1 page.title=Implementing SELinux 2 @jd:body 3 4 <!-- 5 Copyright 2014 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>SELinux is set up to default-deny, which means that every single access for 28 which it has a hook in the kernel must be explicitly allowed by policy. This 29 means a policy file is comprised of a large amount of information regarding 30 rules, types, classes, permissions, and more. A full consideration of SELinux 31 is out of the scope of this document, but an understanding of how to write 32 policy rules is now essential when bringing up new Android devices. There is a 33 great deal of information available regarding SELinux already. See <a 34 href="{@docRoot}devices/tech/security/se-linux.html#supporting_documentation">Supporting 35 documentation</a> for suggested resources.</p> 36 37 <h2 id=summary_of_steps>Summary of steps</h2> 38 39 <p>Here is a brief summary of the steps needed to implement SELinux on your 40 Android device:</p> 41 42 <ol> 43 <li>Add SELinux support in the kernel and configuration. 44 <li>Grant each service (process or daemon) started from <code>init</code> its own domain. 45 <li>Identify these services by: 46 <ul> 47 <li>Reviewing the init file and finding all services. 48 <li>Examining warnings in <code>dmesg</code>. 49 <li>Searching (<code>grep</code>) through processes to see which run in the init domain. 50 </ul> 51 <li>Label all new processes, drivers, sockets, etc. 52 All objects need to be labeled 53 properly to ensure they interact properly with the policies you apply. See the 54 labels used in AOSP for examples to follow in label name creation. 55 <li>Institute security policies that fully cover all labels and restrict 56 permissions to their absolute minimum. 57 </ol> 58 59 <p>Ideally, OEMs start with the policies in the AOSP and then build upon them for 60 their own customizations.</p> 61 62 <h2 id=key_files>Key files</h2> 63 64 <p>SELinux for Android is accompanied by everything you need to enable SELinux 65 now. You merely need to integrate the <a href="https://android.googlesource.com/kernel/common/">latest Android kernel</a> and then incorporate the files found in the <a href="https://android.googlesource.com/platform/external/sepolicy/">external/sepolicy</a> directory:</p> 66 67 <p><a href="https://android.googlesource.com/kernel/common/">https://android.googlesource.com/kernel/common/ </a></p> 68 69 <p><a href="https://android.googlesource.com/platform/external/sepolicy/">https://android.googlesource.com/platform/external/sepolicy/</a></p> 70 71 <p>Those files when compiled comprise the SELinux kernel security policy and cover 72 the upstream Android operating system. You should not need to modify the 73 external/sepolicy files directly. Instead, add your own device-specific policy 74 files within the /device/manufacturer/device-name/sepolicy directory.</p> 75 76 <p>Here are the files you must create or edit in order to implement SELinux:</p> 77 78 <ul> 79 <li><em>New SELinux policy source (*.te) files</em> - Located in the <root>/device/manufacturer/device-name/sepolicy directory. These files define domains and their labels. The new policy files get 80 concatenated with the existing policy files during compilation into a single 81 SELinux kernel policy file. 82 <p class="caution"><strong>Important:</strong> Do not alter the app.te file provided by the Android Open Source Project. 83 Doing so risks breaking all third-party applications.</p> 84 <li><em>Updated BoardConfig.mk makefile</em> - Located in the <device-name> directory containing the sepolicy subdirectory. It must be updated to reference the sepolicy subdirectory once created if it 85 wasnt in initial implementation. 86 <li><em>Updated *_contexts files</em> - Located in the sepolicy subdirectory. These label files and 87 are managed in the userspace. As you create new policies, update these files to 88 reference them. In order to apply new *_contexts, you must run <code>restorecon</code> on the file to be relabeled. 89 </ul> 90 91 <p>Then just update your BoardConfig.mk makefile - located in the directory 92 containing the sepolicy subdirectory - to reference the sepolicy subdirectory 93 and each policy file once created, as shown below. The BOARD_SEPOLICY variables 94 and their meaning is documented in the external/sepolicy/README file.</p> 95 96 <pre> 97 BOARD_SEPOLICY_DIRS += \ 98 <root>/device/manufacturer/device-name/sepolicy 99 100 BOARD_SEPOLICY_UNION += \ 101 genfs_contexts \ 102 file_contexts \ 103 sepolicy.te 104 </pre> 105 106 <p>After rebuilding your device, it is enabled with SELinux. You can now either 107 customize your SELinux policies to accommodate your own additions to the 108 Android operating system as described in <a 109 href="customize.html">Customization</a> or verify your existing setup as 110 covered in <a href="validate.html">Validation</a>.