ucount: use-after-free read in inc_ucount & dec_ucount

Eric W. Biederman ebiederm at xmission.com
Fri Mar 3 16:46:36 UTC 2017


Nikolay Borisov <n.borisov.lkml at gmail.com> writes:

> On  3.03.2017 18:30, Eric W. Biederman wrote:
>> Nikolay Borisov <n.borisov.lkml at gmail.com> writes:
>> 
>>> [Added containers ml, Eric Biederman and Jan Kara]. Please,
>>> next time don't add random people but take the time to see who touched
>>> the code.
>> 
>> Comments below.
>> 
>>> On  3.03.2017 14:16, JongHwan Kim wrote:
>>>> I've got the following report with syzkaller fuzzer
>>>>
>>>>
>>>> Syzkaller hit 'KASAN: use-after-free Read in dec_ucount' bug on commit .
>>>>
>>>> ==================================================================
>>>> BUG: KASAN: use-after-free in __read_once_size
>>>> include/linux/compiler.h:254 [inline] at addr ffff88006d399bc4
>>>> BUG: KASAN: use-after-free in atomic_read
>>>> arch/x86/include/asm/atomic.h:26 [inline] at addr ffff88006d399bc4
>>>> BUG: KASAN: use-after-free in atomic_dec_if_positive
>>>> include/linux/atomic.h:616 [inline] at addr ffff88006d399bc4
>>>> BUG: KASAN: use-after-free in dec_ucount+0x1e5/0x210 kernel/ucount.c:217
>>>> at addr ffff88006d399bc4
>>>> Read of size 4 by task syz-executor3/19713
>>>> CPU: 1 PID: 19713 Comm: syz-executor3 Not tainted 4.10.0+ #4
>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>>>> Ubuntu-1.8.2-1ubuntu1 04/01/2014
>>>> Call Trace:
>>>>  __dump_stack lib/dump_stack.c:15 [inline]
>>>>  dump_stack+0x115/0x1cf lib/dump_stack.c:51
>>>>  kasan_object_err+0x1c/0x70 mm/kasan/report.c:162
>>>>  print_address_description mm/kasan/report.c:200 [inline]
>>>>  kasan_report_error mm/kasan/report.c:289 [inline]
>>>>  kasan_report.part.1+0x20e/0x4e0 mm/kasan/report.c:311
>>>>  kasan_report mm/kasan/report.c:331 [inline]
>>>>  __asan_report_load4_noabort+0x29/0x30 mm/kasan/report.c:331
>>>>  __read_once_size include/linux/compiler.h:254 [inline]
>>>>  atomic_read arch/x86/include/asm/atomic.h:26 [inline]
>>>>  atomic_dec_if_positive include/linux/atomic.h:616 [inline]
>>>>  dec_ucount+0x1e5/0x210 kernel/ucount.c:217
>>>>  dec_inotify_instances fs/notify/inotify/inotify.h:37 [inline]
>>>>  inotify_free_group_priv+0x6c/0x80 fs/notify/inotify/inotify_fsnotify.c:169
>>>>  fsnotify_final_destroy_group fs/notify/group.c:37 [inline]
>>>>  fsnotify_put_group+0x73/0xa0 fs/notify/group.c:110
>>>>  fsnotify_destroy_group+0xec/0x120 fs/notify/group.c:93
>>>>  inotify_release+0x37/0x50 fs/notify/inotify/inotify_user.c:280
>>>>  __fput+0x327/0x7e0 fs/file_table.c:208
>>>>  ____fput+0x15/0x20 fs/file_table.c:244
>>>>  task_work_run+0x18a/0x260 kernel/task_work.c:116
>>>>  exit_task_work include/linux/task_work.h:21 [inline]
>>>>  do_exit+0xa45/0x1b20 kernel/exit.c:873
>>>>  do_group_exit+0x149/0x400 kernel/exit.c:977
>>>>  get_signal+0x7d5/0x1810 kernel/signal.c:2313
>>>>  do_signal+0x94/0x1f30 arch/x86/kernel/signal.c:807
>>>>  exit_to_usermode_loop+0x162/0x1e0 arch/x86/entry/common.c:156
>>>>  prepare_exit_to_usermode arch/x86/entry/common.c:190 [inline]
>>>>  syscall_return_slowpath+0x2b6/0x310 arch/x86/entry/common.c:259
>>>>  entry_SYSCALL_64_fastpath+0xc0/0xc2
>>>
>>> So PID 19713 is exitting and as part of it it's freeing its file
>>> descriptors, one of which is apparently an inotify fd. And this has
>>> already been freed.
>>>
>>>
>>>> RIP: 0033:0x44fb79
>>>> RSP: 002b:00007ffd0f00f6d8 EFLAGS: 00000206 ORIG_RAX: 00000000000000ca
>>>> RAX: fffffffffffffdfc RBX: 0000000000708024 RCX: 000000000044fb79
>>>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000708024
>>>> RBP: 00000000000ae8e6 R08: 0000000000708000 R09: 000000160000000d
>>>> R10: 00007ffd0f00f710 R11: 0000000000000206 R12: 0000000000708000
>>>> R13: 0000000000708024 R14: 00000000000ae8a1 R15: 0000000000000016
>>>> Object at ffff88006d399b88, in cache kmalloc-96 size: 96
>>>> Allocated:
>>>> PID = 19691
>>>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
>>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:502
>>>>  set_track mm/kasan/kasan.c:514 [inline]
