SELinux is set up to default-deny, which means that every single access for which it has a hook in the kernel must be explicitly allowed by policy. This means a policy file is comprised of a large amount of information regarding rules, types, classes, permissions, and more. A full consideration of SELinux is out of the scope of this document, but an understanding of how to write policy rules is now essential when bringing up new Android devices. There is a great deal of information available regarding SELinux already. See Supporting documentation for suggested resources.
To enable SELinux, integrate the latest Android kernel and then incorporate the files found in the system/sepolicy directory. When compiled, those files comprise the SELinux kernel security policy and cover the upstream Android operating system.
In general, you should not modify the
directly. Instead, add or edit your own device-specific policy files in the
directory. In Android 8.0 and higher, the changes you make to these files should
only affect policy in your vendor directory. For more details on separation of
public sepolicy in Android 8.0 and higher, see
Customizing SEPolicy in Android
8.0+. Regardless of Android version, you're still modifying these files:
Files that end with
*.te are SELinux policy source files, which
define domains and their labels. You may need to create new policy files in
but you should try to update existing files where possible.
Context files are where you specify labels for your objects.
file_contextsassigns labels to files and is used by various userspace components. As you create new policies, create or update this file to assign new labels to files. To apply new
file_contexts, rebuild the filesystem image or run
restoreconon the file to be relabeled. On upgrades, changes to
file_contextsare automatically applied to the system and userdata partitions as part of the upgrade. Changes can also be automatically applied on upgrade to other partitions by adding
restorecon_recursivecalls to your init.board.rc file after the partition has been mounted read-write.
genfs_contextsassigns labels to filesystems, such as
vfatthat do not support extended attributes. This configuration is loaded as part of the kernel policy but changes may not take effect for in-core inodes, requiring a reboot or unmounting and re-mounting the filesystem to fully apply the change. Specific labels may also be assigned to specific mounts, such as
property_contextsassigns labels to Android system properties to control what processes can set them. This configuration is read by the
initprocess during startup.
service_contextsassigns labels to Android binder services to control what processes can add (register) and find (lookup) a binder reference for the service. This configuration is read by the
servicemanagerprocess during startup.
seapp_contextsassigns labels to app processes and
/data/datadirectories. This configuration is read by the
zygoteprocess on each app launch and by
seinfotag to apps based on their signature and optionally their package name. The
seinfotag can then be used as a key in the
seapp_contextsfile to assign a specific label to all apps with that
seinfotag. This configuration is read by
After editing or adding policy and context files, update your
makefile to reference the
sepolicy subdirectory and each new policy file.
For more information about the
BOARD_SEPOLICY variables, see
BOARD_SEPOLICY_DIRS += \ <root>/device/manufacturer/device-name/sepolicy BOARD_SEPOLICY_UNION += \ genfs_contexts \ file_contexts \ sepolicy.te
After rebuilding, your device is enabled with SELinux. You can now either customize your SELinux policies to accommodate your own additions to the Android operating system as described in Customization or verify your existing setup as covered in Validation.
When the new policy files and BoardConfig.mk updates are in place, the new policy settings are automatically built into the final kernel policy file. For more information about how sepolicy is built on the device, see Building sepolicy.
To get started with SELinux:
- Enable SELinux in the kernel:
- Change the kernel_cmdline parameter so that:
BOARD_KERNEL_CMDLINE := androidboot.selinux=permissiveThis is only for initial development of policy for the device. After you have an initial bootstrap policy, remove this parameter so your device is enforcing or it will fail CTS.
- Boot up the system in permissive and see what denials are encountered on boot:
On Ubuntu 14.04 or newer:
adb shell su -c dmesg | grep denied | audit2allow -p out/target/product/BOARD/root/sepolicyOn Ubuntu 12.04:
adb pull /sys/fs/selinux/policy adb logcat -b all | audit2allow -p policy
- Evaluate the output for warnings that resemble
init: Warning! Service name needs a SELinux domain defined; please fix!See Validation for instructions and tools.
- Identify devices, and other new files that need labeling.
- Use existing or new labels for your objects. Look at the
*_contextsfiles to see how things were previously labeled and use knowledge of the label meanings to assign a new one. Ideally, this will be an existing label which will fit into policy, but sometimes a new label will be needed, and rules for access to that label will be needed. Add your labels to the appropriate context files.
- Identify domains/processes that should have their own security domains.
You will likely need to write a completely new policy for each. All
services spawned from
init, for instance, should have their own. The following commands help reveal those that remain running (but ALL services need such a treatment):
adb shell su -c ps -Z | grep init
adb shell su -c dmesg | grep 'avc: '
init.device.rcto identify any domains that don't have a domain type. Give them a domain early in your development process to avoid adding rules to
initor otherwise confusing
initaccesses with ones that are in their own policy.
- Set up
BOARD_SEPOLICY_*variables. See the README in
system/sepolicyfor details on setting this up.
- Examine the init.device.rc and fstab.device file and
make sure every use of
mountcorresponds to a properly labeled filesystem or that a
context= mountoption is specified.
- Go through each denial and create SELinux policy to properly handle each. See the examples in Customization.
You should start with the policies in the AOSP and then build upon them for your own customizations. For more information about policy strategy and a closer look at some of these steps, see Writing SELinux Policy.
Here are specific examples of exploits to consider when crafting your own software and associated SELinux policies:
Symlinks - Because symlinks appear as files, they are often
read as files, which can lead to exploits. For instance, some privileged
components, such as
init, change the permissions of certain files,
sometimes to be excessively open.
Attackers might then replace those files with symlinks to code they control, allowing the attacker to overwrite arbitrary files. But if you know your application will never traverse a symlink, you can prohibit it from doing so with SELinux.
System files - Consider the class of system files that
should be modified only by the system server. Still, since
vold run as root, they can access
those system files. So if
netd became compromised, it could
compromise those files and potentially the system server itself.
With SELinux, you can identify those files as system server data files.
Therefore, the only domain that has read/write access to them is system server.
netd became compromised, it could not switch domains to the
system server domain and access those system files although it runs as root.
App data - Another example is the class of functions that must run as root but should not get to access app data. This is incredibly useful as wide-ranging assertions can be made, such as certain domains unrelated to application data being prohibited from accessing the internet.
setattr - For commands such as
chown, you could identify the set of files where the associated
domain can conduct
setattr. Anything outside of that could be
prohibited from these changes, even by root. So an application might run
chown against those labeled
app_data_files but not