Introduction to Android SELinux

Security-Enhanced Linux (SELinux) is a mandatory access control (MAC) system integrated into the Android operating system to enhance security. It operates on the principle of least privilege, ensuring that every process, file, and resource access is explicitly permitted by a defined security policy. While Android ships with a robust default SELinux policy, developing custom Android systems, integrating new hardware, or implementing specific security features often necessitates the modification or extension of this policy. This guide provides an expert-level, step-by-step approach to understanding, diagnosing, and crafting custom SELinux policies for Android.

The Imperative of Security on Android

In the Android ecosystem, SELinux prevents unauthorized processes from accessing sensitive resources, even if a service or application is compromised. It partitions the system into various security domains and types, limiting what each domain can access. Without a properly tuned SELinux policy, custom modifications can inadvertently create security vulnerabilities or, more commonly, lead to frustrating ‘permission denied’ errors that crash applications or prevent system services from functioning correctly.

Understanding SELinux Fundamentals

Before diving into policy development, it’s crucial to grasp basic SELinux concepts:

• Subjects and Objects: Processes are subjects, and files, sockets, devices are objects.
• Types: A label applied to both subjects and objects, defining their security context. E.g., system_server_t for the system server, data_file_t for application data.
• Domains: A special type assigned to a process, defining its security context and what actions it can perform.
• Classes: Categories of system resources, e.g., file, socket, binder.
• Permissions: Specific actions within a class, e.g., read, write, bind.
• Rules: Statements that allow or deny specific permissions for a source type on a target type within a given class. Example: allow system_server_t data_file_t:file { read write };

Prerequisites and Environment Setup

AOSP Build Environment

To develop custom SELinux policies, you’ll need a complete Android Open Source Project (AOSP) build environment. This involves:

• A Linux development machine (Ubuntu LTS recommended).
• Sufficient disk space (300GB+).
• A properly synced AOSP source tree for your target Android version.
• Familiarity with building AOSP images (source build/envsetup.sh; lunch <target>; make -jN).

Essential Tools

Beyond the AOSP build tools, you’ll extensively use:

• adb (Android Debug Bridge) for device interaction.
• audit2allow: A utility to convert SELinux denial messages into policy rules (part of AOSP).
• sesearch: A tool to query compiled SELinux policy (part of AOSP).
• A text editor for policy files.

Navigating the AOSP SELinux Policy Structure

Android’s SELinux policy is distributed across several directories within the AOSP source tree:

• system/sepolicy/: Contains the core platform policy, common to all Android devices. You’ll typically add your custom rules here or in a device-specific overlay.
• device/<vendor>/<device>/sepolicy/: Device-specific policy extensions. This is often the primary location for OEM-specific rules.
• vendor/<vendor_name>/sepolicy/: Vendor-specific policy, relevant for SoC vendors.
• prebuilts/vndk/sepolicy/: Prebuilt VNDK sepolicy.

Within these directories, you’ll find various file types:

• .te (Type Enforcement) files: Define types, domains, and the rules governing them.
• .if (Interface) files: Define public interfaces that other policy modules can use.
• .fc (File Context) files: Map file paths to specific SELinux types.
• BoardConfig.mk or Android.bp files: Control the compilation and inclusion of policy files.

Diagnosing SELinux Denials

The first step in policy development is identifying what’s being denied. SELinux denials, known as AVC (Access Vector Cache) denials, are logged by the kernel. You can observe these denials on a device running in enforcing mode.

Identifying AVC Denials

To observe denials, connect your device via ADB and use:

adb shell su -c 'dmesg | grep avc'

Or for a more persistent log:

adb shell su -c 'logcat -b all | grep avc'

A typical AVC denial message looks like this:

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

        
        
        

            

                
            
            

                
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