Understanding SELinux: A Beginner's Guide for RHCSA Candidates

Security-Enhanced Linux (SELinux) is an integral part of the Linux kernel that offers advanced security controls by enforcing strict security policies. For RHCSA (Red Hat Certified System Administrator) candidates, understanding SELinux is essential for appreciating the importance of maintaining system security. This guide will cover the fundamental concepts of SELinux, its key components, and management techniques.

What is SELinux?

SELinux is a Linux security module designed to enforce access control policies on Linux systems using mandatory access controls (MAC). Unlike traditional discretionary access controls (DAC), where users can manage their own file permissions, SELinux policies are established by the system administrator and enforced by the kernel to safeguard against unauthorized access.

Key Components of SELinux

  • Security Policies

SELinux operates based on security policies, which are sets of rules defining the permissions for different users, processes, and files. These policies determine what subjects (users or processes) can do to objects (files, directories, or resources). The primary policies in SELinux are:

    • Targeted Policy: Applies to specific daemons or applications.
    • MLS (Multi-Level Security) Policy: Implements a strict security model suitable for highly sensitive environments.

  • Security Contexts

Every file, process, and resource in an SELinux-enabled system is labeled with a security context. This context includes:

    • User: The SELinux user identity.
    • Role: Defines what a user or process can do.
    • Type: The type of the object, used for type enforcement.
    • Level: Sensitivity level, used primarily in MLS policies.

  • Access Vector Cache (AVC)

The Access Vector Cache (AVC) stores the decisions made by SELinux for quicker access in the future. When a subject requests access to an object, SELinux checks the AVC to see if there’s already a decision on the permission. If not, the request is sent to the security server.

  • Security Server and Policy Database

The security server evaluates access requests that are not found in the AVC. It uses the policy database to check the security context of both the subject and the object. Based on the rules in the policy database, it grants or denies the access.

SELinux

How SELinux Works

  • Request for Access: When an application or process (subject) requests access to a file or resource (object), SELinux intercepts this request.
  • Check AVC: SELinux first checks the Access Vector Cache (AVC) for a cached decision. If the permission is found, SELinux enforces the cached decision.
  • Query Security Server: If the permission is not cached, SELinux queries the security server.
  • Evaluate Security Context: The security server checks the security context of the subject and the object against the policy database.
  • Decision: The security server grants or denies access based on the security policy. If access is denied, an "avc: denied" message is logged in /var/log/messages.

Managing SELinux

As an RHCSA candidate, you need to know how to manage and troubleshoot SELinux:

  • Checking Status: Use sestatus to check if SELinux is enabled and enforcing policies.
SELinux
  • Changing Modes: SELinux can operate in three modes:
    • Enforcing: SELinux policy is enforced.
    • Permissive: SELinux policy is not enforced but violations are logged.
    • Disabled: SELinux is turned off.
  • Change modes temporarily with setenforce:
    • Change to permissive / disable from enforcing
      • SELinux
    • Change to enforcing  from permissive / disable 
      • SELinux
  • Change modes permanently by editing /etc/selinux/config:
    • SELINUX=enforcing
      • SELinux

Managing Policies

  • Use tools like ‘semanage’ to manage SELinux policies.
  • For example, to allow a process to bind to a specific port:
    • semanage port -a -t http_port_t -p tcp 8080

Viewing Logs

  • Check /var/log/messages or /var/log/audit/audit.log for SELinux-related logs.
  • Use ausearch and audit2allow to troubleshoot and create custom policies.
    • ausearch -m avc -ts recent
    • audit2allow -w -a

For hands-on practice with SELinux and other RHCSA topics, visit RHCSAGuru. Our website offers comprehensive labs and resources designed to help you master SELinux security and prepare effectively for your RHCSA certification.

Conclusion

Understanding and managing SELinux is crucial for maintaining the security and integrity of your Linux systems. As an RHCSA candidate, mastering SELinux concepts and commands will not only help you pass the exam but also equip you with the skills necessary to secure your Linux environment effectively.