policy maps each Linux user to an SELinux user. This allows Linux users to inherit the restrictions of SELinux users.

Important

The MLS policy does not contain the unconfined module, including unconfined users, types, and roles. As a result, users that would be unconfined, including root, cannot access every object and perform every action they could in the targeted policy.

You can customize the permissions for confined users in your SELinux policy according to specific needs by adjusting the booleans in policy. You can determine the current state of these booleans by using the semanage boolean -l command. To list all SELinux users, their SELinux roles, and MLS/MCS levels and ranges, use the semanage user -l command as root.

Table 6.1. Roles of SELinux users in MLS
User	Default role	Additional roles
`guest_u`	`guest_r`
`xguest_u`	`xguest_r`
`user_u`	`user_r`
`staff_u`	`staff_r`	`auditadm_r`
		`secadm_r`
		`sysadm_r`
		`staff_r`
`sysadm_u`	`sysadm_r`
`root`	`staff_r`	`auditadm_r`
		`secadm_r`
		`sysadm_r`
		`system_r`
`system_u`	`system_r`

Note that system_u is a special user identity for system processes and objects, and system_r is the associated role. Administrators must never associate this system_u user and the system_r role to a Linux user. Also, unconfined_u and root are unconfined users. For these reasons, the roles associated to these SELinux users are not included in the following table Types and access of SELinux roles.

Each SELinux role corresponds to an SELinux type and provides specific access rights.

Table 6.2. Types and access of SELinux roles in MLS
Role	Type	Login using X Window System	`su` and `sudo`	Execute in home directory and `/tmp` (default)	Networking
`guest_r`	`guest_t`	no	no	yes	no
`xguest_r`	`xguest_t`	yes	no	yes	web browsers only (Firefox, GNOME Web)
`user_r`	`user_t`	yes	no	yes	yes
`staff_r`	`staff_t`	yes	only `sudo`	yes	yes
`auditadm_r`	`auditadm_t`		yes	yes	yes
`secadm_r`	`secadm_t`		yes	yes	yes
`sysadm_r`	`sysadm_t`	only when the `xdm_sysadm_login` boolean is `on`	yes	yes	yes

By default, the sysadm_r role has the rights of the secadm_r role, which means a user with the sysadm_r role can manage the security policy. If this does not correspond to your use case, you can separate the two roles by disabling the sysadm_secadm module in the policy. For additional information, see Separating system administration from security administration in MLS.
Non-login roles dbadm_r, logadm_r, and webadm_r can be used for a subset of administrative tasks. By default, these roles are not associated with any SELinux user.

6.3. Switching the SELinux policy to MLS

Use the following steps to switch the SELinux policy from targeted to Multi-Level Security (MLS).

Important

Do not use the MLS policy on a system that is running the X Window System. Furthermore, when you relabel the file system with MLS labels, the system may prevent confined domains from access, which prevents your system from starting correctly. Therefore ensure that you switch SELinux to permissive mode before you relabel the files. On most systems, you see a lot of SELinux denials after switching to MLS, and many of them are not trivial to fix.

Procedure

Install the selinux-policy-mls package:
```
# yum install selinux-policy-mls
```
Open the /etc/selinux/config file in a text editor of your choice, for example:
```
# vi /etc/selinux/config
```
Change SELinux mode from enforcing to permissive and switch from the targeted policy to MLS:
```
SELINUX=permissive
SELINUXTYPE=mls
```
Save the changes, and quit the editor.
Before you enable the MLS policy, you must relabel each file on the file system with an MLS label:
```
# fixfiles -F onboot
System will relabel on next boot
```
Restart the system:
```
# reboot
```
Check for SELinux denials:
```
# ausearch -m AVC,USER_AVC,SELINUX_ERR,USER_SELINUX_ERR -ts recent -i
```
Because the previous command does not cover all scenarios, see Troubleshooting problems related to SELinux for guidance on identifying, analyzing, and fixing SELinux denials.
After you ensure that there are no problems related to SELinux on your system, switch SELinux back to enforcing mode by changing the corresponding option in /etc/selinux/config:
```
SELINUX=enforcing
```
Restart the system:
```
# reboot
```

Important

If your system does not start or you are not able to log in after you switch to MLS, add the enforcing=0 parameter to your kernel command line. See Changing SELinux modes at boot time for more information.

Also note that in MLS, SSH logins as the root user mapped to the sysadm_r SELinux role differ from logging in as root in staff_r. Before you start your system in MLS for the first time, consider allowing SSH logins as sysadm_r by setting the ssh_sysadm_login SELinux boolean to 1. To enable ssh_sysadm_login later, already in MLS, you must log in as root in staff_r, switch to root in sysadm_r using the newrole -r sysadm_r command, and then set the boolean to 1.

Verification

Verify that SELinux runs in enforcing mode:
```
# getenforce
Enforcing
```

Check that the status of SELinux returns the mls value:

# sestatus | grep mls
Loaded policy name:             mls

Additional resources

fixfiles(8), setsebool(8), and ssh_selinux(8) man pages on your system

6.4. Establishing user clearance in MLS

After you switch SELinux policy to MLS, you must assign security clearance levels to users by mapping them to confined SELinux users. By default, a user with a given security clearance:

Cannot read objects that have a higher sensitivity level.
Cannot write to objects at a different sensitivity level.

Prerequisites

The SELinux policy is set to mls.
The SELinux mode is set to enforcing.
The policycoreutils-python-utils package is installed.
A user assigned to an SELinux confined user:
- For a non-privileged user, assigned to user_u (example_user in the following procedure).
- For a privileged user, assigned to staff_u (staff in the following procedure) .

Important

Make sure that the users have been created when the MLS policy was active. Users created in other SELinux policies cannot be used in MLS.

Procedure

Optional: To prevent adding errors to your SELinux policy, switch to the permissive SELinux mode, which facilitates troubleshooting:
```
# setenforce 0
```
Note that in permissive mode, SELinux does not enforce the active policy but only logs Access Vector Cache (AVC) messages, which can be then used for troubleshooting and debugging.
Define a clearance range for the staff_u SELinux user. For example, this command sets the clearance range from s1 to s15 with s1 being the default clearance level:
```
# semanage user -m -L s1 -r s1-s15 staff_u
```
Generate SELinux file context configuration entries for user home directories:
```
# genhomedircon
```

Restore file security contexts to default:

# restorecon -R -F -v /home/
Relabeled /home/staff from staff_u:object_r:user_home_dir_t:s0 to staff_u:object_r:user_home_dir_t:s1
Relabeled /home/staff/.bash_logout from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1
Relabeled /home/staff/.bash_profile from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1
Relabeled /home/staff/.bashrc from staff_u:object_r:user_home_t:s0 to staff_u:object_r:user_home_t:s1

Assign a clearance level to the user:
```
# semanage login -m -r s1 example_user
```
Where s1 is the clearance level assigned to the user.
Relabel the user’s home directory to the user’s clearance level:
```
# chcon -R -l s1 /home/example_user
```
Optional: If you previously switched to the permissive SELinux mode, and after you verify that everything works as expected, switch back to the enforcing SELinux mode:
```
# setenforce 1
```

Verification

Verify that the user is mapped to the correct SELinux user and has the correct clearance level assigned:

# semanage login -l
Login Name      SELinux User         MLS/MCS Range        Service
__default__     user_u               s0-s0                *
example_user    user_u               s1                   *
…

Log in as the user within MLS.
Verify that the user’s security level works correctly:
Warning
The files you use for verification should not contain any sensitive information in case the configuration is incorrect and the user actually can access the files without authorization.
1. Verify that the user cannot read a file with a higher-level sensitivity.
2. Verify that the user can write to a file with the same sensitivity.
3. Verify that the user can read a file with a lower-level sensitivity.

