ucount: use-after-free read in inc_ucount & dec_ucount
Dmitry Vyukov
dvyukov at google.com
Sat Mar 4 11:44:47 UTC 2017
On Sat, Mar 4, 2017 at 11:58 AM, Nikolay Borisov
<n.borisov.lkml at gmail.com> wrote:
> [Addressing Dmitry Vyukov to ask for syzkaller clarification]
>
> 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.
>
> Dmitry, can you tell if syzkaller tests sending file descriptors across
> sockets? Since the calltraces here show multiple processes being
> involved in different operations on the exact same file descriptor.
>
> Also JongHwan, can you provide the full, compilable reproducer to try
> and track this issue down?
syzkaller can pass descriptors across sockets, but currently only
within a single multi-threaded process.
Are you sure it's the same descriptor? It seems to me that it's struct
ucounts, which is shared via the global ucounts_hashtable, so no
sharing is required in user processes.
Unless I am missing something, we want:
@@ -154,7 +155,7 @@ static struct ucounts *get_ucounts(struct
user_namespace *ns, kuid_t uid)
ucounts = new;
}
}
- if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
+ if (!atomic_add_unless(&ucounts->count, 1, 0))
ucounts = NULL;
spin_unlock_irq(&ucounts_lock);
return ucounts;
no?
put_ucounts drops the last reference, then get_ucounts finds the
ucounts and successfully increments refcount as it's not INT_MAX (it's
0) and starts using it, meanwhile put_ucounts proceeds to
unconditionally deleting the ucounts.
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