Policy compatibility

This article describes how Android handles the policy compatibility issues with platform OTAs, where new platform SELinux settings may differ from old vendor SELinux settings.

Treble-based SELinux policy design considers a binary distinction between platform and vendor policy; the scheme becomes more complicated if vendor partitions generate dependencies, such as platform < vendor < oem.

In Android 8.0 and higher, SELinux global policy is divided into private and public components. Public components consist of the policy and associated infrastructure, which are guaranteed to be available for a platform version. This policy will be exposed to vendor policy writers to enable vendors to build a vendor policy file, which when combined with the platform-provided policy, results in a fully-functional policy for a device.

• For versioning, the exported platform-public policy will be written as attributes.
• For ease of policy writing, exported types will be transformed into versioned attributes as part of the policy build process. Public types may also be used directly in labeling decisions provided by vendor contexts files.

Android maintains a mapping between exported concrete types in platform policy and the corresponding versioned attributes for each platform version. This ensures that when objects are labeled with a type, it doesn't break behavior guaranteed by the platform-public policy in a previous version. This mapping is maintained by keeping a mapping file up-to-date for each platform version, which keeps attribute membership information for each type exported in public policy.

Object ownership and labeling

When customizing policy in Android 8.0 and higher, ownership must be clearly defined for each object to keep platform and vendor policy separate. For example, if the vendor labels /dev/foo and the platform then labels /dev/foo in a subsequent OTA, there will be undefined behavior. For SELinux, this manifests as a labeling collision. The device node can have only a single label which resolves to whichever label is applied last. As a result:

• Processes that need access to the unsuccessfully applied label will lose access to the resource.
• Processes that gain access to the file may break because the wrong device node was created.

System properties also have potential for naming collisions that could result in undefined behavior on the system (as well as for SELinux labeling). Collisions between platform and vendor labels can occur for any object that has an SELinux label, including properties, services, processes, files, and sockets. To avoid these issues, clearly define ownership of these objects.

In addition to label collisions, SELinux type/attribute names may also collide. A type/attribute name collision will always result in a policy compiler error.

Type/attribute namespacing

SELinux doesn't allow multiple declarations of the same type/attribute. Policy with duplicate declarations will fail to compilation. To avoid type and attribute name collisions, all vendor declarations should be namespaced starting with vendor_.

type foo, domain; → type vendor_foo, domain;

System property and process labeling ownership

Avoiding labeling collisions is best solved using property namespaces. To easily identify platform properties and avoid name conflicts when renaming or adding exported-platform properties, ensure all vendor properties have their own prefixes:

Property type	Acceptable prefixes
control properties	ctl.vendor. ctl.start$vendor. ctl.stop$vendor. init.svc.vendor.
read-writable	vendor.
read-only	ro.vendor. ro.boot. ro.hardware.
persistent	persist.vendor.

Vendors can continue to use ro.boot.* (which comes from the kernel cmdline) and ro.hardware.* (an obvious hardware-related property).

All the vendor services in init rc files should have vendor. for services in init rc files of non-system partitions. Similar rules are applied to the SELinux labels for the vendor properties (vendor_ for the vendor properties).

File ownership

Preventing collisions for files is challenging because platform and vendor policy both commonly provide labels for all filesystems. Unlike type naming, namespacing of files isn't practical since many of them are created by the kernel. To prevent these collisions, follow the naming guidance for filesystems in this section. For Android 8.0, these are recommendations without technical enforcement. In the future, these recommendations will be enforced by the Vendor Test Suite (VTS).

System (/system)

Only the system image must provide labels for /system components through file_contexts, service_contexts, etc. If labels for /system components are added in /vendor policy, a framework-only OTA update might not be possible.

Vendor (/vendor)

The AOSP SELinux policy already labels parts of vendor partition the platform interacts with, which enables writing SELinux rules for platform processes to be able to talk and/or access parts of vendor partition. Examples:

/vendor path	Platform-provided label	Platform processes depending on the label
/vendor(/.*)?	vendor_file	All HAL clients in framework, ueventd, etc.
/vendor/framework(/.*)?	vendor_framework_file	dex2oat, appdomain, etc.
/vendor/app(/.*)?	vendor_app_file	dex2oat, installd, idmap, etc.
/vendor/overlay(/.*)	vendor_overlay_file	system_server, zygote, idmap, etc.

As a result, specific rules must be followed (enforced through neverallows) when labelling additional files in vendor partition:

• vendor_file must be the default label for all files in vendor partition. The platform policy requires this to access passthrough HAL implementations.
• All new exec_types added in vendor partition through vendor SEPolicy must have vendor_file_type attribute. This is enforced through neverallows.
• To avoid conflicts with future platform/framework updates, avoid labelling files other than exec_types in vendor partition.
• All library dependencies for AOSP-identified same process HALs must be labelled as same_process_hal_file.

Procfs (/proc)

Files in /proc may be labeled using only the genfscon label. In Android 7.0, both the platform and vendor policy used genfscon to label files in procfs.

Recommendation: Only platform policy labels /proc. If vendor processes need access to files in /proc that are currently labeled with the default label (proc), vendor policy shouldn't explicitly label them and should instead use the generic proc type to add rules for vendor domains. This allows the platform updates to accommodate future kernel interfaces exposed through procfs and label them explicitly as needed.

Debugfs (/sys/kernel/debug)

Debugfs can be labeled in both file_contexts and genfscon. In Android 7.0 to Android 10, both platform and vendor label debugfs.

In Android 11, debugfs can't be accessed or mounted on production devices. Device manufacturers should remove debugfs.

Tracefs (/sys/kernel/debug/tracing)

Tracefs can be labeled in both file_contexts and genfscon. In Android 7.0, only the platform labels tracefs.

Recommendation: Only platform may label tracefs.

Sysfs (/sys)

Files in /sys may be labeled using both file_contexts and genfscon. In Android 7.0, both platform and vendor use file_contexts and genfscon to label files in sysfs.

Recommendation: The platform may label sysfs nodes that aren't device-specific. Otherwise, only vendor may label files.

tmpfs (/dev)

Files in /dev may be labeled in file_contexts. In Android 7.0, both platform and vendor label files here.

Recommendation: Vendor may label only files in /dev/vendor (for example, /dev/vendor/foo, /dev/vendor/socket/bar).

Rootfs (/)

Files in / may be labeled in file_contexts. In Android 7.0, both platform and vendor label files here.

Recommendation: Only system may label files in /.

Data (/data)

Data is labeled through a combination of file_contexts and seapp_contexts.

Recommendation: Disallow vendor labeling outside /data/vendor. Only platform may label other parts of /data.

Compatibility attributes

SELinux policy is an interaction between source and target types for specific object classes and permissions. Every object (processes, files, etc.) affected by SELinux policy may have only one type, but that type may have multiple attributes.

Policy is written mostly in terms of existing types:

allow source_type target_type:target_class permission(s);

This works because the policy was written with knowledge of all types. However, if the vendor policy and platform policy use specific types, and the label of a specific object changes in only one of those policies, the other may contain policy that gained or lost access previously relied upon. For example:

File_contexts:
/sys/A   u:object_r:sysfs:s0
Platform: allow p_domain sysfs:class perm;
Vendor: allow v_domain sysfs:class perm;

Could be changed to:

File_contexts:
/sys/A   u:object_r:sysfs_A:s0

Although the vendor policy would remain the same, the v_domain would lose access due to the lack of policy for the new sysfs_A type.

By defining a policy in terms of attributes, we can give the underlying object a type that has an attribute corresponding to policy for both the platform and vendor code. This can be done for all types to effectively create an attribute-policy wherein concrete types are never used. In practice, this is required only for the portions of policy that overlap between platform and vendor, which are defined and provided as platform public policy that gets built as part of the vendor policy.

Defining public policy as versioned attributes satisfies two policy compatibility goals:

• Ensure vendor code continues to work after platform update. Achieved by adding attributes to concrete types for objects corresponding to those on which vendor code relied, preserving access.
• Ability to deprecate policy. Achieved by clearly delineating policy sets into attributes that can be removed as soon as the version to which they correspond no longer is supported. Development can continue in the platform, knowing the old policy is still present in the vendor policy and will be automatically removed when/if it upgrades.

Policy writability

To meet the goal of not requiring knowledge of specific version changes for policy development, Android 8.0 includes a mapping between platform-public policy types and their attributes. Type foo is mapped to attribute foo_vN, where N is the version targeted. vN corresponds to the PLATFORM_SEPOLICY_VERSION build variable and is of the form MM.NN, where MM corresponds to the platform SDK number and NN is a platform sepolicy specific version.

Attributes in public policy aren't versioned, but rather exist as an API on which platform and vendor policy can build to keep the interface between the two partitions stable. Both platform and vendor policy writers can continue to write policy as it is written today.

Platform-public policy exported as allow source_foo target_bar:class perm;is included as part of the vendor policy. During compilation (which includes the corresponding version) it is transformed into the policy that will go to the vendor portion of the device (shown in the transformed Common Intermediate Language (CIL)):

 (allow source_foo_vN target_bar_vN (class (perm)))

As vendor policy is never ahead of the platform, it shouldn't be concerned with prior versions. However, platform policy will need to know how far back vendor policy is, include attributes to its types, and set policy corresponding to versioned attributes.

Policy diffs

Automatically creating attributes by adding _vN to the end of each type does nothing without mapping of attributes to types across version diffs. Android maintains a mapping between versions for attributes and a mapping of types to those attributes. This is done in the aforementioned mapping files with statements, such as (CIL):

(typeattributeset foo_vN (foo))

Platform upgrades

The following section details scenarios for platform upgrades.

Same types

This scenario occurs when an object doesn't change labels in policy versions. This is the same for source and target types and can be seen with /dev/binder, which is labeled binder_device across all releases. It is represented in transformed policy as:

binder_device_v1 … binder_device_vN

When upgrading from v1 → v2, the platform policy must contain:

type binder_device; -> (type binder_device) (in CIL)

In the v1 mapping file (CIL):

(typeattributeset binder_device_v1 (binder_device))

In the v2 mapping file (CIL):

(typeattributeset binder_device_v2 (binder_device))

In the v1 vendor policy (CIL):

(typeattribute binder_device_v1)
(allow binder_device_v1 …)

In the v2 vendor policy (CIL):

(typeattribute binder_device_v2)
(allow binder_device_v2 …)

New types

This scenario occurs when the platform has added a new type, which can happen when adding new features or during policy hardening.

• New feature. When the type is labeling an object that was previously non-existent (such as a new service process), the vendor code didn't previously interact with it directly so no corresponding policy exists. The new attribute corresponding to the type doesn't have an attribute in the previous version, and so wouldn't need an entry in the mapping file targeting that version.
• Policy hardening. When the type represents policy hardening, the new type attribute must link back to a chain of attributes corresponding to the previous one (similar to the previous example changing /sys/A from sysfs to sysfs_A). Vendor code relies on a rule enabling access to sysfs, and needs to include that rule as an attribute of the new type.

When upgrading from v1 → v2, the platform policy must contain:

type sysfs_A; -> (type sysfs_A) (in CIL)
type sysfs; (type sysfs) (in CIL)

In the v1 mapping file (CIL):

(typeattributeset sysfs_v1 (sysfs sysfs_A))

In the v2 mapping file (CIL):

(typeattributeset sysfs_v2 (sysfs))
(typeattributeset sysfs_A_v2 (sysfs_A))

In the v1 vendor policy (CIL):

(typeattribute sysfs_v1)
(allow … sysfs_v1 …)

In the v2 vendor policy (CIL):

(typeattribute sysfs_A_v2)
(allow … sysfs_A_v2 …)
(typeattribute sysfs_v2)
(allow … sysfs_v2 …)

Removed types

This (rare) scenario occurs when a type is removed, which can happen when the underlying object:

• Remains but gets a different label.
• Is removed by the platform.

