?? SELinux Security Policies in AOSP??

SELinux (Security-Enhanced Linux) is a game-changer in Linux security, originally developed by the NSA and later adapted by Google for Android 4.4 under SEAndroid. This transition introduced Mandatory Access Control (MAC), making Android devices significantly more secure.

?? Key Concepts Covered:

1?? DAC vs MAC – The Foundation of SELinux

?? DAC (Discretionary Access Control): Traditional Linux security model where a process inherits the same permissions as the user.

?? MAC (Mandatory Access Control): SELinux enforces strict security policies—no implicit access is granted.

?? Example: Even with root access, a process cannot perform actions unless explicitly allowed in security policy files.

2?? Understanding SELinux Policy Language

?? Security Context (SContext) – Every process and file is assigned a security label following the structure:

• user:role:type[:range]
• Example: u:r:init:s0 (Process labeled as init)

?? Core SELinux Concepts:

• Type Enforcement (TE) – Defines what a process (domain) can do.
• Role-Based Access Control (RBAC) – Assigns roles to restrict actions.
• Multi-Level Security (MLS) – Used in high-security environments.

?? Example TE Rule:

allow netd proc:file write;        

This means netd can write files in /proc only if explicitly allowed.

3?? Neverallow Rules & Security Restrictions

?? neverallow ensures no unintended permissions exist.

?? Example: Blocking all app domains from gaining unrestricted kernel access:

neverallow { appdomain -unconfineddomain } self:capability2 *;        

If any conflicting allow rules exist, the security policy build fails.

4?? Domain & Type Transitions (Process & File Labeling)

?? Processes and files need controlled label transitions to prevent privilege escalation.

?? Example: When init spawns a process, it must be assigned a specific domain:

type_transition init_t apache_exec_t : process apache_t;        

?? Macros simplify transitions by combining multiple rules.

5?? Object Classes & Permissions in SEAndroid

?? SELinux controls access at multiple levels via Object Classes such as:

• file, dir, socket, binder, zygote (Android-specific), property_service, etc.

?? Permissions (perm_set) for each class define actions like read, write, execute, ioctl.

6?? Role-Based Access Control (RBAC) & Constrain Rules

?? Defines which roles can switch to others, restricting privilege escalation.

?? Example: A role switch must be explicitly allowed:

allow from_role_id to_role_id;        

?? constrain statements enforce additional restrictions on user, role, or type.

?? Why This Matters?

? If you're working on AOSP, embedded Linux, or Android system security, mastering SEPolicy is crucial.

? SEAndroid is the foundation of Android security—understanding it helps in custom ROMs, security hardening, and compliance.

#SELinux #SEAndroid #AndroidSecurity #AOSP #CyberSecurity #EmbeddedSystems