Re: 2.6.32-rc6 BUG at mm/slab.c:2869!

[Vegard Nossum <vegard.nossum@gmail.com>]

2009/8/21 Bob Copeland <me@bobcopeland.com>:
> On Thu, Aug 20, 2009 at 02:02:49PM +0200, Vegard Nossum wrote:
>> > I'll try that and kmemcheck next.
>>
>> Hm, I'm afraid kmemcheck gives some known false positives related to
>> bitfields in ext4 code, so in the case that something turned up, it
>> might be hard to distinguish it from those false positives.
>
> Well I didn't get anything from ext4 so far.  I did hit one with
> fsnotify:
>
> WARNING: kmemcheck: Caught 32-bit read from freed memory (f34a443c)
> eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee008a06f700011000
>  a a a a a a a a a a a a a a a a a a a a a a a a f f f f f f f f
>                                                         ^
>
> Pid: 2745, comm: fsck.ext4 Not tainted (2.6.31-rc6 #2) MacBook1,1
> EIP: 0060:[<c10f3656>] EFLAGS: 00010217 CPU: 0
> EIP is at inotify_handle_event+0x76/0xc0
> EAX: f34a443c EBX: f34a4438 ECX: 00000000 EDX: f6732000
> ESI: f6559764 EDI: 00000000 EBP: f6733f0c ESP: c1527450
>  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
> CR0: 8005003b CR2: f6c046d4 CR3: 367fb000 CR4: 000026d0
> DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
> DR6: ffff4ff0 DR7: 00000400
>  [<c10f0d78>] fsnotify+0xa8/0x130
>  [<c10c5e11>] __fput+0xb1/0x1e0
>  [<c10c5f55>] fput+0x15/0x20
>  [<c10c2ca7>] filp_close+0x47/0x80
>  [<c10c2d54>] sys_close+0x74/0xc0
>  [<c1002ec8>] sysenter_do_call+0x12/0x36
>  [<ffffffff>] 0xffffffff
>
> I think that is list_empty() here where %eax is list_head
> and event_list->next is the read location... which definitely
> doesn't look like a pointer, if I'm reading it correctly.

I think f34a443c is a valid pointer. On my machine, at least:

[    0.004000]     lowmem  : 0xc0000000 - 0xf73fe000   ( 883 MB)

>
> inotify_fsnotify.o:
>
>        /* did event_priv get attached? */
>        if (list_empty(&fsn_event_priv->event_list))
>  143:   8d 43 04                lea    0x4(%ebx),%eax
>
>        event_priv = kmem_cache_alloc(event_priv_cachep, GFP_KERNEL);
>        if (unlikely(!event_priv))
>                return -ENOMEM;
>
>        fsn_event_priv = &event_priv->fsnotify_event_priv_data;
>  146:   39 43 04                cmp    %eax,0x4(%ebx)     <=== read here
>  149:   74 1d                   je     168 <inotify_handle_event+0x98>

I can see somewhat of a race, I think:

1. userspace calls inotify_read(), where we wait for something to happen:

249         while (1) {
250                 prepare_to_wait(&group->notification_waitq, &wait,
TASK_INTERRUPTIBLE);
251
252                 mutex_lock(&group->notification_mutex);
253                 kevent = get_one_event(group, count);
254                 mutex_unlock(&group->notification_mutex);

2. an event occurs, and inotify_handle_event() calls
fsnotify_add_notify_event():

 64         ret = fsnotify_add_notify_event(group, event, fsn_event_priv);
 65         /* EEXIST is not an error */
 66         if (ret == -EEXIST)
 67                 ret = 0;

3. fsnotify_add_notify_event() adds the fsn_event_priv to the event,
and adds the event to the group, and finally wakes up anybody who is
waiting on &group->notification_waitq:

230         fsnotify_get_event(event);
231         list_add_tail(&holder->event_list, list);
232         if (priv)
233                 list_add_tail(&priv->event_list, &event->private_data_list);
234         spin_unlock(&event->lock);
235         mutex_unlock(&group->notification_mutex);
236
237         wake_up(&group->notification_waitq);

4. inotify_read() wakes up and frees the event:

253                 kevent = get_one_event(group, count);

5. inotify_handle_event() now dereferences the event_priv pointer,
which was already freed:

 69         /* did event_priv get attached? */
 70         if (list_empty(&fsn_event_priv->event_list))


I think that's it. Any thoughts? I put Eric Paris on Cc.


Vegard