Re: [PATCH] bdi: Fix another oops in wb_workfn()

Jens Axboe
2018-06-22T18:08:20Z

On 6/22/18 2:52 AM, Jan Kara wrote:
> On Mon 18-06-18 10:40:14, Tejun Heo wrote:
>> On Mon, Jun 18, 2018 at 03:46:58PM +0200, Jan Kara wrote:
>>> syzbot is reporting NULL pointer dereference at wb_workfn() [1] due to
>>> wb->bdi->dev being NULL. And Dmitry confirmed that wb->state was
>>> WB_shutting_down after wb->bdi->dev became NULL. This indicates that
>>> unregister_bdi() failed to call wb_shutdown() on one of wb objects.
>>>
>>> The problem is in cgwb_bdi_unregister() which does cgwb_kill() and thus
>>> drops bdi's reference to wb structures before going through the list of
>>> wbs again and calling wb_shutdown() on each of them. This way the loop
>>> iterating through all wbs can easily miss a wb if that wb has already
>>> passed through cgwb_remove_from_bdi_list() called from wb_shutdown()
>>> from cgwb_release_workfn() and as a result fully shutdown bdi although
>>> wb_workfn() for this wb structure is still running. In fact there are
>>> also other ways cgwb_bdi_unregister() can race with
>>> cgwb_release_workfn() leading e.g. to use-after-free issues:
>>>
>>> CPU1 CPU2
>>> cgwb_bdi_unregister()
>>> cgwb_kill(*slot);
>>>
>>> cgwb_release()
>>> queue_work(cgwb_release_wq, &wb->release_work);
>>> cgwb_release_workfn()
>>> wb = list_first_entry(&bdi->wb_list, ...)
>>> spin_unlock_irq(&cgwb_lock);
>>> wb_shutdown(wb);
>>> ...
>>> kfree_rcu(wb, rcu);
>>> wb_shutdown(wb); -> oops use-after-free
>>>
>>> We solve these issues by synchronizing writeback structure shutdown from
>>> cgwb_bdi_unregister() with cgwb_release_workfn() using a new mutex. That
>>> way we also no longer need synchronization using WB_shutting_down as the
>>> mutex provides it for CONFIG_CGROUP_WRITEBACK case and without
>>> CONFIG_CGROUP_WRITEBACK wb_shutdown() can be called only once from
>>> bdi_unregister().
>>>
>>> Reported-by: syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>
>>> Signed-off-by: Jan Kara <jack@suse.cz>
>>
>> Acked-by: Tejun Heo <tj@kernel.org>
>
> OK, Jens, can you please pick up the fix? Thanks!

Applied, thanks!

--
Jens Axboe