security/selinux/implement.html

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<p>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 <a
href="/security/selinux#supporting_documentation">Supporting
documentation</a> for suggested resources.</p>

<h2 id=summary_of_steps>Summary of steps</h2>

<p>Here is a brief summary of the steps needed to implement SELinux on your
Android device:</p>

<ol>
  <li>Add SELinux support in the kernel and configuration.
  <li>Grant each service (process or daemon) started from <code>init</code> its own domain.
  <li>Identify these services by:
  <ul>
    <li>Reviewing the init.&lt;device&gt;.rc file and finding all services.
    <li>Examining warnings of the form <em>init:  Warning!  Service name needs a SELinux domain defined; please fix!</em> in <code>dmesg</code> output.
    <li>Checking <code>ps -Z | grep init</code> output to see which services are running in the init domain.
  </ul>
  <li>Label all new processes, drivers, sockets, etc.
All objects need to be labeled
properly to ensure they interact properly with the policies you apply. See the
labels used in AOSP for examples to follow in label name creation.
  <li>Institute security policies that fully cover all labels and restrict
permissions to their absolute minimum.
</ol>

<p>Ideally, OEMs start with the policies in the AOSP and then build upon them for
their own customizations.</p>

<h2 id=key_files>Key files</h2>

<p>SELinux for Android is accompanied by everything you need to enable SELinux
now. You merely need to integrate the <a href="https://android.googlesource.com/kernel/common/">latest Android kernel</a> and then incorporate the files found in the <a href="https://android.googlesource.com/platform/system/sepolicy/">system/sepolicy</a> directory:</p>

<p><a href="https://android.googlesource.com/kernel/common/">https://android.googlesource.com/kernel/common/ </a></p>

<p><a href="https://android.googlesource.com/platform/system/sepolicy/">https://android.googlesource.com/platform/system/sepolicy/</a></p>

<p>Those files when compiled comprise the SELinux kernel security policy and cover
the upstream Android operating system. You should not need to modify the
system/sepolicy files directly. Instead, add your own device-specific policy
files within the /device/manufacturer/device-name/sepolicy directory.</p>

<p>Here are the files you must create or edit in order to implement SELinux:</p>

<ul>
  <li><em>New SELinux policy source (*.te) files</em> - Located in the
<root>/device/manufacturer/device-name/sepolicy directory. These files define
domains and their labels. The new policy files get
concatenated with the existing policy files during compilation into a single
SELinux kernel policy file.
<p class="caution"><strong>Important:</strong> Do not alter the app.te file
provided by the Android Open Source Project.
Doing so risks breaking all third-party applications.</p>
  <li><em>Updated BoardConfig.mk makefile</em> - Located in the <device-name>
directory containing the sepolicy subdirectory. It must be updated to reference
the sepolicy subdirectory once created if it
wasnt in initial implementation.
  <li><em>file_contexts</em> - Located in the sepolicy subdirectory. This file
assigns 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. In order to apply new file_contexts, you must
rebuild the filesystem image or run <code>restorecon</code> on the file to be
relabeled.  On upgrades, changes to file_contexts are 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_recursive calls to your init.<em>board</em>.rc file after the
partition has been mounted read-write.
  <li><em>genfs_contexts</em> - Located in the sepolicy subdirectory. This file
assigns labels to filesystems such as proc or vfat that 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 vfat using the context=
mount option.
  <li><em>property_contexts</em> - Located in the sepolicy subdirectory. This
file assigns labels to Android system properties to control what processes can
set them.  This configuration is read by the init process during startup.
  <li><em>service_contexts</em> - Located in the sepolicy subdirectory. This
file assigns 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 servicemanager process during startup.
  <li><em>seapp_contexts</em> - Located in the sepolicy subdirectory. This file
assigns labels to app processes and /data/data directories.  This configuration
is read by the zygote process on each app launch and by installd during
startup.
  <li><em>mac_permissions.xml</em> - Located in the sepolicy subdirectory. This
file assigns a seinfo tag to apps based on their signature and optionally their
package name.  The seinfo tag can then be used as a key in the seapp_contexts
file to assign a specific label to all apps with that seinfo tag.  This
configuration is read by system_server during startup.
</ul>

<p>Then just update your BoardConfig.mk makefile - located in the directory
containing the sepolicy subdirectory - to reference the sepolicy subdirectory
and each policy file once created, as shown below. The BOARD_SEPOLICY variables
and their meaning is documented in the system/sepolicy/README file.</p>

