ucount: use-after-free read in inc_ucount & dec_ucount
Dmitry Vyukov
dvyukov at google.com
Sat Mar 4 12:15:23 UTC 2017
On Sat, Mar 4, 2017 at 1:10 PM, Nikolay Borisov
<n.borisov.lkml at gmail.com> wrote:
>
>
> On 4.03.2017 14:01, Dmitry Vyukov wrote:
>> On Sat, Mar 4, 2017 at 12:50 PM, Dmitry Vyukov <dvyukov at google.com> wrote:
>>>> <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.
>>>
>>>
>>> It also seems that a concurrent put_ucounts can make get_ucounts fail
>>> _spuriously_, which does not look good.
>>> Don't we want something along the following lines?
>>>
>>> diff --git a/kernel/ucount.c b/kernel/ucount.c
>>> index 8a11fc0cb459..233c8e46acd5 100644
>>> --- a/kernel/ucount.c
>>> +++ b/kernel/ucount.c
>>> @@ -143,19 +143,18 @@ static struct ucounts *get_ucounts(struct
>>> user_namespace *ns, kuid_t uid)
>>>
>>> new->ns = ns;
>>> new->uid = uid;
>>> - atomic_set(&new->count, 0);
>>> + atomic_set(&new->count, 1);
>>>
>>> spin_lock_irq(&ucounts_lock);
>>> ucounts = find_ucounts(ns, uid, hashent);
>>> if (ucounts) {
>>> + atomic_inc(&ucounts->count);
>>> kfree(new);
>>> } else {
>>> hlist_add_head(&new->node, hashent);
>>> ucounts = new;
>>> }
>>> }
>>> - if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
>>> - ucounts = NULL;
>>> spin_unlock_irq(&ucounts_lock);
>>> return ucounts;
>>> }
>>> @@ -166,7 +165,10 @@ static void put_ucounts(struct ucounts *ucounts)
>>>
>>> if (atomic_dec_and_test(&ucounts->count)) {
>>> spin_lock_irqsave(&ucounts_lock, flags);
>>> - hlist_del_init(&ucounts->node);
>>> + if (atomic_read(&ucounts->count) == 0)
>>> + hlist_del_init(&ucounts->node);
>>> + else
>>> + ucounts = NULL;
>>> spin_unlock_irqrestore(&ucounts_lock, flags);
>>>
>>> kfree(ucounts);
>>
>>
>> /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
>>
>> This is broken per se. Need something more elaborate.
>>
>
>
> How about this :
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 8a11fc0cb459..b817ac0e587c 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -166,11 +166,15 @@ static void put_ucounts(struct ucounts *ucounts)
>
> if (atomic_dec_and_test(&ucounts->count)) {
> spin_lock_irqsave(&ucounts_lock, flags);
> - hlist_del_init(&ucounts->node);
> - spin_unlock_irqrestore(&ucounts_lock, flags);
> -
> - kfree(ucounts);
> + if (!atomic_read(&ucounts->count)) {
> + hlist_del_init(&ucounts->node);
> + spin_unlock_irqrestore(&ucounts_lock, flags);
> + kfree(ucounts);
> + return;
> + }
> }
> +
> + spin_unlock_irqrestore(&ucounts_lock, flags);
> }
>
>
>
> This makes the atomic_dec_and_test and hashtable removal atomic in essence.
This won't work.
Consider the following scenario:
Thread 0 decrements count to 0 here:
if (atomic_dec_and_test(&ucounts->count)) {
Then thread 1 calls get_ucounts, increments count to 1, then calls
put_ucounts, decrements count to 0 and unhashes and frees ucounts.
Not thread 0 does:
if (!atomic_read(&ucounts->count)) {
but ucounts is already freed!
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