Additional resources

Switching the SELinux policy to MLS
Adding a new user as an SELinux-confined user
Permanent changes in SELinux states and modes
Troubleshooting problems related to SELinux
Basic SELinux Troubleshooting in CLI Knowledgebase article

6.5. Changing a user’s clearance level within the defined security range in MLS

As a user in Multi-Level Security (MLS), you can change your current clearance level within the range the administrator assigned to you. You can never exceed the upper limit of your range or reduce your level below the lower limit of your range. This allows you, for example, to modify lower-sensitivity files without increasing their sensitivity level to your highest clearance level.

For example, as a user assigned to range s1-s3:

You can switch to levels s1, s2, and s3.
You can switch to ranges s1-s2, and s2-s3.
You cannot switch to ranges s0-s3 or s1-s4.

Switching to a different level opens a new shell with the different clearance. This means you cannot return to your original clearance level in the same way as decreasing it. However, you can always return to the previous shell by entering exit.

Prerequisites

The SELinux policy is set to mls.
SELinux mode is set to enforcing.
You can log in as a user assigned to a range of MLS clearance levels.

Procedure

Log in as the user from a secure terminal.
Secure terminals are defined in the /etc/selinux/mls/contexts/securetty_types file. By default, the console is a secure terminal, but SSH is not.
Check the current user’s security context:
```
$ id -Z
user_u:user_r:user_t:s0-s2
```
In this example, the user is assigned to the user_u SELinux user, user_r role, user_t type, and the MLS security range s0-s2.
Check the current user’s security context:
```
$ id -Z
user_u:user_r:user_t:s1-s2
```
Switch to a different security clearance range within the user’s clearance range:
```
$ newrole -l s1
```
You can switch to any range whose maximum is lower or equal to your assigned range. Entering a single-level range changes the lower limit of the assigned range. For example, entering newrole -l s1 as a user with a s0-s2 range is equivalent to entering newrole -l s1-s2.

Verification

Display the current user’s security context:
```
$ id -Z
user_u:user_r:user_t:s1-s2
```
Return to the previous shell with the original range by terminating the current shell:
```
$ exit
```

Additional resources

Establishing user clearance in MLS
newrole(1) and securetty_types(5) man pages on your system

6.6. Increasing file sensitivity levels in MLS

By default, Multi-Level Security (MLS) users cannot increase file sensitivity levels. However, the security administrator (secadm_r) can change this default behavior to allow users to increase the sensitivity of files by adding the local module mlsfilewrite to the system’s SELinux policy. Then, users assigned to the SELinux type defined in the policy module can increase file classification levels by modifying the file. Any time a user modifies a file, the file’s sensitivity level increases to the lower value of the user’s current security range.

The security administrator, when logged in as a user assigned to the secadm_r role, can change the security levels of files by using the chcon -l s0 /path/to/file command. For more information, see Changing file sensitivity in MLS.

Prerequisites

The SELinux policy is set to mls.
SELinux mode is set to enforcing.
The policycoreutils-python-utils package is installed.
The mlsfilewrite local module is installed in the SELinux MLS policy.
You are logged in as a user in MLS which is:
- Assigned to a defined security range. This example shows a user with a security range s0-s2.
- Assigned to the same SELinux type defined in the mlsfilewrite module. This example requires the (typeattributeset mlsfilewrite (user_t)) module.

Procedure

Optional: Display the security context of the current user:
```
$ id -Z
user_u:user_r:user_t:s0-s2
```
Change the lower level of the user’s MLS clearance range to the level which you want to assign to the file:
```
$ newrole -l s1-s2
```
Optional: Display the security context of the current user:
```
$ id -Z
user_u:user_r:user_t:s1-s2
```

Optional: Display the security context of the file:

$ ls -Z /path/to/file
user_u:object_r:user_home_t:s0 /path/to/file

Change the file’s sensitivity level to the lower level of the user’s clearance range by modifying the file:
```
$ touch /path/to/file
```
Important
The classification level reverts to the default value if the restorecon command is used on the system.
Optional: Exit the shell to return to the user’s previous security range:
```
$ exit
```

Verification

Display the security context of the file:

$ ls -Z /path/to/file
user_u:object_r:user_home_t:s1 /path/to/file

Additional resources

Allowing MLS users to edit files on lower levels.

6.7. Changing file sensitivity in MLS

In the MLS SELinux policy, users can only modify files at their own sensitivity level. This is intended to prevent any highly sensitive information to be exposed to users at lower clearance levels, and also prevent low-clearance users creating high-sensitivity documents. Administrators, however, can manually increase a file’s classification, for example for the file to be processed at the higher level.

Prerequisites

SELinux policy is set to mls.
SELinux mode is set to enforcing.
You have security administration rights, which means that you are assigned to either:
- The secadm_r role.
- If the sysadm_secadm module is enabled, to the sysadm_r role. The sysadm_secadm module is enabled by default.
The policycoreutils-python-utils package is installed.
A user assigned to any clearance level. For additional information, see Establishing user clearance levels in MLS .
In this example, User1 has clearance level s1.
A file with a classification level assigned and to which you have access.
In this example, /path/to/file has classification level s1.

Procedure

Check the file’s classification level:

# ls -lZ /path/to/file
-rw-r-----. 1 User1 User1 user_u:object_r:user_home_t:s1 0 12. Feb 10:43 /path/to/file

Change the file’s default classification level:
```
# semanage fcontext -a -r s2 /path/to/file
```

Force the relabeling of the file’s SELinux context:

# restorecon -F -v /path/to/file
Relabeled /path/to/file from user_u:object_r:user_home_t:s1 to user_u:object_r:user_home_t:s2

Verification

Check the file’s classification level:

# ls -lZ /path/to/file
-rw-r-----. 1 User1 User1 user_u:object_r:user_home_t:s2 0 12. Feb 10:53 /path/to/file

Optional: Verify that the lower-clearance user cannot read the file:
```
$ cat /path/to/file
cat: file: Permission denied
```

Additional resources

Establishing user clearance levels in MLS .

6.8. Separating system administration from security administration in MLS

By default, the sysadm_r role has the rights of the secadm_r role, which means a user with the sysadm_r role can manage the security policy. If you need more control over security authorizations, you can separate system administration from security administration by assigning a Linux user to the secadm_r role and disabling the sysadm_secadm module in the SELinux policy.

Prerequisites

The SELinux policy is set to mls.
The SELinux mode is set to enforcing.
The policycoreutils-python-utils package is installed.
A Linux user which will be assigned to the secadm_r role:
- The user is assigned to the staff_u SELinux user
- A password for this user has been defined.
Warning
Make sure you can log in as the user which will be assigned to the secadm role. If not, you can prevent any future modifications of the system’s SELinux policy.

