page.title=Verified Boot
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    <h2>In this document</h2>
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<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
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 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>Because 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">Verifying
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 <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://gitlab.com/cryptsetup/cryptsetup/wikis/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>