1page.title=Verified Boot 2@jd:body 3 4<!-- 5 Copyright 2015 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18--> 19<div id="qv-wrapper"> 20 <div id="qv"> 21 <h2>In this document</h2> 22 <ol id="auto-toc"> 23 </ol> 24 </div> 25</div> 26 27<h2 id="introduction">Introduction</h2> 28 29<p>Android 4.4 and later supports verified boot through the optional 30device-mapper-verity (dm-verity) kernel feature, which provides transparent 31integrity checking of block devices. dm-verity helps prevent persistent rootkits 32that can hold onto root privileges and compromise devices. This experimental 33feature helps Android users be sure when booting a device it is in the same 34state as when it was last used.</p> 35 36<p>Clever malware with root privileges can hide from detection programs and 37otherwise mask themselves. The rooting software can do this because it is often 38more privileged than the detectors, enabling the software to "lie" to the 39detection programs.</p> 40 41<p>The dm-verity feature lets you look at a block device, the underlying storage 42layer of the file system, and determine if it matches its expected 43configuration. It does this using a cryptographic hash tree. For every block 44(typically 4k), there is a SHA256 hash.</p> 45 46<p>Since the hash values are stored in a tree of pages, only the top-level 47"root" hash must be trusted to verify the rest of the tree. The ability to 48modify any of the blocks would be equivalent to breaking the cryptographic hash. 49See the following diagram for a depiction of this structure.</p> 50 51<img src="../images/dm-verity-hash-table.png" alt="dm-verity-hash-table" id="figure1"/> 52<p class="img-caption"> 53 <strong>Figure 1.</strong> dm-verity hash table 54</p> 55 56<p>A public key is included on the boot partition, which must be verified 57externally by the OEM. That key is used to verify the signature for that hash 58and confirm the device's system partition is protected and unchanged.</p> 59 60<h2 id="prerequisites">Prerequisites</h2> 61 62<h3 id="verified-boot">Establishing a verified boot flow</h3> 63<p>To greatly reduce the risk of compromise, verify the kernel using a key 64burned into the device. For details, see <a href="verified-boot.html">Verified 65boot</a>.</p> 66 67<h3 id="block-otas">Switching to block-oriented OTAs</h3> 68<p>To enable dm-verity for a device, you must use block-based over-the-air 69(OTA) updates to ensure all devices use the same system partition. For details, 70see <a href="{@docRoot}devices/tech/ota/block.html">Block-Based OTAs</a>.</p> 71 72<h3 id="config-dm-verity">Configuring dm-verity</h3> 73 74<p>After switching to block-oriented OTAs, incorporate the latest Android kernel 75or use a stock upstream kernel and enable dm-verity support by including the 76relevant configuration option <code>CONFIG_DM_VERITY</code>.</p> 77 78<p>When using the Android kernel, dm-verity is turned on when the kernel is 79built. For details, see <a href="dm-verity.html">Implementing dm-verity</a>.</p> 80 81<h2 id="supporting-docs">Supporting documentation</h2> 82<p><a href="verified-boot.html">Verifying Boot</a><br/> 83<a href="{@docRoot}devices/tech/ota/block.html">Block-Based OTA</a><br/> 84<a href="dm-verity.html">Implementing dm-verity</a><br/> 85<a href="https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity">cryptsetup - 86dm-verity: device-mapper block integrity checking target</a><br/> 87<a href="http://www.chromium.org/chromium-os/chromiumos-design-docs/verified-boot">The 88Chromium Projects - Verified Boot</a><br/> 89<a 90href="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> 91
