KASAN: use-after-free in move_expired_inodes

KASAN: use-after-free in move_expired_inodes

[Shankara Pailoor]

Hi,

We got the following error:

BUG: KASAN: use-after-free in move_expired_inodes+0xce6/0xdf0
Write of size 8 at addr ffff8800a3a36bf8 by task kworker/u8:0/5

while fuzzing with Syzkaller on 4.14-rc4 on x86_64. Included is the
trace of the crash along with the programs running around the time of
the crash.

Programs can be found here: https://pastebin.com/RYGtNn3z

Stack trace here: https://pastebin.com/SaJXWMg3

We don't have a C reproducer but we will send one if we have it.

Regards,
Shankara

Re: KASAN: use-after-free in move_expired_inodes

[Shankara Pailoor]

Hi Al, etc,

I was unable to find a reproducer but I was looking at
move_expired_inodes (fs/fs-writeback.c 1093.c) and how do you ensure
that the inode can't be freed after retrieving it from the work queue?
Any insights would be appreciated.

Regards,
Shankara

On Tue, Oct 31, 2017 at 9:24 AM, Shankara Pailoor <sp3485@columbia.edu> wrote:
> Hi,
>
> We got the following error:
>
> BUG: KASAN: use-after-free in move_expired_inodes+0xce6/0xdf0
> Write of size 8 at addr ffff8800a3a36bf8 by task kworker/u8:0/5
>
> while fuzzing with Syzkaller on 4.14-rc4 on x86_64. Included is the
> trace of the crash along with the programs running around the time of
> the crash.
>
> Programs can be found here: https://pastebin.com/RYGtNn3z
>
> Stack trace here: https://pastebin.com/SaJXWMg3
>
> We don't have a C reproducer but we will send one if we have it.
>
> Regards,
> Shankara

Re: KASAN: use-after-free in move_expired_inodes

[Jan Kara]

Hi,

On Tue 31-10-17 22:04:49, Shankara Pailoor wrote:
> I was unable to find a reproducer but I was looking at
> move_expired_inodes (fs/fs-writeback.c 1093.c) and how do you ensure
> that the inode can't be freed after retrieving it from the work queue?
> Any insights would be appreciated.

In move_expired_inodes() we hold wb->list_lock which protects the list
inode is in. fs/inode.c:evict() checks for inode being in the list and
removes it from the list blocking on the wb->list_lock as well. Granted
list_empty(&inode->i_io_list) is not protected by any lock so that check
*could* be somewhat stale but it cannot be older than e.g. time when
inode's refcount dropped to 0 at which point inode->i_io_list should be
already stable. But maybe flusher is shuffling inode between lists and
evict() saw some intermediate state. So far I don't see how that could
happen but maybe it could - will look more into that later...

								Honza

> On Tue, Oct 31, 2017 at 9:24 AM, Shankara Pailoor <sp3485@columbia.edu> wrote:
> > Hi,
> >
> > We got the following error:
> >
> > BUG: KASAN: use-after-free in move_expired_inodes+0xce6/0xdf0
> > Write of size 8 at addr ffff8800a3a36bf8 by task kworker/u8:0/5
> >
> > while fuzzing with Syzkaller on 4.14-rc4 on x86_64. Included is the
> > trace of the crash along with the programs running around the time of
> > the crash.
> >
> > Programs can be found here: https://pastebin.com/RYGtNn3z
> >
> > Stack trace here: https://pastebin.com/SaJXWMg3
> >
> > We don't have a C reproducer but we will send one if we have it.
> >
> > Regards,
> > Shankara
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR