android10/system/sepolicy

This directory contains the core Android SELinux policy configuration.
It defines the domains and types for the AOSP services and apps common to
all devices. Device-specific policy should be placed under a
separate device///sepolicy subdirectory and linked
into the policy build as described below.

Policy Generation:

Additional, per device, policy files can be added into the
policy build. These files should have each line including the
final line terminated by a newline character (0x0A). This
will allow files to be concatenated and processed whenever
the m4(1) macro processor is called by the build process.
Adding the newline will also make the intermediate text files
easier to read when debugging build failures. The sets of file,
service and property contexts files will automatically have a
newline inserted between each file as these are common failure
points.

These device policy files can be configured through the use of
the BOARD_SEPOLICY_DIRS variable. This variable should be set
in the BoardConfig.mk file in the device or vendor directories.

BOARD_SEPOLICY_DIRS contains a list of directories to search
for additional policy files. Order matters in this list.
For example, if you have 2 instances of widget.te files in the
BOARD_SEPOLICY_DIRS search path, then the first one found (at the
first search dir containing the file) will be concatenated first.
Reviewing out/target/product//obj/ETC/sepolicy_intermediates/policy.conf
will help sort out ordering issues.

Example BoardConfig.mk Usage:
From the Tuna device BoardConfig.mk, device/samsung/tuna/BoardConfig.mk

BOARD_SEPOLICY_DIRS += device/samsung/tuna/sepolicy

Additionally, OEMs can specify BOARD_SEPOLICY_M4DEFS to pass arbitrary m4
definitions during the build. A definition consists of a string in the form
of macro-name=value. Spaces must NOT be present. This is useful for building modular
policies, policy generation, conditional file paths, etc. It is supported in
the following file types:
* All *.te and SE Linux policy files as passed to checkpolicy
* file_contexts
* service_contexts
* property_contexts
* keys.conf

Example BoardConfig.mk Usage:
BOARD_SEPOLICY_M4DEFS += btmodule=foomatic \
btdevice=/dev/gps

SPECIFIC POLICY FILE INFORMATION

mac_permissions.xml:
ABOUT:
The mac_permissions.xml file is used for controlling the mmac solutions
as well as mapping a public base16 signing key with an arbitrary seinfo
string. Details of the files contents can be found in a comment at the
top of that file. The seinfo string, previously mentioned, is the same string
that is referenced in seapp_contexts.

It is important to note the final processed version of this file
is stripped of comments and whitespace. This is to preserve space on the
system.img. If one wishes to view it in a more human friendly format,
the "tidy" or "xmllint" command will assist you.

TOOLING:
insertkeys.py
Is a helper script for mapping arbitrary tags in the signature stanzas of
mac_permissions.xml to public keys found in pem files. This script takes
a mac_permissions.xml file(s) and configuration file in order to operate.
Details of the configuration file (keys.conf) can be found in the subsection
keys.conf. This tool is also responsible for stripping the comments and
whitespace during processing.

keys.conf
The keys.conf file is used for controlling the mapping of "tags" found in
the mac_permissions.xml signature stanzas with actual public keys found in
pem files. The configuration file is processed via m4.

The script allows for mapping any string contained in TARGET_BUILD_VARIANT
with specific path to a pem file. Typically TARGET_BUILD_VARIANT is either
user, eng or userdebug. Additionally, one can specify "ALL" to map a path to
any string specified in TARGET_BUILD_VARIANT. All tags are matched verbatim
and all options are matched lowercase. The options are "tolowered" automatically
for the user, it is convention to specify tags and options in all uppercase
and tags start with @. The option arguments can also use environment variables
via the familiar $VARIABLE syntax. This is often useful for setting a location
to ones release keys.

Often times, one will need to integrate an application that was signed by a separate
organization and may need to extract the pem file for the insertkeys/keys.conf tools.
Extraction of the public key in the pem format is possible via openssl. First you need
to unzip the apk, once it is unzipped, cd into the META_INF directory and then execute
openssl pkcs7 -inform DER -in CERT.RSA -out CERT.pem -outform PEM -print_certs
On some occasions CERT.RSA has a different name, and you will need to adjust for that.
After extracting the pem, you can rename it, and configure keys.conf and
mac_permissions.xml to pick up the change. You MUST open the generated pem file in a text
editor and strip out anything outside the opening and closing scissor lines. Failure to do
so WILL cause a compile time issue thrown by insertkeys.py

NOTE: The pem files are base64 encoded and PackageManagerService, mac_permissions.xml
and setool all use base16 encodings.