SELinux

How to configure.

Sorry this is such a mess, this is more of a braindump…


General

(from SEAndroid-Aalto presentation):

Subjects are primarily processes. Processes that participate in the access control are assigned a domain

Objects are e.g. files, sockets.. Objects that participate in the access control are assigned a type

 X          N        allow   M                 Y
subjects   domains   rules  types          objects
                                 --------------[]
                                /      --------[]
[]---\        /-------------[]---- ---/--------[]
[]    ------[]------/-------[]--- ---/---------[]
[]----------[]\----/--------[]-\----/----------[]
[]      /  -[]-\------------[]--\--------------[]
[]-----/  /   \ \              \/\             []
[]       /     \------------[]--\-\------------[]
[]------/-                       --\-----------[]
                                    -----------[]

An object is always an OS primitive of some sort. This is not a policy issue, this is reality. An object therefore belongs to one or more classes which are predefined by the system proper. A class can be e.g. file, socket, character device, ….

Objects   Classes      Permissions
[]--           /-----------[]
[]--\-------[]-------------[]
[]-/          \------------[]
[]-

A class is associated with a fixed set of permissions (e.g read, write, open..), often closely mapped to system call functionality. Again, the set of available permission is predefined, policy does not change that But! An allow rule includes also the class, and within that the permissions allowed by the rule. Class and permission names are ”well known” as defined by NSA / SELinux

In general: allow [domain] [type] : [class] {[ allowed permissions ]}

  • [domain]: Subject(a process in a domain)
  • [type]: Object(a file or other resource)
  • [class]: Resource class(from predefined set)
  • [allowed_permissions]: Class-specific permissions that are allowed by this rule

Testing

Install needed tools for the host machine:

apt install setools python-networkx policycoreutils

Testing it out: either mka selinux_policy or make bootimage But to only check if the policy is valid: make -j $(nproc) sepolicy_tests!

Misc

Directories:

  • system/sepolicy
  • device/sony/sepolicy
  • device/sony/<platform>/sepolicy_platform
  • system/bt/vendor_libs/linux/sepolicy
  • system/extras/boottime_tools/bootio/sepolicy
  • file_contexts(“real” files, but not necessarily limited to that), genfs_contexts(“generated” files) -> contexts need to exist!

  • property_contexts -> always need a corresponding type in property.te!

PRODUCT_FULL_TREBLE_OVERRIDE := false
BOARD_USE_ENFORCING_SELINUX := true

Treble and changes from Oreo to Pie binder_in_vendor_violators etc

binder, hwbinder, vndbinder

Cleaning commands:

rm out/target/product/kagura/obj/ETC/*sepolicy* -r
find out/target/product/kagura/ -iname "*sepolicy*" -exec rm -rv {} \;
find out/target/product/kagura/ -iname "*selin*" -exec rm -rv {} \;
find out/target/product/kagura/ -iname "*file_context*" -exec rm -rv {} \;

Tips:

  • restorecon -R /path
  • ls -Z /path or ls -Z /path/file
  • ps -AZ
  • Always try to find out what the macros mean! Check system/sepolicy/pulic/{te_macros,global_macros} and any file you might find that explains a thing that does not fall within the standard rule_name source_type target_type : class perm_set; schema. Especially binder_use() and similar macros pull in neverallows and things you might not expect.
  • Have a copy of the te_macros and global_macros printed out!
  • load_policy to reload policy on device
  • sesearch and seinfo tools to examine policy(must be installed on host, not available on standard android device)
  • Check paths with matchpathcon -P <policy file dir> -f <file_contexts_file> /my/path (Important: -P needs to point to the directory containing the policy file, not the policy file itself!)

Examples:

Different app levels:

  • seapp_contexts
  • platform_app.te
  • service_contexts, hwservice_contexts

From domain.te:

Only service_manager_types can be added to service_manager
# TODO: rework this:
neverallow * ~service_manager_type:service_manager { add find };

From init.te:

Init never adds or uses services via service_manager
neverallow init service_manager_type:service_manager { add find };

Or own domain via own myapp.te, with:

type myapp, domain;
type myapp_exec, exec_type, vendor_file_type, file_type;

init_daemon_domain(myapp);
add_service(myapp, qcrilam_service)

typeattribute myapp binder_in_vendor_violators;
binder_use(myapp)

allow myapp_service self:hwservice_manager find;

(for more, see https://stackoverflow.com/questions/28914215/is-it-possible-to-use-the-package-name-as-the-domain-name-in-android-selinux)

Have a look at system/sepolicy/public/te_macros, system/sepolicy/public/global_macros

