1장. Getting started with 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.

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.

그림 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.