During policy loosening, a type is removed and the object labeled with that type is given a different, already-existing label. This represents a merging of attribute mappings: The vendor code must still be able to access the underlying object by the attribute it used to possess, but the rest of the system must now be able to access it with its new attribute.

If the attribute to which it has been switched is new, then relabeling is the same as in the new type case, except that when an existing label is used, the addition of the old attribute new type would cause other objects also labeled with this type to be newly accessible. This is essentially what is done by the platform and is deemed to be an acceptable tradeoff to maintain compatibility.

(typeattribute sysfs_v1)
(allow … sysfs_v1 …)

Example Version 1: Collapsing types (removing sysfs_A)

When upgrading from v1 → v2, the platform policy must contain:

type sysfs; (type sysfs) (in CIL)

In the v1 mapping file (CIL):

(typeattributeset sysfs_v1 (sysfs))
(type sysfs_A) # in case vendors used the sysfs_A label on objects
(typeattributeset sysfs_A_v1 (sysfs sysfs_A))

In the v2 mapping file (CIL):

(typeattributeset sysfs_v2 (sysfs))

In the v1 vendor policy (CIL):

(typeattribute sysfs_A_v1)
(allow … sysfs_A_v1 …)
(typeattribute sysfs_v1)
(allow … sysfs_v1 …)

In the v2 vendor policy (CIL):

(typeattribute sysfs_v2)
(allow … sysfs_v2 …)

Example Version 2: Removing completely (foo type)

When upgrading from v1 → v2, the platform policy must contain:

# nothing - we got rid of the type

In the v1 mapping file (CIL):

(type foo) #needed in case vendors used the foo label on objects
(typeattributeset foo_v1 (foo))

In the v2 mapping file (CIL):

# nothing - get rid of it

In the v1 vendor policy (CIL):

(typeattribute foo_v1)
(allow foo …)
(typeattribute sysfs_v1)
(allow sysfs_v1 …)

In the v2 vendor policy (CIL):

(typeattribute sysfs_v2)
(allow sysfs_v2 …)

New class/permissions

This scenario occurs when a platform upgrade introduces new policy components that don't exist in previous versions. For example, when Android added the servicemanager object manager that created the add, find, and list permissions, vendor daemons wanting to register with the servicemanager needed permissions that weren't available. In Android 8.0, only the platform policy may add new classes and permissions.

To allow all domains that could have been created or extended by vendor policy to use the new class without obstruction, the platform policy needs to include a rule similar to:

allow {domain -coredomain} *:new_class perm;

This may even require policy allowing access for all interface (public policy) types, to be sure vendor image gains access. If this results in unacceptable security policy (as it may have with the servicemanager changes), a vendor upgrade could potentially be forced.

Removed class/permissions

This scenario occurs when an object manager is removed (such as the ZygoteConnection object manager) and shouldn't cause issues. The object manager class and permissions could remain defined in policy until the vendor version no longer uses it. This is done by adding the definitions to the corresponding mapping file.

Vendor customization for new/relabeled types

New vendor types are at the core of vendor policy development as they are needed to describe new processes, binaries, devices, subsystems, and stored data. As such, it is imperative to allow the creation of vendor-defined types.

As vendor policy is always the oldest on the device, there is no need to automatically convert all vendor types to attributes in policy. The platform doesn't rely on anything labeled in vendor policy because the platform has no knowledge of it; however, the platform will provide the attributes and public types it uses to interact with objects labeled with these types (such as domain, sysfs_type, etc.). For the platform to continue to interact correctly with these objects, the attributes and types must be appropriately applied and specific rules may need to be added to the customizable domains (such as init).

