android for selinux

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NB SEforandroid 1

Security Enhancements for Android

Introduction

This section gives an overview of the enhancements made to Android to add SELinux services to Security Enhancements for Android™ (SE for Android).

The main objective of this document is to provide a reference for the tools, commands, policy building tools and file formats of SE for Android based on the 5.1 release. The builds discussed are from AOSP master and SEAndriod master repositories (as March '15).

The AOSP git repositories can be found at https://android.googlesource.com.

For up to date information on the status of SE for Android the following should be consulted: http://selinuxproject.org/page/SEforAndroid.

Terminology

This section describes how the terms SE for Android, AOSP and SEAndroid are used in this document.

SE for Android

This is the overall programme by Google to improve security on Android. For this document, it is used to describe the implementation of SELinux (MAC) and Middleware MAC (MMAC) on Android.

AOSP

The Android code base distributed by Google (see http://source.android.com/source/downloading.html). Release 5.1 contains SELinux support that is described at http://source.android.com/devices/tech/security/se-linux.html.

AOSP contains the core SELinux MAC functionality with a run-time MMAC framework to support specific labeling of apps as described in the Install/Run-time MMAC Policy section.

AOSP also contains services to allow updating of Intent Firewall policies, however currently no files are installed (although SEAndroid supplies a sample and update tools).

SEAndroid

The SEAndroid project enhancements are decreasing as more features move into AOSP (for example the MAC policy is now the same as AOSP). The additional SEAndroid features are:

Install time MMAC (an enhancement of the AOSP Run-time MMAC service)
Installation of Enterprise Operations (EOps) configuration files.
Sample EOps and Intent Firewall configuration files (the actual services are supplied by AOSP, replacing the SEAndroid Intent MMAC, Content Provider MMAC and Revoke Permissions services that are now obsolete).
Tools to manage bundles for policy, EOps and Intent Firewall updates.

See the SE for Android project page for up-to-date details at http://selinuxproject.org/page/SEforAndroid

Useful Links

The following link describes how to validate SELinux in Android:

http://source.android.com/devices/tech/security/se-linux.html

The http://selinuxproject.org/page/SEforAndroid pages describe the current merge status with AOSP, how to obtain the code, install SEAndroid and the features that have been implemented. It also has useful reference papers with "Security Enhanced (SE) Android: Bringing Flexible MAC to Android" available at http://www.internetsociety.org/sites/default/files/02_4.pdf being a recommended read.

The white paper "An Overview of Samsung KNOX" also gives an overview of how SE for Android is being integrated with other security services (such as secure boot and integrity measurement) to help provide a more secure mobile platform.

Document Sections

The sections that follow cover:

Overview of Android package additions and updates to support MAC
Additional kernel LSM / SELinux support
Android Classes & Permissions
SELinux commands and methods
SELinux extensions for init
Policy construction and build:
- Build file locations
- Policy files
- Build tools
Logging and auditing
Android libselinux additional functions
Configuration file formats

SE for Android Project Updates

This gives a high level view of the new and updated projects to support SE for Android services and covers AOSP with any additional SEAndroid functions noted. These are not a complete set of updates, but give some idea of the scope.

external/libselinuxProvides the SELinux userspace function library that is installed on the device. It is based on the 2.1.0 Linux version but has additional functions to support Android as summarised in the external/libselinux/README.android. Some additional detail is as follows:selinux_android_setcontext

Sets the correct domain context when launching applications using setcon(3). Information contained in the seapp_contexts file is used to compute the correct context.

It is called by frameworks/base/core/jni/com_android_internal_os_Zygote.cpp when forking a new process and the system/core/run-as/run-as.c utility for app debugging.

selinux_android_setfilecon

Sets the correct context on application directory / files using setfilecon(3). Information contained in the seapp_contexts file is used to compute the correct context.