Procedure

Create a new sudoers file in the /etc/sudoers.d directory for the user:
```
# visudo -f /etc/sudoers.d/<sec_adm_user>
```
To keep the sudoers files organized, replace <sec_adm_user> with the Linux user which will be assigned to the secadm role.
Add the following content into the /etc/sudoers.d/<sec_adm_user> file:
```
<sec_adm_user> ALL=(ALL) TYPE=secadm_t ROLE=secadm_r ALL
```
This line authorizes <secadmuser> on all hosts to perform all commands, and maps the user to the secadm SELinux type and role by default.
Log in as the <sec_adm_user> user.
To make sure that the SELinux context (which consists of SELinux user, role, and type) is changed, log in using ssh, the console, or xdm. Other ways, such as su and sudo, cannot change the entire SELinux context.

Verify the user’s security context:

$ id
uid=1000(<sec_adm_user>) gid=1000(<sec_adm_user>) groups=1000(<sec_adm_user>) context=staff_u:staff_r:staff_t:s0-s15:c0.c1023

Run the interactive shell for the root user:

$ sudo -i
[sudo] password for <sec_adm_user>:

Verify the current user’s security context:

# id
uid=0(root) gid=0(root) groups=0(root) context=staff_u:secadm_r:secadm_t:s0-s15:c0.c1023

Disable the sysadm_secadm module from the policy:
```
# semodule -d sysadm_secadm
```
Important
Use the semodule -d command instead of removing the system policy module by using the semodule -r command. The semodule -r command deletes the module from your system’s storage, which means it cannot be loaded again without reinstalling the selinux-policy-mls package.

Verification

As the user assigned to the secadm role, and in the interactive shell for the root user, verify that you can access the security policy data:
```
# seinfo -xt secadm_t

Types: 1
   type secadm_t, can_relabelto_shadow_passwords, (…) userdomain;
```
Log out from the root shell:
```
# logout
```

Log out from the <sec_adm_user> user:

$ logout
Connection to localhost closed.

Display the current security context:

# id
uid=0(root) gid=0(root) groups=0(root) context=root:sysadm_r:sysadm_t:s0-s15:c0.c1023

Attempt to enable the sysadm_secadm module. The command should fail:

# semodule -e sysadm_secadm
SELinux:  Could not load policy file /etc/selinux/mls/policy/policy.31:  Permission denied
/sbin/load_policy:  Can't load policy:  Permission denied
libsemanage.semanage_reload_policy: load_policy returned error code 2. (No such file or directory).
SELinux:  Could not load policy file /etc/selinux/mls/policy/policy.31:  Permission denied
/sbin/load_policy:  Can't load policy:  Permission denied
libsemanage.semanage_reload_policy: load_policy returned error code 2. (No such file or directory).
semodule:  Failed!

Attempt to display the details about the sysadm_t SELinux type. The command should fail:
```
# seinfo -xt sysadm_t
[Errno 13] Permission denied: '/sys/fs/selinux/policy'
```

6.9. Defining a secure terminal in MLS

The SELinux policy checks the type of the terminal from which a user is connected, and allows running of certain SELinux applications, for example newrole, only from secure terminals. Attempting this from a non-secure terminal produces an error: Error: you are not allowed to change levels on a non secure terminal;.

The /etc/selinux/mls/contexts/securetty_types file defines secure terminals for the Multi-Level Security (MLS) policy.

Default contents of the file:

console_device_t
sysadm_tty_device_t
user_tty_device_t
staff_tty_device_t
auditadm_tty_device_t
secureadm_tty_device_t

Warning

Adding terminal types to the list of secure terminals can expose your system to security risks.

Prerequisites

SELinux policy is set to mls.
You are connected from an already secure terminal, or SELinux is in permissive mode.
You have security administration rights, which means that you are assigned to either:
- The secadm_r role.
- If the sysadm_secadm module is enabled, to the sysadm_r role. The sysadm_secadm module is enabled by default.
The policycoreutils-python-utils package is installed.

Procedure

Determine the current terminal type:
```
# ls -Z `tty`
root:object_r:user_devpts_t:s0 /dev/pts/0
```
In this example output, user_devpts_t is the current terminal type.
Add the relevant SELinux type on a new line in the /etc/selinux/mls/contexts/securetty_types file.
Optional: Switch SELinux to enforcing mode:
```
# setenforce 1
```

Verification

Log in from the previously insecure terminal you have added to the /etc/selinux/mls/contexts/securetty_types file.

Additional resources

securetty_types(5) man page on your system

6.10. Allowing MLS users to edit files on lower levels

By default, MLS users cannot write to files which have a sensitivity level below the lower value of the clearance range. If your scenario requires allowing users to edit files on lower levels, you can do so by creating a local SELinux module. However, writing to a file will increase its sensitivity level to the lower value of the user’s current range.

Prerequisites

The SELinux policy is set to mls.
The SELinux mode is set to enforcing.
The policycoreutils-python-utils package is installed.
The setools-console and audit packages for verification.

Procedure

Optional: Switch to permissive mode for easier troubleshooting.
```
# setenforce 0
```
Open a new .cil file with a text editor, for example ~/local_mlsfilewrite.cil, and insert the following custom rule:
```
(typeattributeset mlsfilewrite (_staff_t_))
```
You can replace staff_t with a different SELinux type. By specifying SELinux type here, you can control which SELinux roles can edit lower-level files.
To keep your local modules better organized, use the local_ prefix in the names of local SELinux policy modules.
Install the policy module:
```
# semodule -i ~/local_mlsfilewrite.cil
```
Note
To remove the local policy module, use semodule -r ~/local_mlsfilewrite. Note that you must refer to the module name without the .cil suffix.
Optional: If you previously switched back to permissive mode, return to enforcing mode:
```
# setenforce 1
```

Verification

Find the local module in the list of installed SELinux modules:
```
# semodule -lfull | grep "local_mls"
400 local_mlsfilewrite  cil
```
Because local modules have priority 400, you can list them also by using the semodule -lfull | grep -v ^100 command.
Log in as a user assigned to the type defined in the custom rule, for example, staff_t.
Attempt to write to a file with a lower sensitivity level. This increases the file’s classification level to the user’s clearance level.
Important
The files you use for verification should not contain any sensitive information in case the configuration is incorrect and the user actually can access the files without authorization.

Chapter 7. Using Multi-Category Security (MCS) for data confidentiality

You can use MCS to enhance the data confidentiality of your system by categorizing data, and then granting certain processes and users access to specific categories

7.1. Multi-Category Security (MCS)

Multi-Category Security (MCS) is an access control mechanism that uses categories assigned to processes and files. Files can then be accessed only by processes that are assigned to the same categories. The purpose of MCS is to maintain data confidentiality on your system.

MCS categories are defined by the values c0 to c1023, but you can also define a text label for each category or combination of categories, such as “Personnel”, “ProjectX”, or “ProjectX.Personnel”. The MCS Translation service (mcstrans) then replaces the category values with the appropriate labels in system inputs and outputs, so that users can use these labels instead of the category values.

When users are assigned to categories, they can label any of their files with any of the categories to which they have been assigned.

MCS works on a simple principle: to access a file, a user must be assigned to all of the categories that have been assigned to the file. The MCS check is applied after normal Linux Discretionary Access Control (DAC) and SELinux Type Enforcement (TE) rules, so it can only further restrict existing security configuration.

MCS within Multi-Level Security

You can use MCS on its own as a non-hierarchical system, or you can use it in combination with Multi-Level Security (MLS) as a non-hierarchical layer within a hierarchical system.

An example of MCS within MLS could be a secretive research organization, where files are classified like this:

Table 7.1. Example of combinations of security levels and categories
Security level	Category
Security level	Not specified	Project X	Project Y	Project Z
Unclassified	`s0`	`s0:c0`	`s0:c1`	`s0:c2`
Confidential	`s1`	`s1:c0`	`s1:c1`	`s1:c2`
Secret	`s2`	`s2:c0`	`s2:c1`	`s2:c2`
Top secret	`s3`	`s3:c0`	`s3:c1`	`s3:c2`

Note

A user with a range s0:c0.1023 would be able to access all files assigned to all categories on level s0, unless the access is prohibited by other security mechanisms, such as DAC or type enforcement policy rules.

The resulting security context of a file or process is a combination of:

SELinux user
SELinux role
SELinux type
MLS sensitivity level
MCS category

For example, a non-privileged user with access to sensitivity level 1 and category 2 in an MLS/MCS environment could have the following SELinux context:

user_u:user_r:user_t:s1:c2

Additional resources

Using Multi-Level Security (MLS)

7.2. Configuring Multi-Category Security for data confidentiality

By default, Multi-Category Security (MCS) is active in the targeted and mls SELinux policies but is not configured for users. In the targeted policy, MCS is configured only for:

OpenShift
virt
sandbox
network labeling
containers (container-selinux)

You can configure MCS to categorize users by creating a local SELinux module with a rule that constrains the user_t SELinux type by MCS rules in addition to type enforcement.

Warning

Changing the categories of certain files may render some services non-operational. If you are not an expert, contact your Red Hat sales representative and request consulting services.

Prerequisites

SELinux mode is set to enforcing.
The SELinux policy is set to targeted or mls.
The policycoreutils-python-utils and setools-console packages are installed.

Procedure

Create a new file named, for example, local_mcs_user.cil:
```
# vim local_mcs_user.cil
```

Insert the following rule:

(typeattributeset mcs_constrained_type (user_t))

Install the policy module:
```
# semodule -i local_mcs_user.cil
```

Verification

For each user domain, display additional details for all the components:

# seinfo -xt user_t

Types: 1
type user_t, application_domain_type, nsswitch_domain, corenet_unlabeled_type, domain, kernel_system_state_reader, mcs_constrained_type, netlabel_peer_type, privfd, process_user_target, scsi_generic_read, scsi_generic_write, syslog_client_type, pcmcia_typeattr_1, user_usertype, login_userdomain, userdomain, unpriv_userdomain, userdom_home_reader_type, userdom_filetrans_type, xdmhomewriter, x_userdomain, x_domain, dridomain, xdrawable_type, xcolormap_type;

Additional resources

Creating a local SELinux policy module
For more information about MCS in the context of containers, see the blog posts How SELinux separates containers using Multi-Level Security and Why you should be using Multi-Category Security for your Linux containers.

7.3. Defining category labels in MCS

You can manage and maintain labels for MCS categories, or combinations of MCS categories with MLS levels, on your system by editing the setrans.conf file. In this file, SELinux maintains a mapping between internal sensitivity and category levels and their human-readable labels.

Note

Category labels only make it easier for users to use the categories. MCS works the same whether you define labels or not.

Prerequisites

The SELinux mode is set to enforcing.
The SELinux policy is set to targeted or mls.
The policycoreutils-python-utils and mcstrans packages are installed.

Procedure

Modify existing categories or create new categories by editing the /etc/selinux/<selinuxpolicy>/setrans.conf file in a text editor. Replace <selinuxpolicy> with targeted or mls depending on the SELinux policy you use. For example:
```
# vi /etc/selinux/targeted/setrans.conf
```
In the setrans.conf file for your policy, define the combinations of categories required by your scenario using the syntax s_<security level>_:c_<category number>_=<category.name>, for example:
```
s0:c0=Marketing
s0:c1=Finance
s0:c2=Payroll
s0:c3=Personnel
```
- You can use category numbers from c0 to c1023.
- In the targeted policy, use the s0 security level.
- In the mls policy, you can label each combination of sensitivity levels and categories.
Optional: In the setrans.conf file, you can also label the MLS sensitivity levels.
Save and exit the file.
To make the changes effective, restart the MCS translation service:
```
# systemctl restart mcstrans
```

Verification

Display the current categories:

# chcat -L

The example above produces the following output:

s0:c0                          Marketing
s0:c1                          Finance
s0:c2                          Payroll
s0:c3                          Personnel
s0
s0-s0:c0.c1023                 SystemLow-SystemHigh
s0:c0.c1023                    SystemHigh

Additional resources

setrans.conf(5) man page on your system

7.4. Assigning categories to users in MCS

You can define user authorizations by assigning categories to Linux users. A user with assigned categories can access and modify files that have a subset of the user’s categories. Users can also assign files they own to categories they have been assigned to.

A Linux user cannot be assigned to a category that is outside of the security range defined for the relevant SELinux user.

Note

Category access is assigned during login. Consequently, users do not have access to newly assigned categories until they log in again. Similarly, if you revoke a user’s access to a category, this is effective only after the user logs in again.

Prerequisites

The SELinux mode is set to enforcing.
The SELinux policy is set to targeted or mls.
The policycoreutils-python-utils package is installed.
Linux users are assigned to SELinux confined users:
- Non-privileged users are assigned to user_u.
- Privileged users are assigned to staff_u.

Procedure

Define the security range for the SELinux user.
```
# semanage user -m -rs0:c0,c1-s0:c0.c9 <user_u>
```
Use category numbers c0 to c1023 or category labels as defined in the setrans.conf file. For additional information, see Defining category labels in MCS .
Assign MCS categories to a Linux user. You can specify only a range within the range defined to the relevant SELinux user:
```
# semanage login -m -rs0:c1 <Linux.user1>
```
Note
You can add or remove categories from Linux users by using the chcat command. The following example adds <category1> and removes <category2> from <Linux.user1> and <Linux.user2>:
```
# chcat -l -- +<category1>,-<category2> <Linux.user1>,<Linux.user2>
```
You must specify -- on the command line before using the -<category> syntax. Otherwise, the chcat command misinterprets the category removal as a command option.

Verification

List the categories assigned to Linux users:

# chcat -L -l <Linux.user1>,<Linux.user2>
<Linux.user1>: <category1>,<category2>
<Linux.user2>: <category1>,<category2>

Additional resources

chcat(8) man page on your system

7.5. Assigning categories to files in MCS

You need administrative privileges to assign categories to users. Users can then assign categories to files. To modify the categories of a file, users must have access rights to that file. Users can only assign a file to a category that is assigned to them.

Note

The system combines category access rules with conventional file access permissions. For example, if a user with a category of bigfoot uses Discretionary Access Control (DAC) to block access to a file by other users, other bigfoot users cannot access that file. A user assigned to all available categories still may not be able to access the entire file system.