Attribute changes for Android 9

Devices upgrading to Android 9 can use the following attributes, but devices launching with Android 9 must not.

Violator attributes

Android 9 includes these domain-related attributes:

• data_between_core_and_vendor_violators. Attribute for all domains that violate the requirement of not sharing files by path between vendor and coredomains. Platform and vendor processes shouldn't use on-disk files to communicate (unstable ABI). Recommendation:
  • Vendor code should use /data/vendor.
  • System shouldn't use /data/vendor.
• system_executes_vendor_violators. Attribute for all system domains (except init and shell domains) that violate the requirement of not executing vendor binaries. Execution of vendor binaries has unstable API. Platform shouldn't execute vendor binaries directly. Recommendation:
  • Such platform dependencies on vendor binaries must be behind HIDL HALs.

    OR

  • coredomains that need access to vendor binaries should be moved to the vendor partition and thus, stop being coredomain.

Untrusted attributes

Untrusted apps that host arbitrary code shouldn't have access to HwBinder services, except those considered sufficiently safe for access from such apps (see safe services below). The two main reasons for this are:

1. HwBinder servers don't perform client authentication because HIDL currently doesn't expose caller UID information. Even if HIDL did expose such data, many HwBinder services either operate at a level below that of apps (such as, HALs) or must not rely on app identity for authorization. Thus, to be safe, the default assumption is that every HwBinder service treats all its clients as equally authorized to perform operations offered by the service.
2. HAL servers (a subset of HwBinder services) contain code with higher incidence rate of security issues than system/core components and have access to the lower layers of the stack (all the way down to hardware) thus increasing opportunities for bypassing the Android security model.

Safe services

Safe services include:

• same_process_hwservice. These services (by definition) run in the process of the client and thus have the same access as the client domain in which the process runs.
• coredomain_hwservice. These services don't pose risks associated with reason #2.
• hal_configstore_ISurfaceFlingerConfigs. This service is specifically designed for use by any domain.
• hal_graphics_allocator_hwservice. These operations are also offered by surfaceflinger Binder service, which apps are permitted to access.
• hal_omx_hwservice. This is a HwBinder version of the mediacodec Binder service, which apps are permitted to access.
• hal_codec2_hwservice. This is a newer version of hal_omx_hwservice.

Useable attributes

All hwservices not considered safe have the attribute untrusted_app_visible_hwservice. The corresponding HAL servers have the attribute untrusted_app_visible_halserver. Devices launching with Android 9 MUST NOT use either untrusted attribute.

Recommendation:

• Untrusted apps should instead talk to a system service that talks to the vendor HIDL HAL. For example, apps can talk to binderservicedomain, then mediaserver (which is a binderservicedomain) in turn talks to the hal_graphics_allocator.

  OR

• Apps that need direct access to vendor HALs should have their own vendor-defined sepolicy domain.

File attribute tests

Android 9 includes build time tests that ensure all files in specific locations have the appropriate attributes (such as, all files in sysfs have the required sysfs_type attribute).

Platform-public policy

The platform-public policy is the core of conforming to the Android 8.0 architecture model without simply maintaining the union of platform policies from v1 and v2. Vendors are exposed to a subset of platform policy that contains useable types and attributes and rules on those types and attributes which then becomes part of vendor policy (that is, vendor_sepolicy.cil).

Types and rules are automatically translated in the vendor-generated policy into attribute_vN such that all platform-provided types are versioned attributes (however attributes aren't versioned). The platform is responsible for mapping the concrete types it provides into the appropriate attributes to ensure that vendor policy continues to function and that the rules provided for a particular version are included. The combination of platform-public policy and vendor policy satisfies the Android 8.0 architecture model goal of allowing independent platform and vendor builds.

Mapping to attribute chains

When using attributes to map to policy versions, a type maps to an attribute or multiple attributes, ensuring objects labeled with the type are accessible via attributes corresponding to their previous types.

