Previously, Magisk uses persist or cache for storing modules' custom
sepolicy rules. In this commit, we significantly broaden its
compatibility and also prevent mounting errors.
The persist partition is non-standard and also critical for Snapdragon
devices, so we prefer not to use it by default.
We will go through the following logic to find the best suitable
non-volatile, writable location to store and load sepolicy.rule files:
Unencrypted data -> FBE data unencrypted dir -> cache -> metadata -> persist
This should cover almost all possible cases: very old devices have
cache partitions; newer devices will use FBE; latest devices will use
metadata FBE (which guarantees a metadata parition); and finally,
all Snapdragon devices have the persist partition (as a last resort).
Fix#3179
Due to changes in ec3705f2ed187863efc34af5415495e1ee7775d2, the app can
no longer communicate with the dameon through a socket opened on the
daemon side due to SELinux restrictions. The workaround here is to have
the daemon decide a socket name, send it to the app, have the app create
the socket server, then finally the daemon connects to the app through
the socket.
Introduce new domain `magisk_client` and new file type `magisk_exec`.
Connection to magiskd's always-on socket is restricted to magisk_client
only. Whitelisted process domains can transit to magisk_client through
executing files labelled magisk_exec. The main magisk binary shall be
the only file labelled as magisk_exec throughout the whole system.
All processes thus are no longer allowed to connect to magiskd directly
without going through the proper magisk binary.
Connection failures are silenced from audit logs with dontaudit rules,
so crazy processes which traverse through all unix domain sockets to try
connection can no longer check logcat to know the actual reason behind
EACCES, leaking the denied process policy (which is u:r:magisk:s0).
This also allows us to remove many rules that open up holes in
untrusted_app domains that were used to make remote shell work properly.
Since all processes establishing the remote shell are now restricted to
the magisk_client domain, all these rules are moved to magisk_client.
This makes Magisk require fewer compromises in Android's security model.
Note: as of this commit, requesting new root access via Magisk Manager
will stop working as Magisk Manager can no longer communicate with
magiskd directly. This will be addressed in a future commit that
involves changes in both native and application side.
It is possible that a module is breaking the device so bad that zygote
cannot even be started. In this case, system_server cannot start and
detect the safe mode key combo, set the persist property, and reboot.
Also on old Android versions, the system directly goes to safe mode
after detecting a key combo without rebooting, defeating the purpose of
Magisk's safe mode protection if we only check for the persist property.
Directly adding key combo check natively in magiskd allows us to enter
Magisk safe mode before the system is even aware of it.
When detecting device is booting as Safe Mode, disable all modules and
MagiskHide and skip all operations. The only thing that'll be available
in this state is root (Magisk Manager will also be disabled by system).
Since the next normal boot will also have all modules disabled, this can
be used to rescue a device in the case when a rogue module causes
bootloop and no custom recovery is available (or recoveries without
the ability to decrypt data).
Rewrite the whole module mounting logic from scratch.
Even the algorithm is different compared to the old one.
This new design focuses on a few key points:
- Modular: Custom nodes can be injected into the mount tree.
It's the main reason for starting the rewrite (needed for Android 11)
- Efficient: Compared to the existing implementation, this is the most
efficient (both in terms of computation and memory usage) design I
currently can come up with.
- Accurate: The old mounting logic relies on handling specifically every
edge case I can think of. During this rewrite I actually found some
cases that the old design does not handle properly. This new design is
architected in a way (node types and its rankings) that it should
handle edge cases all by itself when constructing mount trees.