>>>>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:605
>>>>  kmem_cache_alloc_trace+0xfb/0x280 mm/slub.c:2745
>>>>  kmalloc include/linux/slab.h:490 [inline]
>>>>  kzalloc include/linux/slab.h:663 [inline]
>>>>  get_ucounts kernel/ucount.c:140 [inline]
>>>>  inc_ucount+0x538/0xa70 kernel/ucount.c:195
>>>>  inotify_new_group+0x309/0x410 fs/notify/inotify/inotify_user.c:655
>>>>  SYSC_inotify_init1 fs/notify/inotify/inotify_user.c:682 [inline]
>>>>  SyS_inotify_init1 fs/notify/inotify/inotify_user.c:669 [inline]
>>>>  sys_inotify_init+0x17/0x80 fs/notify/inotify/inotify_user.c:696
>>>>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>>>
>>> However, it has been actually allocated by a different process with pid
>>> 19691.
>>>
>>>
>>>> Freed:
>>>> PID = 19708
>>>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
>>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:502
>>>>  set_track mm/kasan/kasan.c:514 [inline]
>>>>  kasan_slab_free+0x73/0xc0 mm/kasan/kasan.c:578
>>>>  slab_free_hook mm/slub.c:1357 [inline]
>>>>  slab_free_freelist_hook mm/slub.c:1379 [inline]
>>>>  slab_free mm/slub.c:2961 [inline]
>>>>  kfree+0xe8/0x2c0 mm/slub.c:3882
>>>>  put_ucounts+0x1dd/0x270 kernel/ucount.c:172
>>>>  dec_ucount+0x172/0x210 kernel/ucount.c:220
>>>>  dec_inotify_instances fs/notify/inotify/inotify.h:37 [inline]
>>>>  inotify_free_group_priv+0x6c/0x80 fs/notify/inotify/inotify_fsnotify.c:169
>>>>  fsnotify_final_destroy_group fs/notify/group.c:37 [inline]
>>>>  fsnotify_put_group+0x73/0xa0 fs/notify/group.c:110
>>>>  fsnotify_destroy_group+0xec/0x120 fs/notify/group.c:93
>>>>  inotify_release+0x37/0x50 fs/notify/inotify/inotify_user.c:280
>>>>  __fput+0x327/0x7e0 fs/file_table.c:208
>>>>  ____fput+0x15/0x20 fs/file_table.c:244
>>>>  task_work_run+0x18a/0x260 kernel/task_work.c:116
>>>>  exit_task_work include/linux/task_work.h:21 [inline]
>>>>  do_exit+0xa45/0x1b20 kernel/exit.c:873
>>>>  do_group_exit+0x149/0x400 kernel/exit.c:977
>>>>  get_signal+0x7d5/0x1810 kernel/signal.c:2313
>>>>  do_signal+0x94/0x1f30 arch/x86/kernel/signal.c:807
>>>>  exit_to_usermode_loop+0x162/0x1e0 arch/x86/entry/common.c:156
>>>>  prepare_exit_to_usermode arch/x86/entry/common.c:190 [inline]
>>>>  syscall_return_slowpath+0x2b6/0x310 arch/x86/entry/common.c:259
>>>>  entry_SYSCALL_64_fastpath+0xc0/0xc2
>>>
>>> And yet we have a third process which freed it, PID 19708. So there is
>>> some dance happening with this fd, being allocated by one process,
>>> handed over to 2 more, which are freeing it. Is this a valid usage
>>> scenario of inotify descriptors?
>> 
>> They are file descriptors so passing them around is valid.  That is
>> something unix domain sockets have allowed since the dawn of linux.
>> 
>> The dance would need to be the fd being passed to the addtional
>> processes and then closed in the original before being closed
>> in the processes the fd was passed to.
>> 
>> If those additional processes last longer than the original process this
>> is easy to achieve.
>> 
>> My guess is that someone just taught syskallzer to pass file descriptors
>> around.  So this may be an old bug.  Either that or syskallzer hasn't
>> been looking at linux-next with KASAN enabled in the kernel.
>> 
>> This is definitely a weird usage pattern.
>
> If we assume that passing the fds is the problem then this *might*
> explain the first bug. However, if you look closely into the second set
> of stacktraces, you see how the crash occurs on inotify fd
> initialization, and according to the trace this line is the culprit:
> tns = iter->ns;
>
> And this points to a deeper problem, if ucounts can disappear in the
> middle of iterating through them.

Ugh.  Yes.  That does sound like a data structure corruption problem.
I had not seen that.  It could just be a reference counting problem
as children are supposed to have references to their parents, but
this looks more like a memory stomp.

Grrr.

Tracking a memory stomp really needs a reproducer because those are
ugly.

Eric


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