Google’s guides:

Gotchas

  • Confusing domains, types vs attributes(?), labels, objects, contexts, …
  • service_manager (from add_service(domain, service)) vs servicemanager (from binder_use(domain)). Yes, this is extremely confusing!
  • Confusing public/vendor and private policy folders/“APIs”
  • Confusing objects(mostly files) and running subjects like “apps”(the Java ones) or “services”(mostly C binaries that run from /system/bin or /vendor/bin, but can also be Java apps OR HAL/HIDL implementations)
  • Things can be active(searching/finding something) or passive(being labeled something and then being interacted with). Attributes are mostly passive, while subjects and their actions are active.

seapp

Is seapp_contexts a labeling mechanism? Or a sanity check through neverallows? Can you leave out everything apart from name=com.my.app?

seinfo is mainly used to enforce signing requirements, e.g. you have mac_permissions.xml(MAC = Mandatory Access Control) that specifies that anything with seinfo=platform has to be signed by the platform key:

<?xml version="1.0" encoding="utf-8"?><policy>
[...]
<!-- Platform dev key in AOSP -->
<signer signature="@PLATFORM" >
  <seinfo value="platform" />
</signer>
</policy>

Taming a service: QcRilAm

THIS IS OUT OF DATE!

QcRilAm is our own little app/service that proxies requests from rild to android.hardware.radio.am. It was written by oshmoun and is needed to get in-call audio to work. It consists of two parts: One HIDL implementation and one Java app that acts as a service. The two talk via binder.

# Establish vnd_qcril_audio_hwservice
# hwservice_contexts
vendor.qti.hardware.radio.am::IQcRilAudio           u:object_r:vnd_qcril_audio_hwservice:s0
vendor.qti.hardware.radio.am::IQcRilAudioCallback   u:object_r:vnd_qcril_audio_hwservice:s0

The HIDL service(called vendor.qti.hardware.radio.am) gets accessed by rild via hwbinder calls(since it is a hardware service).

# Allow rild to access vendor.qti.hardware.radio.am HIDL services
# rild.te
hwbinder_use(rild)
add_hwservice(rild, vnd_qcril_audio_hwservice)

The persistent android service part of QcRilAm is a java app that will also appear in a user’s app listing called com.sony.qcrilam. It only serves to run its one service called com.sony.qcrilam.QcRilAmService.

Right now, it appears in the process list as a platform_app:

$ ps -AZ | grep qcrilam
u:r:platform_app:s0:c512,c768  u0_a62        2045   585 3685212  80452 SyS_epoll_wait      0 S com.sony.qcrilam

To use a more restricted and thus safer domain:

# Assign com.sony.qcrilam.QcRilAmService a domain
# service_contexts
com.sony.qcrilam.QcRilAmService         u:object_r:qcrilam_service:s0

But wait! We need to define what a qcrilam_service is first!

# service.te
type qcrilam_service, service_manager_type;

Same goes for vnd_qcril_audio_hwservice:

# hwservice.te
type vnd_qcril_audio_hwservice,    hwservice_manager_type;

Alright, now we need to assign qcrilam_service to a domain. We could borrow from an already existing one, but AOSP doesn’t quite provide one that suits us. So we create our own:

# qcrilam.te
type qcrilam, domain;
add_service(qcrilam, qcrilam_service)
# the add_service() macro is equivalent to this:
#allow qcrilam qcrilam_service:service_manager { add find };
# TODO: Not needed with vnd change!
#typeattribute qcrilam binder_in_vendor_violators;
#binder_use(qcrilam_service)
vndbinder_use(qcrilam_service)
binder_call(qcrilam, vnd_qcril_audio_hwservice)

com.sony.qcrilam got assigned the platform_app label by SELinux automatically. This is a very high privilege level, as opposed to untrusted_app contexts. We want qcrilam to find its HIDL cousin via hwservicemanager in addition to being able to call them via binder:

# platform_app.te
allow platform_app vnd_qcril_audio_hwservice:hwservice_manager find;

A final thing to keep in mind is that rild is very greedy with permissions. Modems are a black box we have to contend with. Allow platform apps to talk to rild via binder and vice versa:

# rild.te
binder_call(platform_app, rild);
binder_call(rild, platform_app);

Wrapping up

Something to keep in mind is: You are only allowing or disallowing things, not influencing the flow of programs1. Even if te macros can look like little function statements, they only dictate what gets allowed, not how a program behaves.


  1. Ok that one is not 100% correct, some programs will try to do things and when they get denied switch to a different strategy. But in general, you will not incluence control flow much with sepolicy. [return]