Introduction: Securing Your Custom Android ROM with SELinux

In the vibrant world of custom Android ROMs, developers often focus on features, performance, and user experience. However, a critical aspect that sometimes receives less attention is system security, especially with regard to SELinux (Security-Enhanced Linux). SELinux is a mandatory access control (MAC) security mechanism that provides a robust layer of protection, preventing unauthorized processes from accessing system resources, even if a traditional discretionary access control (DAC) check would permit it. For custom ROM developers, understanding and writing proper SELinux policies isn’t just a best practice; it’s essential for maintaining system integrity, preventing exploits, and ensuring user privacy.

This guide will demystify SELinux policy writing for custom ROMs, providing a step-by-step approach from understanding AVC denials to integrating your custom policies into the build system. We’ll focus on practical examples, arming you with the knowledge to harden your custom Android builds effectively.

Understanding SELinux Modes: Permissive vs. Enforcing

SELinux operates primarily in two modes:

• Permissive Mode: In this mode, SELinux will log Access Vector Cache (AVC) denials to the kernel log but will not actually enforce them. Processes are allowed to perform actions that would otherwise be denied. This mode is invaluable for debugging and developing new policies, as it allows you to identify what needs to be permitted without breaking system functionality.
• Enforcing Mode: This is the secure, production mode. SELinux strictly enforces all policy rules. Any action not explicitly permitted by the policy will be denied, and an AVC denial will be logged. This is the desired state for any released ROM.

You can check the current SELinux status on your device using getenforce and temporarily switch modes (requires root) using setenforce 0 (permissive) or setenforce 1 (enforcing).

adb shellgetenforceadb shellsu -c 'setenforce 0' # Switch to permissiveadb shellsu -c 'setenforce 1' # Switch to enforcing

Identifying and Interpreting AVC Denials

The first step in writing policy is to identify what SELinux is denying. When a process attempts an action that violates the SELinux policy, an AVC denial is logged. These denials are your roadmap to what policies need to be written.

Where to Find Denials:

• Kernel Log (dmesg): The most common place to find AVC denials.
• Logcat: Sometimes, specific Android services will log SELinux-related errors here.
• Audit Log (on rooted devices): Located at /sys/fs/selinux/audit/audit_log.

To view denials:

adb shell dmesg | grep 'avc: denied'adb shell logcat | grep 'selinux' # Less common for direct denials, more for service errors

Example AVC Denial:

An AVC denial typically looks something like this:

avc: denied { read } for pid=1234 comm=

        
        
        

            

                
            
            

                
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