1 page.title=Media Framework Hardening 2 @jd:body 3 4 <!-- 5 Copyright 2016 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 <p>To improve device security, Android 7.0 breaks up the monolithic 28 <code>mediaserver</code> process into multiple processes with permissions and 29 capabilities restricted to only those required by each process. These changes 30 mitigate media framework security vulnerabilities by:</p> 31 <ul> 32 <li>Splitting AV pipeline components into app-specific sandboxed processes.</li> 33 <li>Enabling updatable media components (extractors, codecs, etc.).</li> 34 </ul> 35 36 <p>These changes also improve security for end users by significantly reducing 37 the severity of most media-related security vulnerabilities, keeping end user 38 devices and data safe.</p> 39 40 <p>OEMs and SoC vendors need to update their HAL and framework changes to make 41 them compatible with the new architecture. Specifically, because vendor-provided 42 Android code often assumes everything runs in the same process, vendors must 43 update their code to pass around native handles (<code>native_handle</code>) 44 that have meaning across processes. For a reference implementation of changes 45 related to media hardening, refer to <code>frameworks/av</code> and 46 <code>frameworks/native</code>.</p> 47 48 <h2 id=arch_changes>Architectural changes</h2> 49 <p>Previous versions of Android used a single, monolithic 50 <code>mediaserver</code> process with great many permissions (camera access, 51 audio access, video driver access, file access, network access, etc.). Android 52 7.0 splits the <code>mediaserver</code> process into several new processes that 53 each require a much smaller set of permissions:</p> 54 55 <p><img src="images/ape_media_split.png" alt="mediaserver hardening"></p> 56 <p class="img-caption"><strong>Figure 1.</strong> Architecture changes for 57 mediaserver hardening</p> 58 59 <p>This new architecture ensures that even if a process is compromised, 60 malicious code does not have access to the full set of permissions previously 61 held by mediaserver. Processes are restricted by SElinux and seccomp policies. 62 </p> 63 64 <p class=note><strong>Note:</strong> Because of vendor dependencies, some codecs 65 still run in the <code>mediaserver</code> and consequently grant 66 <code>mediaserver</code> more permissions than necessary. Specifically, Widevine 67 Classic continues to run in the <code>mediaserver</code> for Android 7.0.</p> 68 69 <h3 id=mediaserver-changes>MediaServer changes</h3> 70 <p>In Android 7.0, the <code>mediaserver</code> process exists for driving 71 playback and recording, e.g. passing and synchronizing buffers between 72 components and processes. Processes communicate through the standard Binder 73 mechanism.</p> 74 <p>In a standard local file playback session, the application passes a file 75 descriptor (FD) to <code>mediaserver</code> (usually via the MediaPlayer Java 76 API), and the <code>mediaserver</code>:</p> 77 <ol> 78 <li>Wraps the FD into a Binder DataSource object that is passed to the extractor 79 process, which uses it to read from the file using Binder IPC. (The 80 mediaextractor doesn't get the FD but instead makes Binder calls back to the 81 <code>mediaserver</code> to get the data.)</li> 82 <li>Examines the file, creates the appropriate extractor for the file type 83 (e.g. MP3Extractor, or MPEG4Extractor), and returns a Binder interface for the 84 extractor to the <code>mediaserver</code> process.</li> 85 <li>Makes Binder IPC calls to the extractor to determine the type of data in the 86 file (e.g. MP3 or H.264 data).</li> 87 <li>Calls into the <code>mediacodec</code> process to create codecs of the 88 required type; receives Binder interfaces for these codecs.</li> 89 <li>Makes repeated Binder IPC calls to the extractor to read encoded samples, 90 uses the Binder IPC to send encoded data to the <code>mediacodec</code> process 91 for decoding, and receives decoded data.</li> 92 </ol> 93 <p>In some use cases, no codec is involved (such as an offloaded playback where 94 encoded data is sent directly to the output device), or the codec may render the 95 decoded data directly instead of returning a buffer of decoded data (video 96 playback).</p> 97 98 <h3 id=mediacodecservice_changes>MediaCodecService changes</h3> 99 <p>The codec service is where encoders and decoders live. Due to vendor 100 dependencies, not all codecs live in the codec process yet. In Android 7.0:</p> 101 <ul> 102 <li>Non-secure decoders and software encoders live in the codec process.</li> 103 <li>Secure decoders and hardware encoders live in the <code>mediaserver</code> 104 (unchanged).</li> 105 </ul> 106 107 <p>An application (or mediaserver) calls the codec process to create a codec of 108 the required type, then calls that codec to pass in encoded data and retrieve 109 decoded data (for decoding) or to pass in decoded data and retrieve encoded data 110 (for encoding). Data transfer to and from codecs uses shared memory already, so 111 that process is unchanged.</p> 112 113 <h3 id=mediadrmserver_changes>MediaDrmServer changes</h3> 114 <p>The DRM server is used when playing DRM-protected content, such as movies in 115 Google Play Movies. It handles decrypting the encrypted data in a secure way, 116 and as such has access to certificate and key storage and other sensitive 117 components. Due to vendor dependencies, the DRM process is not used in all cases 118 yet.