Prerequisites

The SELinux mode is set to enforcing.
The SELinux policy is set to targeted or mls.
The policycoreutils-python-utils package is installed.
Access and permissions to a Linux user that is:
- Assigned to an SELinux user.
- Assigned to the category to which you want to assign the file. For additional information, see Assigning categories to users in MCS .
Access and permissions to the file you want to add to the category.
For verification purposes: Access and permissions to a Linux user not assigned to this category

Procedure

Add categories to a file:
```
$ chcat -- +<category1>,+<category2> <path/to/file1>
```
Use category numbers c0 to c1023 or category labels as defined in the setrans.conf file. For additional information, see Defining category labels in MCS .
You can remove categories from a file by using the same syntax:
```
$ chcat -- -<category1>,-<category2> <path/to/file1>
```
Note
When removing a category, you must specify -- on the command line before using the -<category> syntax. Otherwise, the chcat command misinterprets the category removal as a command option.

Verification

Display the security context of the file to verify that it has the correct categories:
```
$ ls -lZ <path/to/file>
-rw-r--r--  <LinuxUser1> <Group1> root:object_r:user_home_t:_<sensitivity>_:_<category>_ <path/to/file>
```
The specific security context of the file may differ.
Optional: Attempt to access the file when logged in as a Linux user not assigned to the same category as the file:
```
$ cat <path/to/file>
cat: <path/to/file>: Permission Denied
```

Additional resources

semanage(8) and chcat(8) man pages on your system

Chapter 8. Writing a custom SELinux policy

To run your applications confined by SELinux, you must write and use a custom policy.

8.1. Custom SELinux policies and related tools

An SELinux security policy is a collection of SELinux rules. A policy is a core component of SELinux and is loaded into the kernel by SELinux user-space tools. The kernel enforces the use of an SELinux policy to evaluate access requests on the system. By default, SELinux denies all requests except for requests that correspond to the rules specified in the loaded policy.

Each SELinux policy rule describes an interaction between a process and a system resource:

ALLOW apache_process apache_log:FILE READ;

You can read this example rule as: The Apache process can read its logging file. In this rule, apache_process and apache_log are labels. An SELinux security policy assigns labels to processes and defines relations to system resources. This way, a policy maps operating-system entities to the SELinux layer.

SELinux labels are stored as extended attributes of file systems, such as ext2. You can list them using the getfattr utility or a ls -Z command, for example:

$ ls -Z /etc/passwd
system_u:object_r:passwd_file_t:s0 /etc/passwd

Where system_u is an SELinux user, object_r is an example of the SELinux role, and passwd_file_t is an SELinux domain.

The default SELinux policy provided by the selinux-policy packages contains rules for applications and daemons that are parts of Red Hat Enterprise Linux 8 and are provided by packages in its repositories. Applications not described in this distribution policy are not confined by SELinux. To change this, you have to modify the policy using a policy module, which contains additional definitions and rules.

In Red Hat Enterprise Linux 8, you can query the installed SELinux policy and generate new policy modules using the sepolicy tool. Scripts that sepolicy generates together with the policy modules always contain a command using the restorecon utility. This utility is a basic tool for fixing labeling problems in a selected part of a file system.

Additional resources

sepolicy(8) and getfattr(1) man pages on your system
Quick start to write a custom SELinux policy Knowledgebase article

8.2. Creating and enforcing an SELinux policy for a custom application

You can confine applications by SELinux to increase the security of host systems and users' data. Because each application has specific requirements, modify this example procedure for creating an SELinux policy that confines a simple daemon according to your use case.

Prerequisites

The selinux-policy-devel package and its dependencies are installed on your system.

Procedure

For this example procedure, prepare a simple daemon that opens the /var/log/messages file for writing:

Create a new file, and open it in a text editor of your choice:
```
$ vi mydaemon.c
```

Insert the following code:

#include <unistd.h>
#include <stdio.h>

FILE *f;

int main(void)
{
while(1) {
f = fopen("/var/log/messages","w");
        sleep(5);
        fclose(f);
    }
}

Compile the file:
```
$ gcc -o mydaemon mydaemon.c
```

Create a systemd unit file for your daemon:

$ vi mydaemon.service
[Unit]
Description=Simple testing daemon

[Service]
Type=simple
ExecStart=/usr/local/bin/mydaemon

[Install]
WantedBy=multi-user.target

Install and start the daemon:

# cp mydaemon /usr/local/bin/
# cp mydaemon.service /usr/lib/systemd/system
# systemctl start mydaemon
# systemctl status mydaemon
● mydaemon.service - Simple testing daemon
   Loaded: loaded (/usr/lib/systemd/system/mydaemon.service; disabled; vendor preset: disabled)
   Active: active (running) since Sat 2020-05-23 16:56:01 CEST; 19s ago
 Main PID: 4117 (mydaemon)
    Tasks: 1
   Memory: 148.0K
   CGroup: /system.slice/mydaemon.service
           └─4117 /usr/local/bin/mydaemon

May 23 16:56:01 localhost.localdomain systemd[1]: Started Simple testing daemon.

Check that the new daemon is not confined by SELinux:

$ ps -efZ | grep mydaemon
system_u:system_r:unconfined_service_t:s0 root 4117    1  0 16:56 ?        00:00:00 /usr/local/bin/mydaemon

Generate a custom policy for the daemon:

$ sepolicy generate --init /usr/local/bin/mydaemon
Created the following files:
/home/example.user/mysepol/mydaemon.te # Type Enforcement file
/home/example.user/mysepol/mydaemon.if # Interface file
/home/example.user/mysepol/mydaemon.fc # File Contexts file
/home/example.user/mysepol/mydaemon_selinux.spec # Spec file
/home/example.user/mysepol/mydaemon.sh # Setup Script

Rebuild the system policy with the new policy module using the setup script created by the previous command:

# ./mydaemon.sh
Building and Loading Policy
+ make -f /usr/share/selinux/devel/Makefile mydaemon.pp
Compiling targeted mydaemon module
Creating targeted mydaemon.pp policy package
rm tmp/mydaemon.mod.fc tmp/mydaemon.mod
+ /usr/sbin/semodule -i mydaemon.pp
...

Note that the setup script relabels the corresponding part of the file system using the restorecon command:

restorecon -v /usr/local/bin/mydaemon /usr/lib/systemd/system

Restart the daemon, and check that it now runs confined by SELinux:

# systemctl restart mydaemon
$ ps -efZ | grep mydaemon
system_u:system_r:mydaemon_t:s0 root        8150       1  0 17:18 ?        00:00:00 /usr/local/bin/mydaemon

Because the daemon is now confined by SELinux, SELinux also prevents it from accessing /var/log/messages. Display the corresponding denial message:

# ausearch -m AVC -ts recent
...
type=AVC msg=audit(1590247112.719:5935): avc:  denied  { open } for  pid=8150 comm="mydaemon" path="/var/log/messages" dev="dm-0" ino=2430831 scontext=system_u:system_r:mydaemon_t:s0 tcontext=unconfined_u:object_r:var_log_t:s0 tclass=file permissive=1
...

You can get additional information also using the sealert tool:

$ sealert -l "*"
SELinux is preventing mydaemon from open access on the file /var/log/messages.