The function is used by the package installer within frameworks/native/cmds/installd/commands.c via the package install() and make_user_data() functions.

selinux_android_restorecon

selinux_android_restorecon_pkgdir

Basically these functions are used to label files and directories based on entries from the file_contexts and/or seapp_contexts files. They call a common handler (selinux_android_restorecon_common()) that will then relabel the requested directories and files. It will also handle recursive labeling of directories and files should a new app, file_contexts or seapp_contexts be installed (see the Checking File Labels section for further information).

The selinux_android_restorecon function is used by:

frameworks/native/cmds/installd/installd.c when installing a new app.

frameworks/base/core/jni/android_os_SELinux.cpp for the Java native_restorecon method.

frameworks/native/cmds/dumpstate/utils.c when dumping Dalvik and stack traces to ensure correct label.

The selinux_android_restorecon_pkgdir function is used by:

frameworks/native/cmds/installd/commands.c for the package restorecon_data() and make_user_data() functions.

selinux_android_seapp_context_reload

Loads the seapp_contexts file for frameworks/native/cmds/installd/installd.c when the package installer is loaded.

selinux_android_load_policy

Mounts the SELinux filesystem if SELinux is enabled and then calls selinux_android_reload_policy to load the policy into the kernel. Used by system/core/init/init.c to initialise SELinux.

selinux_android_reload_policy

Reloads the policy into the kernel. Used by system/core/init/init.c selinux_reload_policy() to reload policy after setting the selinux.reload_policy property.

selinux_android_use_data_policy

Used by system/core/init/init.c to decide which policy directory to load the property_contexts file from.

There is also a new labeling service for selabel_lookup(3) to query the Android property_contexts and service_contexts files.

Various Android services will also call (not a complete list):

selinux_status_updated(3), is_selinux_enabled(3), to check whether anything changed within the SELinux environment (e.g. updated configuration files).

selinux_check_access(3) to check if the source context has access permission for the class on the target context.

selinux_label_open(3), selabel_lookup(3), selinux_android_file_context_handle, selinux_android_prop_context_handle, setfilecon(3), setfscreatecon(3) to manage file labeling.

selinux_lookup_best_match called by system/core/init/devices.c when ueventd creates a device node as it may also create one or more symlinks (for block and PCI devices). Therefore a "best match" look-up for a device node is based on its real path, plus any links that may have been created (see commits https://android.googlesource.com/platform/system/core/+/b0ab94b7d5a888f0b6920b156e5c6a075fa0741a, https://android.googlesource.com/platform/system/core/+/b4c5200f51c3568f604a4557119ab545a6ddac94 and https://android.googlesource.com/platform/external/libselinux/+/be7f5e8814c4954aca51d3f95455c5d9d527658c).

external/libsepol

Provides the policy userspace library for building policy on the host and is not available on the device. There are no specific updates to support Android except an Android.mk file.

external/checkpolicy

Provides the policy build tool. Added support for MacOS X. Not available on the device as policy rebuilds are done in the development environment.

external/sepolicy

This is a policy specifically for the core components of Android that looks much like the reference policy, but is contained in one directory that has the policy modules (*.te files), class / permission files etc.. The policy is built by the Android.mk file and the resulting policy is installed on the target device (as sepolicy) along with its supporting configuration files.

Device specific policy may be defined under the device directory as discussed in the Processing Device Policy section.

The policy can be updated along with its configuration files as discussed in the Updating Policy section.

The policy files are discussed in the SELinux Policy Files section and support tools in Policy Build Tools.

The Android specific object classes are described in the Android Classes and Permissions section.

The directory also contains the MMAC configuration files.

packages/apps/SEAdmin

This is an example Android application to manage the SE for Android environment (such as loading a new policy). Only available on SEAndroid build.

packages/apps/Settings

SELinux settings for the settings manager application.

bionic

Bionic is the Android libc that is a derived from the BSD standard C library code. It contains enhancements to support security providers such as SELinux.

bootable/recovery

Changes to manage file labeling on recovery plus a recovery init.rc file in the etc directory.

build

Changes to build SE for Android and manage file labeling on images and OTA (over the air) target files.

frameworks/base

JNI - Add SELinux support functions such as isSELinuxEnabled and setFSCreateCon.

