Sepolicy Android: SELinux security architecture

Users who go deeper into modifying their devices often come across the abbreviation sepolicy when working with custom firmware or obtaining root access. This is not just a technical term, but a fundamental security mechanism built into the operating system kernel. Understanding how it works Security-Enhanced Linuxis critical for properly configuring a smartphone and preventing critical failures.

Unlike conventional access rights, sepolicy implements mandatory access control, which strictly regulates the interaction between processes. If you have problems launching certain modules or see errors in the logs related to avc: deniedthen the reason most often lies in the restrictions of security policies. Let's look at how this system protects your device from the inside.

Many enthusiasts try to disable these checks for the sake of convenience, but this opens a security hole in the entire device. Android relies on these rules to isolate applications from each other and from system services. Without understanding the structure policy any changes in the system can lead to unpredictable consequences, up to a complete failure of the device to boot.

Principles of mandatory access control

The basis sepolicy is the concept in which each process and file system object has a special label, called context. This label determines which domain zone the entity belongs to. The system core checks these labels against a pre-written set of rules before allowing any action. This creates an abstraction layer that is independent of user rights root.

The traditional UNIX model, based on the rights of the owner, group and other users, fades into the background here. Even if the process is running as the superuser, it will not be able to perform the action if policy explicitly prohibits it. This approach minimizes the damage in the event of exploitation of a vulnerability in one of the applications, since the attacker will not be able to go beyond the limits allocated to him. domain.

There are two main modes of operation of this subsystem: Enforcing and Permissive. In the first mode, rule violations are blocked and recorded in a log, which is standard for serial devices. In the second mode, violations are only logged, but not blocked, which is convenient for developers, but extremely dangerous for normal use. Switching between modes often requires rebuilding the kernel or changing boot parameters.

⚠️ Attention: Constant operation in the mode Permissive reduces the overall security of the device and may interfere with the operation of banking applications and highly secure services, such as Google Pay or SafetyNet.

Custom firmware developers must carefully balance security and functionality. Rules that are too strict can break third-party modules, while rules that are too weak put user data at risk. This is why there is so much discussion in the community around the optimal configuration sepolicy for specific tasks.

Structure of policy files and domains

Policy files are compiled into a binary format, which is loaded by the kernel at startup. The source code of the rules is stored in text files with the extension .te (Type Enforcement). These files describe file types, process domains, and access rules between them. Understanding the syntax of these files is necessary to create your own Magisk modules or change the behavior of the system.

Each rule defines which subject (domain) can perform which actions (read, write, execute) on which object (file, socket, device). For example, a rule may allow a domain system_app to read files from the domain system_data_filebut deny writing. Violation of this rule will result in the generation of an error message in the system log dmesg or logcat.

You can use command line utilities to analyze the current configuration. The command getenforce will show the current protection status, and ls -Z will display security contexts for files in the directory. This allows you to quickly diagnose why an application is not able to access the required resource. Often the problem is solved by adding one single rule to the local policy.

adb shell getenforce
adb shell ls -Z /data/local/tmp

It is important to note that the structure of policies in Android is constantly becoming more complex with each new release of the operating system. The introduction of project sections (Project Treble) and other architectural changes led to the separation of policies into system and vendor ones. This means that the rules can be located in different sections of memory and loaded sequentially.

Diagnostics of violations and analysis of logs

When an application or service cannot perform the required action, an entry like avc: deniedappears in the logs. These messages contain all the necessary information to resolve the problem: request source (scontext), request target (tcontext), object class (tclass), and action type (perm). Analysis of these fields allows you to accurately determine which rule is missing in the configuration.

There are many tools for automatically analyzing such logs. The utility audit2allow is capable of generating a ready-made rule based on the failure record. However, blindly applying the generated rules is not recommended, as this may open up unnecessary holes in the protection. It is necessary to manually check whether the requested action is indeed legitimate for the given process.

Often users are faced with a situation where, after installing a module, some system function stops working. In 90% of cases, this is due to the fact that the module is trying to access a resource that is not accessible by the current module. The solution is to create a file sepolicy. The solution is to create a file sepolicy.rule inside the module that adds the necessary exceptions without changing the system partition.

Log component	Description	Example value
scontext	Request source context (process)	u:r:untrusted_app:s0
tcontext	Context of the request target (file/resource)	u:object_r:system_data_file:s0
tclass	Access object class	file, socket, process
perm	Requested action	read, write, execute

For in-depth analysis, you can use the utility supolicy from the developer Chainfire, which allows you to view and modify the policy in real time on devices with root access. This is a powerful tool that requires caution, since an error in the command syntax can lead to system instability during operation.

