Chapter 21. SELinux

SELinux provides a flexible mandatory access control (MAC) system built into the Linux kernel. Under standard Linux discretionary access control (DAC), an application or process running as a user (UID or SUID) has the user's permissions to objects such as files, sockets, and other processes. Running an SELinux MAC kernel protects the system from malicious or flawed applications that can damage or destroy the system. SELinux defines the access and transition rights of every user, application, process, and file on the system. SELinux then governs the interactions of these subjects and objects using a security policy that specifies how strict or lenient a given Red Hat Enterprise Linux installation should be.

For the most part, SELinux is almost completely invisible to system users. Only system administrators must worry about how strict a policy to implement for their server environment. The policy can be as strict or lenient as needed, and is very finely detailed. This detail gives the SELinux kernel complete, granular control over the entire system.

When a subject such as an application attempts to access an object such as a file, the policy enforcement server in the kernel checks an access vector cache (AVC), where subject and object permissions are cached. If a decision cannot be made based on data in the AVC, the request continues to the security server, which looks up the security context of the application and the file in a matrix. Permission is then granted or denied, with an avc: denied message detailed in /var/log/messages. Subjects and objects gain their security context from installed policy, which also provides the information to populate the security server's matrix.

In addition to running in an enforcing mode, SELinux can run in a permissive mode, where the AVC is checked and denials are logged, but SELinux does not enforce the policy.

For more information about how SELinux works, refer to Section 21.3, “ Additional Resources ”.