SELinux Java class and method definitions.

Checking Zygote connection contexts.

Managing file permissions for the package manager and wallpaper services.

SELinux additions to support run time MMAC and for SEAndroid the additional MMAC services.

system/core

SELinux support services for toolbox (e.g. load_policy, runcon).

SELinux support for system initialisation (e.g. init, init.rc).

SELinux support for auditing avc's (auditd).

system/extras

SELinux support for the ext4 file system. Note that the make_ext4fs utility is used to build these file systems and relies on the file_contexts file having all the relevant entries, if not, it will be unable to set the security.selinux xattr on the inode and fail.

kernel

All Android kernels support the Linux Security Module (LSM) and SELinux services, however they are based on various versions (currently 3.4 for Goldfish used by the emulator), therefore the latest SELinux enhancements may not always be present. The Kernel LSM / SELinux Support section describes the Andriod kernel changes.

deviceBuild information for each device, details regarding SEAndroid supported devices can be found at:

http://seandroid.bitbucket.org/BuildingKernels.html#9

Device specific policy can be added as discussed in the Building the Policy and Processing Device Policy sections.

Kernel LSM / SELinux Support

The paper "Security Enhanced (SE) Android: Bringing Flexible MAC to Android" available at http://www.internetsociety.org/sites/default/files/02_4.pdf gives a good review of what did and didn't change in the kernel to support Android. This section briefly describes the only major change that was to support the Binder IPC service that consists of the following:

LSM hooks in the binder code (drivers/staging/android/binder.c) and (include/linux/security.h)
Default support for capabilities (security/capability.c) in case no other module is loaded.
Hooks in the LSM security module (security/security.c).
SELinux support for the binder object class and permissions (security/selinux/include/classmap.h) that are shown in the Android Classes and Permissions section. Support for these permission checks are added to security/selinux/hooks.c.

Android Classes and Permissions

Additional classes have been added to Android and are listed in the following tables with descriptions of their permissions. The policy files external/sepolicy/security_classes and external/sepolicy/access_vectors contain the complete list with descriptions available at: http://selinuxproject.org/page/NB_ObjectClassesPermissions. However, note that while the security_classes file contains many entries, not all are required for Android.

binder class - This is a kernel object to manage the Binder IPC service.
Permission	Description (4 unique permissions)
call	Perform a binder IPC to a given target process (can A call B?).
impersonate	Perform a binder IPC on behalf of another process (can A impersonate B on an IPC?). Not currently used in policy but kernel (selinux/hooks.c) checks permission in selinux_binder_transaction call.
set_context_mgr	Register self as the Binder Context Manager aka servicemanager (global name service). Can A set the context manager to B, where normally A == B. See policy module servicemanager.te.
transfer	Transfer a binder reference to another process (can A transfer a binder reference to B?).

property_service class - This is a userspace object to manage the Android Property Service. See check_mac_perms() in system/core/init/property_service.c
Permission	Description (1 unique permission)
set	Set a property.

service_manager class - This is a userspace object to manage Android services. See check_mac_perms() in frameworks/native/cmds/servicemanager/service_manager.c
Permission	Description (3 unique permission)
add	Add a service.
find	Find a service.
list	List services.

keystore_key class - This is a userspace object to manage the Android keystore (see system/security/keystore/keystore.cpp).
Permission	Description (16 unique permissions)
test	Test if keystore okay.
get	Get key.
insert	Insert/update key.
delete	Delete key.
exist	Check if key exists.
saw	Search for matching string.
reset	Reset keystore.
password	Generate new keystore password.
lock	Lock keystore.
unlock	Unlock keystore.
zero	Check if keystore empty.
sign	Sign data.
verify	Verify data.
grant	Add or remove access.
duplicate	Duplicate the key.
clear_uid	Clear keys for this uid.

debuggerd class - This is a userspace object to allow file dumps (see system/core/debuggerd/debuggerd.cpp).
Permission	Description (2 unique permissions)
dump_tombstone	Write tombstone file.
dump_backtrace	Write backtrace file.

drmservice class - This is a userspace object to allow finer access control of the Digital Rights Management services (see frameworks/av/drm/drmserver/DrmManagerService.cpp).
Permission	Description (8 unique permissions)
consumeRights	Consume rights for content.
setPlaybackStatus	Set the playback state.
openDecryptSession	Open the DRM session for the requested DRM plugin.
closeDecryptSession	Close DRM session.
initializeDecrypSession	Initialise the decrypt resources.
decrypt	Decrypt data stream.
finalizeDecryptUnit	Release DRM resources.
pread	Read the data stream.