Maintaining a goal to hide version information from the policy writer means automatically generating the versioned attributes and assigning them to the appropriate types. In the common case of static types, this is straightforward: type_foo maps to type_foo_v1.

For an object label change such as sysfs → sysfs_A or mediaserver → audioserver, creating this mapping is non-trivial (and is described in the examples above). Platform policy maintainers must determine how to create the mapping at transition points for objects, which requires understanding the relationship between objects and their assigned labels and determining when this occurs. For backwards compatibility, this complexity needs to be managed on the platform side, which is the only partition that may uprev.

Version uprevs

For simplicity, the Android platform releases an sepolicy version when a new release branch is cut. As described above, the version number is contained in PLATFORM_SEPOLICY_VERSION and is of the form MM.nn, where MM corresponds to the SDK value and nn is a private value maintained in /platform/system/sepolicy. For example, 19.0 for Kitkat, 21.0 for Lollipop, 22.0 for Lollipop-MR1 23.0 for Marshmallow, 24.0 for Nougat, 25.0 for Nougat-MR1, 26.0 for Oreo, 27.0 for Oreo-MR1, and 28.0 for Android 9. Uprevs aren't always whole numbers. For example, if an MR bump to a versions necessitates an incompatible change in system/sepolicy/public but not an API bump, then that sepolicy version could be: vN.1. The version present in a development branch is a never-to-be-used-in-shipping-devices 10000.0.

Android may deprecate oldest version when upreving. For input on when to deprecate a version, Android may collect the number of devices with vendor policies running that Android version and still receiving major platform updates. If the number is less than a certain threshold, that version is deprecated.

Performance impact of multiple attributes

As described in https://github.com/SELinuxProject/cil/issues/9, a large number of attributes assigned to a type result in performance issues in the event of a policy cache miss.

This was confirmed to be an issue in Android, so changes were made to Android 8.0 to remove attributes added to the policy by the policy compiler, as well as to remove unused attributes. These changes resolved performance regressions.

system_ext public and product public policy

Starting in Android 11, the system_ext and product partitions are allowed to export their designated public types to the vendor partition. Like platform public policy, the vendor uses types and rules automatically translated into the versioned attributes, for example, from type into type_N, where N is the version of the platform which the vendor partition is built against.

When the system_ext and product partitions are based on the same platform version N, the build system generates base mapping files to system_ext/etc/selinux/mapping/N.cil and product/etc/selinux/mapping/N.cil, which contain identity mappings from type to type_N. The vendor can access type with the versioned attribute type_N.

In case that only the system_ext and product partitions are updated, say N to N+1 (or later), while the vendor stays at N, the vendor may lose access to the types of the system_ext and product partitions. To prevent breakage, the system_ext and product partitions should provide mapping files from concrete types into type_N attributes. Each partner is responsible for maintaining the mapping files, if they are going to support N vendor with N+1 (or later) system_ext and product partitions.

To do that, partners are expected to:

1. Copy the generated base mapping files from N system_ext and product partitions to their source tree.
2. Amend the mapping files as needed.
3. Install the mapping files to N+1 (or later) system_ext and product partitions.

For example, suppose that N system_ext has one public type named foo_type. Then system_ext/etc/selinux/mapping/N.cil in the N system_ext partition will look like:

(typeattributeset foo_type_N (foo_type))
(expandtypeattribute foo_type_N true)
(typeattribute foo_type_N)

If bar_type is added to N+1 system_ext, and if bar_type should be mapped to foo_type for N vendor, N.cil can be updated from

(typeattributeset foo_type_N (foo_type))

to

(typeattributeset foo_type_N (foo_type bar_type))

and then installed to N+1 system_ext's partition. N vendor can continue accessing to N+1 system_ext's foo_type and bar_type.

SELinux contexts labeling

To support the distinction between platform and vendor sepolicy, the system builds SELinux context files differently to keep them separate.