Modification of policies through Magisk

Framework Magisk has become the de facto standard for making changes to sepolicy without modifying the system section. The mechanism magiskpolicy allows you to load additional rules when the device boots. This is achieved by creating a script in the module folder, which is executed early in the system initialization.

The process of adding rules is as follows: the developer determines the necessary permissions and writes them to a file sepolicy.rule. When loaded, Magisk applies these rules to the policy loaded into memory. This allows you to maintain the integrity of the system partition, which is important for passing integrity checks and the operation of OTA updates.

• 🛠️ Create a file sepolicy.rule in the root of your Magisk module.
• 📝 Write the rules in a format understandable to the utility magiskpolicy.
• 🔄 Make sure that the module is loaded after the kernel is initialized.
• ✅ Test the application after restarting the device.

If the system already has an explicit prohibition, adding the permission may not work depending on the kernel version and implementation. SELinux. In such cases, deeper intervention or patching of the policy itself is required before it is loaded.

⚠️ Attention: Incorrect syntax in the file sepolicy.rule may cause Magisk to be unable to boot and the device to go into an endless reboot (bootloop). Always check the syntax before installation.

Consequences of disabling SELinux protection

Full disabling SELinux puts the system into a mode where mandatory access control does not operate. Although this solves all compatibility problems with older modules and tweaks, this practice is considered extremely unsafe in modern conditions. Malicious software that has gained access to the system will be able to freely read data from other applications and change system files.

Many modern applications, especially banking and gaming with anti-cheat, check the status SELinux at startup. Detection of a mode Permissive or disabled protection often leads to immediate closure of the application or blocking of the account. This is done to protect user data and prevent cheating in online games.

In addition, disabling protection may disrupt the operation of data encryption and profile isolation mechanisms. In a corporate environment, the use of disabled devices is typically prohibited by security policies. For the average user, the risk of data loss or theft of personal information increases many times over. SELinux usually prohibited by security policies. For the average user, the risk of data loss or theft of personal information increases many times over.

Performance Myth

There is an opinion that disabling SELinux improves performance. On modern processors, the overhead for checking policies is negligible and does not affect the speed of the interface or games.

Instead of completely disabling, experts recommend fine-tuning the rules. This allows you to maintain a high level of security, allowing access only to those resources that are really necessary for the operation of specific software. This approach requires more time to set up, but guarantees stability and protection.

Relevance of policies in new versions of Android

With the release of new versions Android security policies become more and more strict. Google is implementing new restrictions, such as blocking access to certain system calls or tightening rules for background processes. Modules running on older versions may stop functioning on new builds precisely because of changes in sepolicy.

Custom firmware developers have to constantly update rule sets to meet the requirements of the new operating system. This sometimes results in some features having to be temporarily disabled until a safe implementation method can be found. Users should be prepared that some tweaks may no longer work after a firmware update.

It is also worth considering differences in implementations from different manufacturers. Samsung, Xiaomi and Google may use slightly different default rule sets. There is no universal solution for all devices, and the policy often has to be adapted to a specific smartphone model.

⚠️ Attention: Interfaces and commands may differ depending on the version of Android and the manufacturer’s shell. Always check the latest documentation for your specific device model before making changes to system files.

Frequently asked questions (FAQ)

Is it safe to change the SELinux mode to Permissive?

For everyday use it is not safe. The mode Permissive disables blocking of violations, which allows malicious code to operate freely. Use this mode only temporarily to debug your own developments or modules.

Why does the application crash with an access error, although there is Root?

root access give user authority, but do not cancel mandatory control SELinux. If there is no rule in the policy that allows a specific process to access the file, the operation will be blocked regardless of the user's rights. You need to add the corresponding rule to the policy.

How to return the standard policy after changes?

If you changed the policy through the Magisk module, just disable or delete this module in the Magisk Manager application and reboot the device. If you flashed a modified kernel image or boot.img, you will need to reflash the stock image via Fastboot or Recovery.

Does Sepolicy affect the operation of the camera and NFC?

Yes, access to hardware modules such as the camera and NFC is strictly controlled by security policies. Incorrect configuration may result in applications not being able to initialize these devices, even if the drivers are loaded correctly.

Is it possible to update OTA with a changed policy?

Usually no. Integrity checking systems will detect changes to system files or the boot image and refuse to install the update. In some cases, installing OTA is possible, but the first activation after the update may not take place due to a conflict between policy versions.