SELinux Commands

A subset of the Linux SELinux commands have been implemented in Android and are listed in Table 1. Some are available as Toolbox commands (see system/core/toolbox) and can be run via adb shell, for example:

adb shell su 0 setenforce permissive

Table 1: SELinux enabled commands

Command	Comment
getenforce	Returns the current enforcing mode.
setenforce	Modify the SELinux enforcing mode: setenforce [enforcing\|permissive\|1\|0]
load_policy	Load new policy into kernel: load_policy policy-file
ls	Supports -Z option to display security context.
ps	Supports -Z option to display security context.
restorecon	Restore file default security context as defined in the file_contexts or seapp_contexts files. The options are: D - data files, F - Force reset, n - do not change, R/r - Recursive change, v - Show changes. restorecon [-DFnrRv] pathname
chcon	Change security context of file. The options are: h - Change symlinks, R - Recurse into subdirectories, v - Verbose output. chcon [-hRv] context file...
runcon	Run command in specified security context: runcon context program args...
id	If SELinux is enabled then the security context is automatically displayed.
getsebool	Deprecated as policy booleans no longer supported. Returns SELinux boolean value(s): getsebool [-a \| boolean_name]
setsebool	Deprecated as policy booleans no longer supported. Set SELinux boolean to a value, does not set the boolean across reboots: setsebool boolean_name [1\|true\|on\|0\|false\|off]

SELinux Public Methods

The public methods implemented are equivalent to libselinux functions and shown in Table 2. They have been taken from frameworks/base/core/java/android/os/SELinux.java.

The SELinux class and its methods are not available in the Android SDK, however if developing SELinux enabled apps within AOSP then Reflection would be used (see the proguard.flags and Android.mk files in packages/apps/SEAdmin).

Table 2: SELinux class public methods

boolean isSELinuxEnabled()

   Determine whether SELinux is enabled or disabled. 
   Return true if SELinux is enabled.

boolean isSELinuxEnforced()

   Determine whether SELinux is permissive or enforcing.
   Returns true if SELinux is enforcing.

boolean setSELinuxEnforce(boolean value)

   Set whether SELinux is in permissive or enforcing modes.
   value of true sets SELinux to enforcing mode.
   Returns true if the desired mode was set.

boolean setFSCreateContext(String context)

   Sets the security context for newly created file objects.
   context is the security context to set.
   Returns true if the operation succeeded.

boolean setFileContext(String path, String context)

   Change the security context of an existing file object.
   path represents the path of file object to relabel.
   context is the new security context to set .
   Returns true if the operation succeeded.

String getFileContext(String path)

   Get the security context of a file object.
   path the pathname of the file object.
   Returns the requested security context or null.

String getPeerContext(FileDescriptor fd)

   Get the security context of a peer socket.
   FileDescriptor is the file descriptor class of the peer socket.
   Returns the peer socket security context or null.

String getContext()

   Gets the security context of the current process.
   Returns the current process security context or null.

String getPidContext(int pid)

   Gets the security context of a given process id.
   pid an int representing the process id to check.
   Returns the security context of the given pid or null.

Deprecated as policy booleans no longer supported.

String[] getBooleanNames()

   Gets a list of the SELinux boolean names.
   Return an array of strings containing the SELinux boolean names.

Deprecated as policy booleans no longer supported.

boolean getBooleanValue(String name)

   Gets the value for the given SELinux boolean name.
   name is the name of the SELinux boolean.
   Returns true or false indicating whether the SELinux boolean is set or not.

