On Thu 03-05-18 18:26:26, Jan Kara wrote:
> Syzbot has reported that it can hit a NULL pointer dereference in
> wb_workfn() due to wb->bdi->dev being NULL. This indicates that
> wb_workfn() was called for an already unregistered bdi which should not
> happen as wb_shutdown() called from bdi_unregister() should make sure
> all pending writeback works are completed before bdi is unregistered.
> Except that wb_workfn() itself can requeue the work with:
>
> mod_delayed_work(bdi_wq, &wb->dwork, 0);
>
> and if this happens while wb_shutdown() is waiting in:
>
> flush_delayed_work(&wb->dwork);
>
> the dwork can get executed after wb_shutdown() has finished and
> bdi_unregister() has cleared wb->bdi->dev.
>
> Make wb_workfn() use wakeup_wb() for requeueing the work which takes all
> the necessary precautions against racing with bdi unregistration.
>
> CC: Tetsuo Handa <penguin-ker...@i-love.sakura.ne.jp>
> CC: Tejun Heo <t...@kernel.org>
> Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
> Reported-by: syzbot <syzbot+9873874c735f2892e...@syzkaller.appspotmail.com>
> Signed-off-by: Jan Kara <j...@suse.cz>
> ---
> fs/fs-writeback.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
Jens, can you please pick up this patch? Probably for the next merge window
(I don't see a reason to rush this at this point in release cycle). Thanks!
Honza
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 47d7c151fcba..471d863958bc 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1961,7 +1961,7 @@ void wb_workfn(struct work_struct *work)
> }
>
> if (!list_empty(&wb->work_list))
> - mod_delayed_work(bdi_wq, &wb->dwork, 0);
> + wb_wakeup(wb);
> else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
> wb_wakeup_delayed(wb);
>
> --
> 2.13.6
>
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
