Chapter 3. Managing confined and unconfined users

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:

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.

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.

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.

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.

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:

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:

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.

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.

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.

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.