On Sat, Mar 4, 2017 at 12:44 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Sat, Mar 4, 2017 at 11:58 AM, Nikolay Borisov
> <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);
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