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<h2 id=introduction>Introduction</h2>

<p>The Android security model is based in part on the concept of application
sandboxes. Each application runs in its own sandbox. Prior to Android 4.3,
these sandboxes were defined by the creation of a unique Linux UID for each
application at time of installation. Starting with Android 4.3,
Security-Enhanced Linux (SELinux) is used to further define the boundaries of
the Android application sandbox.</p>

<p>As part of the Android <a href="/security/index.html">
security model</a>, Android uses SELinux to enforce mandatory access control
(MAC) over all processes, even processes running with root/superuser privileges
(a.k.a. Linux capabilities). SELinux enhances Android security by confining
privileged processes and automating security policy creation.</p>

<p>Contributions to it have been made by a number
of companies and organizations; all Android code
and contributors are publicly available for review on <a
href="https://android.googlesource.com/">android.googlesource.com</a>. With
SELinux, Android can better protect and confine system services, control
access to application data and system logs, reduce the effects of malicious
software, and protect users from potential flaws in code on mobile devices.</p>

<p>Android includes SELinux in enforcing mode and a
corresponding security policy that works by default across the <a
href="https://android.googlesource.com/">Android Open Source Project</a>. In
enforcing mode, illegitimate actions are prevented and all attempted violations
are logged by the kernel to <code>dmesg</code> and <code>logcat</code>. Android
device manufacturers should gather information about errors so they may
refine their software and SELinux policies before enforcing them.</p>

<h2 id=background>Background</h2>

<p>SELinux operates on the ethos of default denial. Anything that is not
explicitly allowed is denied. SELinux can operate in one of two global modes:
permissive mode, in which permission denials are logged but not enforced, and
enforcing mode, in which denials are both logged and enforced. SELinux also
supports a per-domain permissive mode in which specific domains (processes) can
be made permissive while placing the rest of the system in global enforcing
mode. A domain is simply a label identifying a process or set of processes in
the security policy, where all processes labeled with the same domain are
treated identically by the security policy. Per-domain permissive mode enables
incremental application of SELinux to an ever-increasing portion of the system.
Per-domain permissive mode also enables policy development for new services
while keeping the rest of the system enforcing.</p>

<p>In the Android 5.0 (L) release, Android moves to full enforcement of
SELinux. This builds upon the permissive release of 4.3 and the partial
enforcement of 4.4. In short, Android is shifting from enforcement on a
limited set of crucial domains (<code>installd</code>, <code>netd</code>,
<code>vold</code> and <code>zygote</code>) to everything (more than 60
domains). This means manufacturers will have to better understand and scale
their SELinux implementations to provide compatible devices. Understand
that:</p>


<ul>
<li>Everything is in enforcing mode in the 5.0 release</li>
<li> No processes other than <code>init</code> should run in the
<code>init</code> domain</li>
<li> Any generic denial (for a block_device, socket_device, default_service,
etc.) indicates that device needs a special domain</li>
</ul>

<h2 id=supporting_documentation>Supporting documentation</h2>

<p>See the documentation below for details on constructing useful policies:</p>

<p><a href="https://events.linuxfoundation.org/sites/events/files/slides/abs2014_seforandroid_smalley.pdf">
https://events.linuxfoundation.org/sites/events/files/slides/
abs2014_seforandroid_smalley.pdf</a></p>

<p><a href="https://www.internetsociety.org/sites/default/files/02_4.pdf">
https://www.internetsociety.org/sites/default/files/02_4.pdf</a></p>

<p><a href="http://freecomputerbooks.com/books/The_SELinux_Notebook-4th_Edition.pdf">
http://freecomputerbooks.com/books/The_SELinux_Notebook-4th_Edition.pdf</a></p>

<p><a href="http://selinuxproject.org/page/ObjectClassesPerms">
http://selinuxproject.org/page/ObjectClassesPerms</a></p>

<p><a href="https://www.nsa.gov/resources/everyone/digital-media-center/publications/research-papers/assets/files/implementing-selinux-as-linux-security-module-report.pdf">
https://www.nsa.gov/resources/everyone/digital-media-center/publications/
research-papers/assets/files/
implementing-selinux-as-linux-security-module-report.pdf</a></p>

<p><a href="https://www.nsa.gov/resources/everyone/digital-media-center/publications/research-papers/assets/files/configuring-selinux-policy-report.pdf">
https://www.nsa.gov/resources/everyone/digital-media-center/publications/
research-papers/assets/files/configuring-selinux-policy-report.pdf</a></p>

<p><a href="https://www.gnu.org/software/m4/manual/index.html">
https://www.gnu.org/software/m4/manual/index.html</a></p>

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