src/security/selinux/validate.jd

page.title=Validating SELinux
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
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<p>Android strongly encourages OEMs to test their SELinux implementations
thoroughly. As manufacturers implement SELinux, they should apply the new
policy to a test pool of devices first.</p>

<p>Once applied, make sure SELinux is running in the correct mode on the device by
issuing the command:getenforce</p>

<p>This will print the global SELinux mode: either Enforcing or
Permissive. Please note, this command shows only the global SELinux mode. To
determine the SELinux mode for each domain, you must examine the corresponding
files or run the latest version of <code>sepolicy-analyze</code> with the appropriate (-p) flag, present in /platform/system/sepolicy/tools/.</p>

<h2 id=reading_denials>Reading denials</h2>

<p>Then check for errors. Errors are routed as event logs to dmesg and <code>logcat</code> and are viewable locally on the device. Manufacturers should examine the
SELinux output to dmesg on these devices and refine settings prior to public
release in permissive mode and eventual switch to enforcing mode. SELinux log
messages contain "avc:" and so may easily be found with <code>grep</code>. It is
possible to capture the ongoing denial logs by running <code>cat /proc/kmsg</code>
or to capture denial logs from the previous boot by running cat <code>/proc/last_kmsg</code>.</p>

<p>With this output, manufacturers can readily identify when system users or
components are in violation of SELinux policy. Manufacturers can then repair
this bad behavior, either by changes to the software, SELinux policy, or both.</p>

<p>Specifically, these log messages indicate what processes would fail under
enforcing mode and why. Here is an example:</p>

<pre>
avc: denied  { connectto } for  pid=2671 comm="ping" path="/dev/socket/dnsproxyd"
scontext=u:r:shell:s0 tcontext=u:r:netd:s0 tclass=unix_stream_socket
</pre>

<p>Interpret this output like so:</p>

<ul>
  <li> The <code>{ connectto }</code> above represents the action being taken. Together with the
<code>tclass</code> at the end (<code>unix_stream_socket</code>), it tells you roughly what was being done
to what. In this case, something was trying to connect to a unix stream socket.
  <li> The <code>scontext (u:r:shell:s0)</code> tells you what context initiated the action. In
this case this is something running as the shell.
  <li> The <code>tcontext (u:r:netd:s0)</code> tells you the context of the action’s target. In
this case, that’s a unix_stream_socket owned by <code>netd</code>.
  <li> The <code>comm="ping"</code> at the top gives you an additional hint about what was being
run at the time the denial was generated. In this case, it’s a pretty good hint.
</ul>

<p>And here is another example:</p>

<pre>
$ adb shell su root dmesg | grep 'avc: '
&lt;5> type=1400 audit: avc:  denied  { read write } for  pid=177
comm="rmt_storage" name="mem" dev="tmpfs" ino=6004 scontext=u:r:rmt:s0
tcontext=u:object_r:kmem_device:s0 tclass=chr_file
</pre>


<p>Here are the key elements from this denial:</p>

<ul>
  <li><em>Action</em> - the attempted action is highlighted in brackets, <code>read write</code> or <code>setenforce</code>. 
  <li><em>Actor</em> - The <code>scontext</code> (source context) entry represents the actor, in this case the<code> rmt_storage</code> daemon.
  <li><em>Object</em> - The <code>tcontext</code> (target context) entry represents the object being acted upon, in this case
kmem.
  <li><em>Result</em> - The <code>tclass</code> (target class) entry indicates the type of object being acted upon, in this
case a <code>chr_file</code> (character device).
</ul>

<h2 id=switching_to_permissive>Switching to permissive</h2>

<p class="caution"><strong>Important:</strong> Permissive mode is not supported on production devices. CTS tests confirm
enforcing mode is enabled.</p>

<p>To turn a device’s SELinux enforcement into globally permissive via ADB, as
root issue:</p>

<pre>
$ adb shell su root setenforce 0
</pre>

<p>Or at the kernel command line (during early device bring-up):</p>

<pre>
androidboot.selinux=permissive
androidboot.selinux=enforcing
</pre>

<h2 id=using_audit2allow>Using audit2allow</h2>

<p>The <code>selinux/policycoreutils/audit2allow</code> tool takes <code>dmesg</code> denials and converts them into corresponding SELinux policy statements. As
such, it can greatly speed SELinux development. <code>audit2allow</code> is shipped as part of the Android source tree and
is compiled automatically when you build Android from source.</p>

<p>To use it, run:</p>

<pre>
$ adb shell su root dmesg | audit2allow -p $OUT/root/sepolicy
</pre>

<p>Nevertheless, care must be taken to examine each potential addition for
overreaching permissions. For example, feeding audit2allow the <code>rmt_storage</code> denial shown earlier results in the following suggested SELinux policy
statement:</p>

<pre>
#============= shell ==============
allow shell kernel:security setenforce;
#============= rmt ==============
allow rmt kmem_device:chr_file { read write };
</pre>


<p>This would grant <code>rmt</code> the ability to write kernel memory, a glaring security hole. Often the <code>audit2allow</code> statements are only a starting point, after which changes to the source
domain, the label of the target and the incorporation of proper macros may be
required to arrive at a good policy. Sometimes the denial being examined should
not result in any policy changes at all, but rather the offending application
should be changed.</p>