*****  Plugin catchall (100. confidence) suggests   **************************

If you believe that mydaemon should be allowed open access on the messages file by default.
Then you should report this as a bug.
You can generate a local policy module to allow this access.
Do allow this access for now by executing:
# ausearch -c 'mydaemon' --raw | audit2allow -M my-mydaemon
# semodule -X 300 -i my-mydaemon.pp

Additional Information:
Source Context                system_u:system_r:mydaemon_t:s0
Target Context                unconfined_u:object_r:var_log_t:s0
Target Objects                /var/log/messages [ file ]
Source                        mydaemon
…

Use the audit2allow tool to suggest changes:

$ ausearch -m AVC -ts recent | audit2allow -R

require {
	type mydaemon_t;
}

#============= mydaemon_t ==============
logging_write_generic_logs(mydaemon_t)

Because rules suggested by audit2allow can be incorrect for certain cases, use only a part of its output to find the corresponding policy interface. Inspect the logging_write_generic_logs(mydaemon_t) macro with the macro-expander tool, to see all allow rules the macro provides:

$ macro-expander "logging_write_generic_logs(mydaemon_t)"
allow mydaemon_t var_t:dir { getattr search open };
allow mydaemon_t var_log_t:dir { getattr search open read lock ioctl };
allow mydaemon_t var_log_t:dir { getattr search open };
allow mydaemon_t var_log_t:file { open { getattr write append lock ioctl } };
allow mydaemon_t var_log_t:dir { getattr search open };
allow mydaemon_t var_log_t:lnk_file { getattr read };

In this case, you can use the suggested interface, because it only provides read and write access to log files and their parent directories. Add the corresponding rule to your type enforcement file:
```
$ echo "logging_write_generic_logs(mydaemon_t)" >> mydaemon.te
```
Alternatively, you can add this rule instead of using the interface:
```
$ echo "allow mydaemon_t var_log_t:file { open write getattr };" >> mydaemon.te
```

Reinstall the policy:

# ./mydaemon.sh
Building and Loading Policy
+ make -f /usr/share/selinux/devel/Makefile mydaemon.pp
Compiling targeted mydaemon module
Creating targeted mydaemon.pp policy package
rm tmp/mydaemon.mod.fc tmp/mydaemon.mod
+ /usr/sbin/semodule -i mydaemon.pp
...

Verification

Check that your application runs confined by SELinux, for example:

$ ps -efZ | grep mydaemon
system_u:system_r:mydaemon_t:s0 root        8150       1  0 17:18 ?        00:00:00 /usr/local/bin/mydaemon

Verify that your custom application does not cause any SELinux denials:
```
# ausearch -m AVC -ts recent
<no matches>
```

Additional resources

sepolgen(8), ausearch(8), audit2allow(1), audit2why(1), sealert(8), and restorecon(8) man pages on your system
Quick start to write a custom SELinux policy Knowledgebase article

8.3. Additional resources

SELinux Policy Workshop

Chapter 9. Creating SELinux policies for containers

Red Hat Enterprise Linux 8 provides a tool for generating SELinux policies for containers using the udica package. With udica, you can create a tailored security policy for better control of how a container accesses host system resources, such as storage, devices, and network. This enables you to harden your container deployments against security violations and it also simplifies achieving and maintaining regulatory compliance.

9.1. Introduction to the udica SELinux policy generator

To simplify creating new SELinux policies for custom containers, RHEL 8 provides the udica utility. You can use this tool to create a policy based on an inspection of the container JavaScript Object Notation (JSON) file, which contains Linux-capabilities, mount-points, and ports definitions. The tool consequently combines rules generated using the results of the inspection with rules inherited from a specified SELinux Common Intermediate Language (CIL) block.

The process of generating SELinux policy for a container using udica has three main parts:

Parsing the container spec file in the JSON format
Finding suitable allow rules based on the results of the first part
Generating final SELinux policy

During the parsing phase, udica looks for Linux capabilities, network ports, and mount points.

Based on the results, udica detects which Linux capabilities are required by the container and creates an SELinux rule allowing all these capabilities. If the container binds to a specific port, udica uses SELinux user-space libraries to get the correct SELinux label of a port that is used by the inspected container.

Afterward, udica detects which directories are mounted to the container file-system name space from the host.

The CIL’s block inheritance feature allows udica to create templates of SELinux allow rules focusing on a specific action, for example:

allow accessing home directories
allow accessing log files
allow accessing communication with Xserver.

These templates are called blocks and the final SELinux policy is created by merging the blocks.

Additional resources

Generate SELinux policies for containers with udica Red Hat Blog article

9.2. Creating and using an SELinux policy for a custom container

To generate an SELinux security policy for a custom container, follow the steps in this procedure.

Prerequisites

The podman tool for managing containers is installed. If it is not, use the yum install podman command.
A custom Linux container - ubi8 in this example.

Procedure

Install the udica package:
```
# yum install -y udica
```
Alternatively, install the container-tools module, which provides a set of container software packages, including udica:
```
# yum module install -y container-tools
```
Start the ubi8 container that mounts the /home directory with read-only permissions and the /var/spool directory with permissions to read and write. The container exposes the port 21.
```
# podman run --env container=podman -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash
```
Note that now the container runs with the container_t SELinux type. This type is a generic domain for all containers in the SELinux policy and it might be either too strict or too loose for your scenario.

Open a new terminal, and enter the podman ps command to obtain the ID of the container:

# podman ps
CONTAINER ID   IMAGE                                   COMMAND   CREATED          STATUS              PORTS   NAMES
37a3635afb8f   registry.access.redhat.com/ubi8:latest  bash      15 minutes ago   Up 15 minutes ago           heuristic_lewin

Create a container JSON file, and use udica for creating a policy module based on the information in the JSON file:

# podman inspect 37a3635afb8f > container.json
# udica -j container.json my_container
Policy my_container with container id 37a3635afb8f created!
[…]

Alternatively:

# podman inspect 37a3635afb8f | udica my_container
Policy my_container with container id 37a3635afb8f created!

Please load these modules using:
# semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}

Restart the container with: "--security-opt label=type:my_container.process" parameter

As suggested by the output of udica in the previous step, load the policy module:

# semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}

Stop the container and start it again with the --security-opt label=type:my_container.process option:

# podman stop 37a3635afb8f
# podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash

Verification

Check that the container runs with the my_container.process type:

# ps -efZ | grep my_container.process
unconfined_u:system_r:container_runtime_t:s0-s0:c0.c1023 root 2275 434  1 13:49 pts/1 00:00:00 podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it ubi8 bash
system_u:system_r:my_container.process:s0:c270,c963 root 2317 2305  0 13:49 pts/0 00:00:00 bash

Verify that SELinux now allows access the /home and /var/spool mount points:

[root@37a3635afb8f /]# cd /home
[root@37a3635afb8f home]# ls
username
[root@37a3635afb8f ~]# cd /var/spool/
[root@37a3635afb8f spool]# touch test
[root@37a3635afb8f spool]#

Check that SELinux allows binding only to the port 21:

[root@37a3635afb8f /]# yum install nmap-ncat
[root@37a3635afb8f /]# nc -lvp 21
…
Ncat: Listening on :::21
Ncat: Listening on 0.0.0.0:21
^C
[root@37a3635afb8f /]# nc -lvp 80
…
Ncat: bind to :::80: Permission denied. QUITTING.

