page.title=Verified Boot
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    <h2>In this document</h2>
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<h2 id="introduction">Introduction</h2>

<p>Android 4.4 and later supports verified boot through the optional device-mapper-verity 
(dm-verity) kernel feature, which provides transparent integrity checking of 
block devices. dm-verity helps prevent persistent rootkits that can hold onto 
root privileges and compromise devices. This experimental feature helps Android 
users be sure when booting a device it is in the same state as when it was last 
used.</p>

<p>Clever malware with root privileges can hide from detection programs and 
otherwise mask themselves. The rooting software can do this because it is often 
more privileged than the detectors, enabling the software to "lie" to to the 
detection programs.</p>

<p>The dm-verity feature lets you look at a block device, the underlying storage 
layer of the file system, and determine if it matches its expected 
configuration. It does this using a cryptographic hash tree. For every block 
(typically 4k), there is a SHA256 hash.</p>

<p>And since the hash values are stored in a tree of pages, only the top-level 
"root" hash must be trusted to verify the rest of the tree. The ability to 
modify any of the blocks would be equivalent to breaking the cryptographic hash. 
See the following diagram for a depiction of this structure.</p>

<img src="../images/dm-verity-hash-table.png" alt="dm-verity-hash-table" id="figure1"/>
<p class="img-caption">
  <strong>Figure 1.</strong> dm-verity hash table
</p>

<p>
A public key is included on the boot partition, which must be verified 
externally by the OEM. That key is used to verify the signature for that hash 
and confirm the device's system partition is protected and unchanged.</p>

<h2 id="prerequisites">Prerequisites</h2>

<h3 id="verified-boot">Establishing a verified boot flow</h3>
<p>To greatly reduce the risk of compromise, verify the kernel using a key
burned into the device. For details, see <a href="verified-boot.html">Verified boot</a>.</p>

<h3 id="block-otas">Switching to block-oriented OTAs</h3>
<p>To enable dm-verity for a device, you must use block-based over-the-air
(OTA) updates to ensure all devices use the same system partition. For details,
see <a href="{@docRoot}devices/tech/ota/block.html">Block-Based OTAs</a>.</p>

<h3 id="config-dm-verity">Configuring dm-verity</h3>

<p>After switching to block-oriented OTAs, incorporate the latest Android kernel or
use a stock upstream kernel and enable dm-verity support by including the
relevant configuration option:<br/>
<code>CONFIG_DM_VERITY
</code></p>
<p>When using the Android kernel, dm-verity is turned on when the kernel is built. For details, see <a href="dm-verity.html">Implementing dm-verity</a>.</p>

<h2 id="supporting-docs">Supporting documentation</h2>
<p><a href="verified-boot.html">Verifying Boot</a><br/>
<a href="{@docRoot}devices/tech/ota/block.html">Block-Based OTA</a><br/>
<a href="dm-verity.html">Implementing dm-verity</a><br/>
<a href="https://code.google.com/p/cryptsetup/wiki/DMVerity">cryptsetup - dm-verity: device-mapper block integrity checking 
target</a><br/>
<a href="http://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot">The Chromium Projects - Verified 
Boot</a><br/>
<a
href="http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/device-mapper/verity.txt">Linux Kernel Documentation: 
verity.txt</a></p>

<p>For additional assistance, contact <a href="mailto:security@android.com">security@android.com</a>.</p>