Deprecated as policy booleans no longer supported.

boolean setBooleanValue(String name, boolean value)

   Sets the value for the given SELinux boolean name. Note that this will be set the boolean permanently across reboots.
   name is the name of the SELinux boolean.
   value is the new value of the SELinux boolean.
   Returns true if the operation succeeded.

boolean checkSELinuxAccess(String scon, String tcon, String tclass, String perm)

   Check permissions between two security contexts.
   scon is the source or subject security context.
   tcon is the target or object security context.
   tclass is the object security class name.
   perm is the permission name.
   Returns true if permission was granted.

boolean restorecon(String pathname)

   Restores a file to its default SELinux security context. If the system is not compiled with SELinux, then true is automatically returned. If SELinux is compiled in, but disabled, then true is returned.
   pathname is the pathname of the file to be relabeled.
   Returns true if the relabeling succeeded.
   exception NullPointerException if the pathname is a null object.

boolean restorecon(File file)

   Restores a file to its default SELinux security context. If the system is not compiled with SELinux, then true is automatically returned. If SELinux is compiled in, but disabled, then true is returned.
   file is the file object representing the path to be relabeled. 
   Returns true if the relabeling succeeded.
   exception NullPointerException if the file is a null object.

boolean restoreconRecursive(File file)

   Recursively restores all files under the given path to their default SELinux security context. If the system is not compiled with SELinux, then true is automatically returned. If SELinux is compiled in, but disabled, then true is returned.
   pathname is the pathname of the file to be relabeled.
   Returns a boolean indicating whether the relabeling succeeded.

Android Init Language SELinux Extensions

The Android init process language has been expanded to support SELinux as shown in Table 3. The complete Android init language description is available in the system/core/init/readme.txt file.

Table 3: SELinux init extensions

seclabel <securitycontext>

service option: Change to security context before exec'ing this service. Primarily for use by services run from the rootfs, e.g. ueventd, adbd. Services on the system partition can instead use policy defined transitions based on their file security context. If not specified and no transition is defined in policy, defaults to the init context.

restorecon <path>

action command: Restore the file named by <path> to the security context specified in the file_contexts configuration. Not required for directories created by the init.rc as these are automatically labeled correctly by init.

restorecon_recursive <path> [ <path> ]*

action command: Recursively restore the directory tree named by <path> to the security context specified in the file_contexts configuration. Do NOT use this with paths leading to shell-writable or app-writable directories, e.g. /data/local/tmp, /data/data or any prefix thereof.

See the Checking File Labels section for further details.

setcon <securitycontext>

action command: Set the current process security context to the specified string. This is typically only used from early-init to set the init context before any other process is started (see init.rc example above).

setenforce 0|1

action command: Set the SELinux system-wide enforcing status. 0 is permissive (i.e. log but do not deny), 1 is enforcing.

setsebool <name> <value>

Deprecated as booleans no longer supported.

action command: Set SELinux boolean <name> to <value>.

<value> may be 1|true|on or 0|false|off

Examples of their usage are shown in the following init.rc file segments:

system/core/rootdir/init.rc
...

on early-init
    ...

    # Set the security context for the init process.
    # This should occur before anything else (e.g. ueventd) is started.
    setcon u:r:init:s0

    # Set the security context of /adb_keys if present.
    restorecon /adb_keys

    start ueventd
    ...

on post-fs-data
...
    # Reload policy from /data/security if present.
    setprop selinux.reload_policy 1

    # Set SELinux security contexts on upgrade or policy update.</nowiki>
    restorecon_recursive /data
    ...
service ueventd /sbin/ueventd
    class core
    critical
    seclabel u:r:ueventd:s0

Device Policy File Locations

Table 4 shows the Android policy files with their default location when the device is built, and their alternate locations when devices are updated by other methods (such as OTA or via adb). The alternate locations are always checked first as if present they override the default location as discussed in the comments section of Table 4.

The init process will initially load the SELinux set of policy files from root (/). Once the /data partition setup has been completed (see init.rc) a policy reload is performed. This will check whether there is a valid policy at /data/security/current and load that if valid.