Additional resources

udica(8) and podman(1) man pages on your system
udica - Generate SELinux policies for containers
Building, running, and managing containers

Chapter 10. Deploying the same SELinux configuration on multiple systems

You can deploy your verified SELinux configuration on multiple systems by using one of the following methods:

Using RHEL system roles and Ansible
Using the RHEL web console
Using semanage export and import commands in your scripts

10.1. Introduction to the `selinux` RHEL system role

RHEL system roles is a collection of Ansible roles and modules that provide a consistent configuration interface to remotely manage multiple RHEL systems. You can perform the following actions by using the selinux RHEL system role:

Cleaning local policy modifications related to SELinux booleans, file contexts, ports, and logins.
Setting SELinux policy booleans, file contexts, ports, and logins.
Restoring file contexts on specified files or directories.
Managing SELinux modules.

The /usr/share/doc/rhel-system-roles/selinux/example-selinux-playbook.yml example playbook installed by the rhel-system-roles package demonstrates how to set the targeted policy in enforcing mode. The playbook also applies several local policy modifications and restores file contexts in the /tmp/test_dir/ directory.

Additional resources

/usr/share/ansible/roles/rhel-system-roles.selinux/README.md file
/usr/share/doc/rhel-system-roles/selinux/ directory

10.2. Using the `selinux` RHEL system role to apply SELinux settings on multiple systems

With the selinux RHEL system role, you can prepare and apply an Ansible playbook with your verified SELinux settings.

Prerequisites

You have prepared the control node and the managed nodes
You are logged in to the control node as a user who can run playbooks on the managed nodes.
The account you use to connect to the managed nodes has sudo permissions on them.

Procedure

Prepare your playbook. You can either start from scratch or modify the example playbook installed as a part of the rhel-system-roles package:
```
# cp /usr/share/doc/rhel-system-roles/selinux/example-selinux-playbook.yml <my-selinux-playbook.yml>
# vi <my-selinux-playbook.yml>
```
Change the content of the playbook to fit your scenario. For example, the following part ensures that the system installs and enables the selinux-local-1.pp SELinux module:
```
selinux_modules:
- { path: "selinux-local-1.pp", priority: "400" }
```
Save the changes, and exit the text editor.
Validate the playbook syntax:
```
$ ansible-playbook <my-selinux-playbook.yml> --syntax-check
```
Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

Run your playbook:

$ ansible-playbook <my-selinux-playbook.yml>

Additional resources

/usr/share/ansible/roles/rhel-system-roles.selinux/README.md file
/usr/share/doc/rhel-system-roles/selinux/ directory
SELinux hardening with Ansible Knowledgebase article

10.3. Managing ports by using the `selinux` RHEL system role

You can automate managing port access in SELinux consistently across multiple systems by using the selinux RHEL system role. This might be useful, for example, when configuring an Apache HTTP server to listen on a different port. You can do this by creating a playbook with the selinux RHEL system role that assigns the http_port_t SELinux type to a specific port number. After you run the playbook on the managed nodes, specific services defined in the SELinux policy can access this port.

You can automate managing port access in SELinux either by using the seport module, which is quicker than using the entire role, or by using the selinux RHEL system role, which is more useful when you also make other changes in SELinux configuration. The methods are equivalent, in fact the selinux RHEL system role uses the seport module when configuring ports. Each of the methods has the same effect as entering the command semanage port -a -t http_port_t -p tcp <port_number> on the managed node.

Prerequisites

You have prepared the control node and the managed nodes
You are logged in to the control node as a user who can run playbooks on the managed nodes.
The account you use to connect to the managed nodes has sudo permissions on them.
Optional: To verify port status by using the semanage command, the policycoreutils-python-utils package must be installed.

Procedure

To configure just the port number without making other changes, use the seport module:

- name: Allow Apache to listen on tcp port <port_number>
  community.general.seport:
    ports: <port_number>
    proto: tcp
    setype: http_port_t
    state: present

Replace <port_number> with the port number to which you want to assign the http_port_t type.

For more complex configuration of the managed nodes that involves other customizations of SELinux, use the selinux RHEL system role. Create a playbook file, for example, ~/playbook.yml, and add the following content:

---
- name: Modify SELinux port mapping example
  hosts: all
  vars:
    # Map tcp port <port_number> to the 'http_port_t' SELinux port type
    selinux_ports:
      - ports: <port_number>
        proto: tcp
        setype: http_port_t
        state: present

  tasks:
    - name: Include selinux role
      ansible.builtin.include_role:
        name: rhel-system-roles.selinux

Replace <port_number> with the port number to which you want to assign the http_port_t type.

Verification

Verify that the port is assigned to the http_port_t type:

# semanage port --list | grep http_port_t
http_port_t                	tcp  	<port_number>, 80, 81, 443, 488, 8008, 8009, 8443, 9000

Additional resources

/usr/share/ansible/roles/rhel-system-roles.selinux/README.md file
/usr/share/doc/rhel-system-roles/selinux/ directory

10.4. Creating an SELinux configuration Ansible playbook in the web console

In the web console, you can generate a shell script or an Ansible playbook of your SELinux configuration. In case of the Ansible playbook, you can conveniently apply the configuration on multiple systems.

Prerequisites

You have installed the RHEL 8 web console.
For instructions, see Installing and enabling the web console.

Procedure

Log in to the RHEL 8 web console.
For details, see Logging in to the web console.
Click SELinux.
Click View the automation script.
A window with the generated script opens. You can navigate between a shell script and an Ansible playbook generation options tab.
Click the Copy to clipboard button to select the script or playbook and apply it.

As a result, you have an automation script that you can apply to more machines.

Additional resources

Troubleshooting problems related to SELinux
Deploying the same SELinux configuration on multiple systems
ansible-playbook(1) man page on your system

10.5. Transferring SELinux settings to another system with semanage

Use the following steps for transferring your custom and verified SELinux settings between RHEL 8-based systems.

Prerequisites

The policycoreutils-python-utils package is installed on your system.

Procedure

Export your verified SELinux settings:

# semanage export -f ./my-selinux-settings.mod

Copy the file with the settings to the new system:

# scp ./my-selinux-settings.mod new-system-hostname:

Log in on the new system:
```
$ ssh root@new-system-hostname
```

Import the settings on the new system:

new-system-hostname# semanage import -f ./my-selinux-settings.mod

Additional resources

semanage-export(8) and semanage-import(8) man pages on your system

Chapter 11. Configuring polyinstantiated directories

By default, all programs, services, and users use the /tmp, /var/tmp, and home directories for temporary storage. This makes these directories vulnerable to race condition attacks and information leaks based on file names. You can make /tmp/, /var/tmp/, and the home directory instantiated so that they are no longer shared between all users, and each user’s /tmp-inst and /var/tmp/tmp-inst is separately mounted to the /tmp and /var/tmp directory.

