On 4.03.2017 14:01, Dmitry Vyukov wrote:
> On Sat, Mar 4, 2017 at 12:50 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>> <n.borisov.lkml@xxxxxxxxx> wrote:
>>>> [Addressing Dmitry Vyukov to ask for syzkaller clarification]
>>>>
>>>> On 3.03.2017 18:30, Eric W. Biederman wrote:
>>>>> Nikolay Borisov <n.borisov.lkml@xxxxxxxxx> 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.
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