Mastering audit2allow & sesearch: Advanced SELinux Policy Development for Android Kernels

Security-Enhanced Linux (SELinux) is a critical security mechanism in Android, providing mandatory access control (MAC) over all processes, files, and resources. While it significantly hardens the operating system, developing and debugging custom SELinux policies for Android kernels, especially in custom ROMs like LineageOS or when porting new hardware, can be a daunting task. This guide delves into advanced usage of two indispensable tools – audit2allow and sesearch – to streamline SELinux policy development and debugging, focusing on understanding and manipulating SELinux contexts.

Understanding SELinux Fundamentals in Android

Before diving into the tools, a solid grasp of SELinux fundamentals in Android is crucial. SELinux operates on the principle of contexts, which are labels assigned to every subject (process) and object (file, socket, IPC, etc.) on the system. These contexts dictate what interactions are permitted or denied.

The Role of Contexts

An SELinux context is typically represented as user:role:type:sensitivity. In Android, the most significant component for policy development is the type (or domain for processes). For example:

u:object_r:system_file:s0: A file type.
u:r:untrusted_app:s0: A process domain for untrusted applications.

The SELinux policy defines rules that specify which source types (scontext) are allowed to perform certain operations (e.g., read, write, execute) on which target types (tcontext) for a given class of object (tclass).

Policy Files and Their Structure

Android’s SELinux policy is compiled from several source files:

.te files (Type Enforcement): Define types, attributes, and allow/deny rules.
file_contexts: Maps file paths to their corresponding SELinux contexts.
genfs_contexts: Defines contexts for pseudo-filesystems like proc and sysfs.
initial_contexts: Defines default contexts for processes.

These files are compiled into a binary policy that the kernel enforces.

Identifying SELinux Denials

The first step in any SELinux debugging scenario is to identify what’s being denied. When an operation is blocked by SELinux, an Access Vector Cache (AVC) denial message is logged in the kernel ring buffer.

Capturing AVC Denials

You can capture these denials using adb:

adb logcat -b all | grep 'avc: denied'

Or directly from the kernel ring buffer (requires root):

adb shell su -c
        
        
        
            
                
            
            
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