If safe mode, then only the root policy files will be loaded. A factory reset will wipe /data and will therefore revert to the original root policy files.

Table 4: Policy file locations

Default Location	Alternate Location	Comments
/sepolicy /file_contexts /seapp_contexts /property_contexts /service_contexts /selinux_version /mac_permissions.xml	/data/security/current	Any or all these files may be in the alternate directory as each conponent that requires them will look in the alternate first and then the default, however: During a policy reload, if there is an selinux_version file in the alternate location, then the default location will be over-ridden. If the policy has been updated via the buildsebundle / SEAdmin app process then this would be the case. The alternate directory may be a symbolic link to another directory. For example the buildsebundle / SEAdmin app process adds a link to /data/security/context that holds the policy files If the policy has been updated via the buildsebundle / SEAdmin app process, then the following will also be present: /data/security/bundle will contain the sepolicy_bundle (the packed files) and a metadata directory containing a version file holding the last version number. There will be *_backup policy files of the previous version that could be restored if required. See the Build Bundle Tools - buildsebundle section for a worked example.
/system/etc/security/eops.xml	/data/security/eops	If the policy has been updated via the buildeopbundle / SEAdmin app process, then the following will also be present in the alternative location: /data/security/eops/eops_metadata/version file holding the last version number. See the Build Bundle Tools - buildeopbundle section for a worked example.
/data/system/ifw/ifw.xml	/data/secure/system/ifw (default for encrypted systems)	This file is not installed by default and note that the Intent Firewall service will read any file from /data/system/ifw/ so long as it has an .xml extension. If required would be built and delivered by the buildifwbundle / SEAdmin app process, with the following also present in the default location: /data/system/ifw/metadata/gservices.version file holding the last version number. See the Build Bundle Tools - buildifwbundle section for a worked example.
/system/etc/sepolicy.recovery	none	Only used for recovery.

Building the Policy

This section covers building of SELinux MAC and Install-time MMAC policies. The file formats of Android specific configuration files are detailed in Policy Configuration File Formats with examples.

SELinux Policy Files

The core policy files are contained in external/sepolicy, with device specific policy in device/<vendor>/<device>/sepolicy[1] (see the Processing Device Policy section). Once generated, the policy and its supporting configuration files will be installed on the device as part of the build process.

Core Policy Files

The following files (along with any device specific policy) are used to build the kernel binary policy file named <tt>sepolicy and installed by default in the root directory.

access_vectors

security_classes

These have been modified to support the new Android classes and permissions (although they still contain the unused Linux userspace items).

initial_sids

initial_sids_contexts

Contains the system initialisation (before policy is loaded) and failsafe (for objects that would not otherwise have a valid label).

fs_use

genfs_contexts

port_contexts

For flexibility of policy building, these files have been separated to allow additional policy files to be defined for specific devices as discussed below.

users

roles

These define the only user (u) and role (r) used by the policy.

mls

Contains the constraints to be applied to the defined classes and permissions.

global_macros

mls_macro

te_marcos

These contain the m4 macros that expand the policy files to build a policy in the kernel policy language as described in http://selinuxproject.org/page/PolicyLanguage. The policy will then be compiled by checkpolicy(8).

attributes

Contains the attribute names (forming the attribute statements) that will be used to group type identifiers defined by the policy.

policy_capabilities

Contains the policy capabilities enabled for the kernel policy (see policycap statement).

*.te

The *.te files are the core policy module definition files. These are the same format as the standard reference policy and are expanded by the m4 macros. There is (generally) one .te file for each domain/service defined containing the policy rules.

Policy Configuration Files

These files (along with any device specific files) will be installed on the device and used to compute security contexts (see the Checking File Labels section for further information).

file_contextsContains default file contexts for setting the SELinux extended file attributes (attr(1)). The format of this file is defined in the file_contexts section. The file is installed by default in the root directory. Android services (such as restorecon) will first check for this file at:

/data/security/current/file_contexts