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

From: Dave Chinner <david@fromorbit.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] bdi: Fix oops in wb_workfn()
Date: Fri, 4 May 2018 07:55:58 +1000	[thread overview]
Message-ID: <20180503215558.GB10363@dastard> (raw)
In-Reply-To: <20180503162626.27753-1-jack@suse.cz>

On Thu, May 03, 2018 at 06:26:26PM +0200, 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-kernel@I-love.SAKURA.ne.jp>
> CC: Tejun Heo <tj@kernel.org>
> Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
> Reported-by: syzbot <syzbot+9873874c735f2892e7e9@syzkaller.appspotmail.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/fs-writeback.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 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);

Yup, looks fine - I can't see any more of these open coded wakeup,
either, so we should be good here.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

As an aside, why is half the wb infrastructure in fs/fs-writeback.c
and the other half in mm/backing-dev.c? it seems pretty random as to
what is where e.g. wb_wakeup() and wb_wakeup_delayed() are almost
identical, but are in completely different files...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com