Introduction to Android SELinux and Mandatory Access Control

Android’s security architecture is robust, and a cornerstone of this strength is SELinux (Security-Enhanced Linux). Operating at the kernel level, SELinux implements Mandatory Access Control (MAC), a security model that dictates access decisions based on a predefined security policy, regardless of user identity. This stands in contrast to Discretionary Access Control (DAC), where resource owners can grant or deny permissions. On Android, SELinux ensures that even if a service or application is compromised, its ability to interact with the rest of the system is strictly limited by its assigned SELinux context and the overarching policy.

Understanding and customizing Android SELinux policies is crucial for device manufacturers, security researchers, and advanced developers looking to harden a system beyond its stock configuration or integrate new hardware and services securely. This guide will walk you through the practical aspects of identifying policy violations, crafting custom rules, and integrating them into an Android build.

Understanding SELinux on Android

Every process, file, and IPC mechanism on an Android device has an associated SELinux context, typically formatted as `user:role:type:level`. The `type` (e.g., `init`, `app_data_file`) is the most commonly used component for defining access rules. SELinux policies define which `type` can perform specific operations (like `read`, `write`, `execute`, `connect`) on other `types` or objects.

Key SELinux Concepts:

• Subjects and Objects: A subject is typically a process, and an object is a file, socket, or other system resource. Both have SELinux contexts.
• Types (Domains): Labels applied to subjects (processes) and objects (files, devices).
• Permissions: Specific operations (e.g., `read`, `write`, `exec`) on objects.
• Rules: Directives like `allow source_type target_type:class permission;`
• Attributes: Collections of types, simplifying policy writing.
• Policy Modules: Collections of rules (`.te` files) that are compiled into a binary policy.

Identifying SELinux Violations (AVC Denials)

The first step in hardening or customizing SELinux is to identify current policy violations. When an unpermitted action occurs, the kernel generates an Access Vector Cache (AVC) denial. These denials are logged and are your primary source of information.

Tools for Monitoring:

1. dmesg: Kernel messages, including SELinux denials.
2. logcat: Android’s main logging system, often showing denials from system services.
3. audit2allow: A tool (part of `sepolicy-tools` on Linux) that can parse AVC denials and suggest SELinux rules.

Example: Capturing an AVC Denial

Let’s say a custom application (`my_app`) attempts to write to a log file (`/data/local/tmp/mylog.txt`) without proper permissions. You might see something like this in `dmesg` or `logcat`:

adb shell dmesg | grep 'avc: denied'

audit: avc: denied { write } for pid=1234 comm=

        
        
        

            

                
            
            

                
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