File contexts

Android 8.0 introduced the following changes for file_contexts:

• To avoid additional compilation overhead on device during boot, file_contexts cease to exist in the binary form. Instead, they are readable, regular expression text file such as {property, service}_contexts (as they were pre-7.0).
• The file_contexts are split between two files:
  • plat_file_contexts
    • Android platform file_context that has no device-specific labels, except for labeling parts of /vendor partition that must be labeled precisely to ensure proper functioning of the sepolicy files.
    • Must reside in system partition at /system/etc/selinux/plat_file_contexts on device and be loaded by init at the start along with the vendor file_context.
  • vendor_file_contexts
    • Device-specific file_context built by combining file_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk files.
    • Must be installed at /vendor/etc/selinux/vendor_file_contexts in vendor partition and be loaded by init at the start along with the platform file_context.

Property contexts

In Android 8.0, the property_contexts is split between two files:

• plat_property_contexts
  • Android platform property_context that has no device-specific labels.
  • Must reside in system partition at /system/etc/selinux/plat_property_contexts and be loaded by init at the start along with the vendor property_contexts.
• vendor_property_contexts
  • Device-specific property_context built by combining property_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in device's Boardconfig.mk files.
  • Must reside in vendor partition at /vendor/etc/selinux/vendor_property_contexts and be loaded by init at the start along with the platform property_context

Service contexts

In Android 8.0, the service_contexts is split between the following files:

• plat_service_contexts
  • Android platform-specific service_context for the servicemanager. The service_context has no device-specific labels.
  • Must reside in system partition at /system/etc/selinux/plat_service_contexts and be loaded by servicemanager at the start along with the vendor service_contexts.
• vendor_service_contexts
  • Device-specific service_context built by combining service_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk files.
  • Must reside in vendor partition at /vendor/etc/selinux/vendor_service_contexts and be loaded by servicemanager at the start along with the platform service_contexts.
  • Although servicemanager looks for this file at boot time, for a fully compliant TREBLE device, the vendor_service_contexts MUST NOT exist. This is because all interaction between vendor and system processes MUST go through hwservicemanager/hwbinder.
• plat_hwservice_contexts
  • Android platform hwservice_context for hwservicemanager that has no device-specific labels.
  • Must reside in system partition at /system/etc/selinux/plat_hwservice_contexts and be loaded by hwservicemanager at the start along with the vendor_hwservice_contexts.
• vendor_hwservice_contexts
  • Device-specific hwservice_context built by combining hwservice_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk files.
  • Must reside in vendor partition at /vendor/etc/selinux/vendor_hwservice_contexts and be loaded by hwservicemanager at the start along with the plat_service_contexts.
• vndservice_contexts
  • Device-specific service_context for the vndservicemanager built by combining vndservice_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk.
  • This file must reside in vendor partition at /vendor/etc/selinux/vndservice_contexts and be loaded by vndservicemanager at the start.

Seapp contexts

In Android 8.0, the seapp_contexts is split between two files:

• plat_seapp_contexts
  • Android platform seapp_context that has no device-specific changes.
  • Must reside in system partition at /system/etc/selinux/plat_seapp_contexts.
• vendor_seapp_contexts
  • Device-specific extension to platform seapp_context built by combining seapp_contexts found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk files.
  • Must reside in vendor partition at /vendor/etc/selinux/vendor_seapp_contexts.

MAC permissions

In Android 8.0, the mac_permissions.xml is split between two files:

• Platform mac_permissions.xml
  • Android platform mac_permissions.xml that has no device-specific changes.
  • Must reside in system partition at /system/etc/selinux/.
• Non-Platform mac_permissions.xml
  • Device-specific extension to platform mac_permissions.xml built from mac_permissions.xml found in the directories pointed to by BOARD_SEPOLICY_DIRS in the device's Boardconfig.mk files.
  • Must reside in vendor partition at /vendor/etc/selinux/.