Introduction to SELinux on Android

SELinux (Security-Enhanced Linux) is a mandatory access control (MAC) system that enhances the security of Android devices by confining programs and processes to the minimum set of privileges they need. Unlike traditional discretionary access control (DAC) where permissions are based on user and group IDs, SELinux policies define explicit rules for what processes can access which resources, regardless of their user context. When developing custom services or daemons for Android, understanding and writing correct SELinux policy is paramount to ensure both security and functionality.

This guide will walk you through the process of creating a new SELinux policy for a hypothetical custom service from the ground up, covering the essential concepts, debugging techniques, and best practices.

Prerequisites

• An Android Open Source Project (AOSP) build environment.
• Basic familiarity with Linux command-line operations.
• A rooted Android device or an AOSP emulator to test policies.
• Understanding of the Android build system (Android.bp, init.rc).

Understanding SELinux Basics for Android

SELinux operates on a principle of ‘everything is denied unless explicitly allowed’. Key concepts include:

• Subjects (Processes): These are labeled with a domain or type.
• Objects (Files, Sockets, IPC): These are labeled with a type.
• Rules: Define interactions between subjects and objects (e.g., allow source_type target_type:class permissions;).
• Contexts: A string label (e.g., u:r:myservice_daemon:s0) applied to processes, files, and other objects. For Android, the user (u) and role (r) are typically fixed, while the type (myservice_daemon) is the primary component we modify.

When an unallowed action occurs, SELinux generates an Audit Violation Message (AVC denial), which is crucial for debugging.

Scenario: A Custom ‘myservice’ Daemon

Let’s imagine we’re adding a simple native C daemon named myservice to Android. This service will run in the background, perform some operations, and potentially log to a file. Our goal is to create a secure SELinux policy that allows myservice to function correctly without granting it excessive privileges.

Step 1: Create the Custom Service Executable

First, create a simple C file, myservice.c:

#include <stdio.h> #include <stdlib.h> #include <unistd.h> int main(int argc, char *argv[]) { printf(

        
        
        

            

                
            
            

                
Android Mobile Specs & Compare Directory
                
Are you researching mobile hardware properties, processor SoCs, GPU chipsets, or RAM configurations? Access our complete specs catalog to compare up to 5 devices side-by-side!
                Compare Devices Specs →