</p> 111 112 <p>Once the new policy files and BoardConfig.mk updates are in place, the new 113 policy settings are automatically built into the final kernel policy file.</p> 114 115 <h2 id=use_cases>Use cases</h2> 116 117 <p>Here are specific examples of exploits to consider when crafting your own 118 software and associated SELinux policies:</p> 119 120 <p><strong>Symlinks</strong> - Because symlinks appear as files, they are often read just as that. This can 121 lead to exploits. For instance, some privileged components such as init change 122 the permissions of certain files, sometimes to be excessively open.</p> 123 124 <p>Attackers might then replace those files with symlinks to code they control, 125 allowing the attacker to overwrite arbitrary files. But if you know your 126 application will never traverse a symlink, you can prohibit it from doing so 127 with SELinux.</p> 128 129 <p><strong>System files</strong> - Consider the class of system files that should only be modified by the 130 system server. Still, since netd, init, and vold run as root, they can access 131 those system files. So if netd became compromised, it could compromise those 132 files and potentially the system server itself.</p> 133 134 <p>With SELinux, you can identify those files as system server data files. 135 Therefore, the only domain that has read/write access to them is system server. 136 Even if netd became compromised, it could not switch domains to the system 137 server domain and access those system files although it runs as root.</p> 138 139 <p><strong>App data</strong> - Another example is the class of functions that must run as root but should 140 not get to access app data. This is incredibly useful as wide-ranging 141 assertions can be made, such as certain domains unrelated to application data 142 being prohibited from accessing the internet.</p> 143 144 <p><strong>setattr</strong> - For commands such as chmod and chown, you could identify the set of files 145 where the associated domain can conduct setattr. Anything outside of that could 146 be prohibited from these changes, even by root. So an application might run 147 chmod and chown against those labeled app_data_files but not shell_data_files 148 or system_data_files.</p> 149 150 <h2 id=steps_in_detail>Steps in detail</h2> 151 152 <p>Here is a detailed view of how Android recommends you employ and customize 153 SELinux to protect your devices:</p> 154 155 <ol> 156 <li>Enable SELinux in the kernel: 157 <code>CONFIG_SECURITY_SELINUX=y</code> 158 <li>Change the kernel_cmdline parameter so that:<br/> 159 <code>BOARD_KERNEL_CMDLINE := androidboot.selinux=permissive</code> 160 <li>Boot up the system in permissive and see what denials are encountered on boot:<br/> 161 <code>su -c dmesg | grep denied > ~/t.tmp su -c dmesg | grep denied | audit2allow</code> 162 <li>Evaluate the output. See <a href="validate.html">Validation</a> for instructions and tools. 163 <li>Identify devices, and other new files that need labeling.Identify devices, and 164 other new files that need labeling. 165 <li>Use existing or new labels for your objects. 166 Look at the *_contexts files to 167 see how things were previously labeled and use knowledge of the label meanings 168 to assign a new one. Ideally, this will be an existing label which will fit 169 into policy, but sometimes a new label will be needed, and rules for access to 170 that label will be needed, as well. 171 <li>Identify domains/processes that should have their own security domains. A policy will likely need to be written for each of these from scratch. All services spawned from <code>init</code>, for instance, should have their own. The following commands help reveal those that remain running (but ALL services need such a treatment):<br/> 172 <code>$ adb shell su -c ps -Z | grep init</code><br/> 173 <code>$ adb shell su -c dmesg | grep 'avc: '</code> 174 <li>Review init.<device>.rc to identify any which are without a type. 175 These should 176 be given domains EARLY in order to avoid adding rules to init or otherwise 177 confusing <code>init</code> accesses with ones that are in their own policy. 178 <li>Set up <code>BOARD_CONFIG.mk</code> to use <code>BOARD_SEPOLICY_UNION</code> and <code>BOARD_SEPOLICY_DIRS</code>. See 179 the README in /sepolicy for details on setting this up. 180 <li> Examine the init.<device>.rc file and make sure every use of mount 181 corresponds to a properly labeled filesystem. 182 <li> Go through each denial and create SELinux policy to properly handle each. See 183 the examples within <a href="customize.html">Customization</a>. 184 </ol> 185