<pre class="devsite-click-to-copy">
BOARD_SEPOLICY_DIRS += \
        &lt;root>/device/manufacturer/device-name/sepolicy

BOARD_SEPOLICY_UNION += \
        genfs_contexts \
        file_contexts \
        sepolicy.te
</pre>

<p class="note"><strong>Note:</strong> As of the M release,
BOARD_SEPOLICY_UNION is no longer required as all policy files found within any
directory included in the BOARD_SEPOLICY_DIRS variable are joined with the
base policy automatically.</p>

<p>After rebuilding your device, it 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 <a
href="customize.html">Customization</a> or verify your existing setup as
covered in <a href="validate.html">Validation</a>.</p>

<p>Once the new policy files and BoardConfig.mk updates are in place, the new
policy settings are automatically built into the final kernel policy file.</p>

<h2 id=use_cases>Use cases</h2>

<p>Here are specific examples of exploits to consider when crafting your own
software and associated SELinux policies:</p>

<p><strong>Symlinks</strong> - Because symlinks appear as files, they are often read just as that. This can
lead to exploits. For instance, some privileged components such as init change
the permissions of certain files, sometimes to be excessively open.</p>

<p>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.</p>

<p><strong>System files</strong> - Consider the class of system files that should only be modified by the
system server. Still, since netd, init, and 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.</p>

<p>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.
Even if netd became compromised, it could not switch domains to the system
server domain and access those system files although it runs as root.</p>

<p><strong>App data</strong> - 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.</p>

<p><strong>setattr</strong> - For commands such as chmod and 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
chmod and chown against those labeled app_data_files but not shell_data_files
or system_data_files.</p>

<h2 id=steps_in_detail>Steps in detail</h2>

<p>Here is a detailed view of how Android recommends you employ and customize
SELinux to protect your devices:</p>

<ol>
  <li>Enable SELinux in the kernel:
<code>CONFIG_SECURITY_SELINUX=y</code>
  <li>Change the kernel_cmdline parameter so that:<br/>
<pre class="devsite-click-to-copy">
BOARD_KERNEL_CMDLINE := androidboot.selinux=permissive
</pre>
<br/>
This is only for initial development of policy for the device.  Once you have
an initial bootstrap policy, remove this parameter so that your device is
enforcing or it will fail CTS.
  <li>Boot up the system in permissive and see what denials are encountered on boot:<br/>
On Ubuntu 14.04 or newer:
<br/>
<pre class="devsite-terminal devsite-click-to-copy">
adb shell su -c dmesg | grep denied | audit2allow -p out/target/product/<var>BOARD</var>/root/sepolicy
</pre>
<br/>
On Ubuntu 12.04:<br/>
<pre class="devsite-terminal devsite-click-to-copy">
adb shell su -c dmesg | grep denied | audit2allow
</pre>
  <li>Evaluate the output. See <a href="validate.html">Validation</a> for instructions and tools.
  <li>Identify devices, and other new files that need labeling.
  <li>Use existing or new labels for your objects.
Look at the *_contexts files 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, as well.
  <li>Identify domains/processes that should have their own security domains. A policy will likely need to be written for each of these from scratch. All services spawned from <code>init</code>, for instance, should have their own. The following commands help reveal those that remain running (but ALL services need such a treatment):<br/>
<pre class="devsite-terminal devsite-click-to-copy">
adb shell su -c ps -Z | grep init
</pre>
<pre class="devsite-terminal devsite-click-to-copy">
adb shell su -c dmesg | grep 'avc: '
</pre>
  <li>Review init.&lt;device&gt;.rc to identify any which are without a type.
These should
be given domains EARLY in order to avoid adding rules to init or otherwise
confusing <code>init</code> accesses with ones that are in their own policy.
  <li>Set up <code>BOARD_CONFIG.mk</code> to use <code>BOARD_SEPOLICY_*</code> variables. See
the README in system/sepolicy for details on setting this up.
  <li> Examine the init.&lt;device&gt;.rc and fstab.&lt;device&gt; file and make sure every use of mount
corresponds to a properly labeled filesystem or that a context= mount option is specified.
  <li> Go through each denial and create SELinux policy to properly handle each. See
the examples within <a href="customize.html">Customization</a>.
</ol>

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