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

[Jens Axboe <axboe@kernel.dk>]

On 5/3/18 3:55 PM, Dave Chinner wrote:
> 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...

That's always bothered me too, it's due for a cleanup and bringing it
all into one location.

-- 
Jens Axboe