Procedure

Enable polyinstantiation in SELinux:
```
# setsebool -P allow_polyinstantiation 1
```
You can verify that polyinstantiation is enabled in SELinux by entering the getsebool allow_polyinstantiation command.
Create the directory structure for data persistence over reboot with the necessary permissions:
```
# mkdir /tmp-inst /var/tmp/tmp-inst --mode 000
```

Restore the entire security context including the SELinux user part:

# restorecon -Fv /tmp-inst /var/tmp/tmp-inst
Relabeled /tmp-inst from unconfined_u:object_r:default_t:s0 to system_u:object_r:tmp_t:s0
Relabeled /var/tmp/tmp-inst from unconfined_u:object_r:tmp_t:s0 to system_u:object_r:tmp_t:s0

If your system uses the fapolicyd application control framework, allow fapolicyd to monitor file access events on the underlying file system when they are bind mounted by enabling the allow_filesystem_mark option in the /etc/fapolicyd/fapolicyd.conf configuration file.
```
allow_filesystem_mark = 1
```
Enable instantiation of the /tmp, /var/tmp/, and users' home directories:
Important
Use /etc/security/namespace.conf instead of a separate file in the /etc/security/namespace.d/ directory because the pam_namespace_helper program does not read additional files in /etc/security/namespace.d.
1. On a system with multi-level security (MLS), uncomment the last three lines in the /etc/security/namespace.conf file:
```
/tmp     /tmp-inst/   		   level 	 root,adm
/var/tmp /var/tmp/tmp-inst/    level 	 root,adm
$HOME    $HOME/$USER.inst/     level
```
2. On a system without multi-level security (MLS), add the following lines in the /etc/security/namespace.conf file:
```
/tmp     /tmp-inst/            user 	 root,adm
/var/tmp /var/tmp/tmp-inst/    user 	 root,adm
$HOME    $HOME/$USER.inst/     user
```

Verify that the pam_namespace.so module is configured for the session:

$ grep namespace /etc/pam.d/login
session    required     pam_namespace.so

Optional: Enable cloud users to access the system with SSH keys:
1. Install the openssh-keycat package.
2. Create a file in the /etc/ssh/sshd_config.d/ directory with the following content:
```
AuthorizedKeysCommand /usr/libexec/openssh/ssh-keycat
AuthorizedKeysCommandRunAs root
```
3. Verify that public key authentication is enabled by checking that the PubkeyAuthentication variable in sshd_config is set to yes. By default, PubkeyAuthentication is set to yes, even though the line in sshd_config is commented out.
```
$ grep -r PubkeyAuthentication /etc/ssh/
/etc/ssh/sshd_config:#PubkeyAuthentication yes
```
Add the session required pam_namespace.so unmnt_remnt entry into the module for each service for which polyinstantiation should apply, after the session include system-auth line. For example, in /etc/pam.d/su, /etc/pam.d/sudo, /etc/pam.d/ssh, and /etc/pam.d/sshd:
```
[...]
session        include        system-auth
session        required    pam_namespace.so unmnt_remnt
[...]
```

Verification

Log in as a non-root user. Users that were logged in before polyinstantiation was configured must log out and log in before the changes take effect for them.
Check that the /tmp/ directory is mounted under /tmp-inst/:
```
$ findmnt --mountpoint /tmp/
TARGET SOURCE                 	FSTYPE OPTIONS
/tmp   /dev/vda1[/tmp-inst/<user>] xfs	rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
```
The SOURCE output differs based on your environment. * On virutal systems, it shows /dev/vda_<number>_. * On bare-metal systems it shows /dev/sda_<number>_ or /dev/nvme*

Additional resources

/usr/share/doc/pam/txts/README.pam_namespace readme file installed with the pam package.

Prevent users and processes from performing unauthorized interactions with files and devices by using Security-Enhanced Linux (SELinux)

Providing feedback on Red Hat documentation

Chapter 1. Getting started with SELinux

1.1. Introduction to SELinux

1.2. Benefits of running SELinux

1.3. SELinux examples

1.4. SELinux architecture and packages

1.5. SELinux states and modes

Chapter 2. Changing SELinux states and modes

2.1. Permanent changes in SELinux states and modes

2.2. Changing SELinux to permissive mode

2.3. Changing SELinux to enforcing mode

2.4. Enabling SELinux on systems that previously had it disabled

2.5. Disabling SELinux

2.6. Changing SELinux modes at boot time

Chapter 3. Managing confined and unconfined users

3.1. Confined and unconfined users in SELinux

3.2. Roles and access rights of SELinux users

3.3. Confined non-administrator roles in SELinux

3.4. Confined administrator roles in SELinux

3.5. Adding a new user automatically mapped to the SELinux unconfined_u user

3.6. Adding a new user as an SELinux-confined user

3.7. Confining regular users in SELinux

3.8. Confining an administrator by mapping to sysadm_u

3.9. Confining an administrator by using sudo and the sysadm_r role

3.10. Additional resources

Chapter 4. Configuring SELinux for applications and services with non-standard configurations

4.1. Customizing the SELinux policy for the Apache HTTP server in a non-standard configuration

4.2. Adjusting the policy for sharing NFS and CIFS volumes by using SELinux booleans

4.3. Finding the correct SELinux type for managing access to non-standard directories

4.4. Managing access to non-standard shared directories for unprivileged SELinux users

Chapter 5. Troubleshooting problems related to SELinux

5.1. Identifying SELinux denials

5.2. Analyzing SELinux denial messages

5.3. Fixing analyzed SELinux denials

5.4. Creating a local SELinux policy module

5.5. SELinux denials in the Audit log

5.6. Additional resources

Chapter 6. Using Multi-Level Security (MLS)

6.1. Multi-Level Security (MLS)

6.2. SELinux roles in MLS

6.3. Switching the SELinux policy to MLS

6.4. Establishing user clearance in MLS

6.5. Changing a user’s clearance level within the defined security range in MLS

6.6. Increasing file sensitivity levels in MLS

6.7. Changing file sensitivity in MLS

6.8. Separating system administration from security administration in MLS

6.9. Defining a secure terminal in MLS

6.10. Allowing MLS users to edit files on lower levels

Chapter 7. Using Multi-Category Security (MCS) for data confidentiality

7.1. Multi-Category Security (MCS)

MCS within Multi-Level Security

7.2. Configuring Multi-Category Security for data confidentiality

7.3. Defining category labels in MCS

7.4. Assigning categories to users in MCS

7.5. Assigning categories to files in MCS

Chapter 8. Writing a custom SELinux policy

8.1. Custom SELinux policies and related tools

8.2. Creating and enforcing an SELinux policy for a custom application

8.3. Additional resources

Chapter 9. Creating SELinux policies for containers

9.1. Introduction to the udica SELinux policy generator

9.2. Creating and using an SELinux policy for a custom container

Chapter 10. Deploying the same SELinux configuration on multiple systems

10.1. Introduction to the selinux RHEL system role

10.2. Using the selinux RHEL system role to apply SELinux settings on multiple systems

10.3. Managing ports by using the selinux RHEL system role

10.4. Creating an SELinux configuration Ansible playbook in the web console

10.5. Transferring SELinux settings to another system with semanage

Chapter 11. Configuring polyinstantiated directories

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3.8. Confining an administrator by mapping to `sysadm_u`

10.1. Introduction to the `selinux` RHEL system role

10.2. Using the `selinux` RHEL system role to apply SELinux settings on multiple systems

10.3. Managing ports by using the `selinux` RHEL system role