</p> 119 120 <h3 id=audioserver_changes>AudioServer changes</h3> 121 <p>The AudioServer process hosts audio related components such as audio input 122 and output, the policymanager service that determines audio routing, and FM 123 radio service. For details on Audio changes and implementation guidance, see 124 <a href="{@docRoot}devices/audio/implement.html">Implementing Audio</a>.</p> 125 126 <h3 id=cameraserver_changes>CameraServer changes</h3> 127 <p>The CameraServer controls the camera and is used when recording video to 128 obtain video frames from the camera and then pass them to 129 <code>mediaserver</code> for further handling. For details on changes and 130 implementation guidance for CameraServer changes, refer to 131 <a href="{@docRoot}devices/camera/versioning.html#hardening">Camera Framework 132 Hardening</a>.</p> 133 134 <h3 id=extractor_service_changes>ExtractorService changes</h3> 135 <p>The extractor service hosts the <em>extractors</em>, components that parse 136 the various file formats supported by the media framework. The extractor service 137 is the least privileged of all the services—it can't read FDs so instead 138 it makes calls onto a Binder interface (provided to it by the 139 <code>mediaserver for</code> each playback session) to access files.</p> 140 <p>An application (or <code>mediaserver</code>) makes a call to the extractor 141 process to obtain an <code>IMediaExtractor</code>, calls that 142 <code>IMediaExtractor</code> to obtain<code> IMediaSources</code> for the track 143 contained in the file, and then calls <code>IMediaSources</code> to read data 144 from them.</p> 145 <p>To transfer the data between processes, the application (or 146 <code>mediaserver</code>) includes the data in the reply-Parcel as part of the 147 Binder transaction or uses shared memory:</p> 148 149 <ul> 150 <li>Using <strong>shared memory</strong> requires an extra Binder call to 151 release the shared memory but is faster and uses less power for large buffers. 152 </li> 153 <li>Using <strong>in-Parcel</strong> requires extra copying but is faster and 154 uses less power for buffers smaller than 64KB.</li> 155 </ul> 156 157 <h2 id=implementation>Implementation</h2> 158 <p>To support the move of <code>MediaDrm</code> and <code>MediaCrypto</code> 159 components into the new <code>mediadrmserver</code> process, vendors must change 160 the allocation method for secure buffers to allow buffers to be shared between 161 processes.</p> 162 <p>In previous Android releases, secure buffers are allocated in 163 <code>mediaserver</code> by <code>OMX::allocateBuffer</code> and used during 164 decryption in the same process, as shown below:</p> 165 166 <p><img src="images/ape_media_buffer_alloc_pren.png"></p> 167 <p class="img-caption"><strong>Figure 2.</strong> Android 6.0 and lower buffer 168 allocation in mediaserver.</p> 169 170 <p>In Android 7.0, the buffer allocation process has changed to a new mechanism 171 that provides flexibility while minimizing the impact on existing 172 implementations. With <code>MediaDrm</code> and <code>MediaCrypto</code> stacks 173 in the new <code>mediadrmserver</code> process, buffers are allocated 174 differently and vendors must update the secure buffer handles so they can be 175 transported across binder when <code>MediaCodec</code> invokes a decrypt 176 operation on <code>MediaCrypto</code>.</p> 177 178 <p><img src="images/ape_media_buffer_alloc_n.png"></p> 179 <p class="img-caption"><strong>Figure 3.</strong> Android 7.0 and higher buffer 180 allocation in mediaserver.</p> 181 182 <h3 id=native_handles>Using native handles</h3> 183 <p>The <code>OMX::allocateBuffer</code> must return a pointer to a 184 <code>native_handle</code> struct, which contains file descriptors (FDs) and 185 additional integer data. A <code>native_handle</code> has all of the advantages 186 of using FDs, including existing binder support for 187 serialization/deserialization, while allowing more flexibility for vendors who 188 don't currently use FDs.</p> 189 <p>Use <code>native_handle_create()</code> to allocate the native handle. 190 Framework code takes ownership of the allocated <code>native_handle</code> 191 struct and is responsible for releasing resources in both the process where 192 the <code>native_handle</code> is originally allocated and in the process where 193 it is deserialized. The framework releases native handles with 194 <code>native_handle_close()</code> followed by 195 <code>native_handle_delete()</code> and serializes/deserializes the 196 <code>native_handle</code> using 197 <code>Parcel::writeNativeHandle()/readNativeHandle()</code>. 198 </p> 199 <p>SoC vendors who use FDs to represent secure buffers can populate the FD in 200 the <code>native_handle</code> with their FD. Vendors who don't use FDs can 201 represent secure buffers using additional fields in the 202 <code>native_buffer</code>.</p> 203 204 <h3 id=decrypt_location>Setting decryption location</h3> 205 <p>Vendors must update the OEMCrypto decrypt method that operates on the 206 <code>native_handle</code> to perform any vendor-specific operations necessary 207 to make the <code>native_handle</code> usable in the new process space (changes 208 typically include updates to OEMCrypto libraries).</p> 209 <p>As <code>allocateBuffer</code> is a standard OMX operation, Android 7.0 210 includes a new OMX extension 211 (<code>OMX.google.android.index.allocateNativeHandle</code>) to query for this 212 support and an <code>OMX_SetParameter</code> call that notifies the OMX 213 implementation it should use native handles.</p> 214
