Re: [PATCH] block: don't dereference request after flush insertion

[Ming Lei <ming.lei@redhat.com>]

On Sun, Oct 17, 2021 at 08:16:25PM -0600, Jens Axboe wrote:
> On 10/17/21 8:11 PM, Ming Lei wrote:
> > On Mon, Oct 18, 2021 at 10:02:32AM +0800, Ming Lei wrote:
> >> On Sun, Oct 17, 2021 at 07:50:24PM -0600, Jens Axboe wrote:
> >>> On 10/17/21 7:49 PM, Ming Lei wrote:
> >>>> On Sat, Oct 16, 2021 at 07:35:39PM -0600, Jens Axboe wrote:
> >>>>> We could have a race here, where the request gets freed before we call
> >>>>> into blk_mq_run_hw_queue(). If this happens, we cannot rely on the state
> >>>>> of the request.
> >>>>>
> >>>>> Grab the hardware context before inserting the flush.
> >>>>>
> >>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>>
> >>>>> ---
> >>>>>
> >>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
> >>>>> index 2197cfbf081f..22b30a89bf3a 100644
> >>>>> --- a/block/blk-mq.c
> >>>>> +++ b/block/blk-mq.c
> >>>>> @@ -2468,9 +2468,10 @@ void blk_mq_submit_bio(struct bio *bio)
> >>>>>  	}
> >>>>>  
> >>>>>  	if (unlikely(is_flush_fua)) {
> >>>>> +		struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> >>>>>  		/* Bypass scheduler for flush requests */
> >>>>>  		blk_insert_flush(rq);
> >>>>> -		blk_mq_run_hw_queue(rq->mq_hctx, true);
> >>>>> +		blk_mq_run_hw_queue(hctx, true);
> >>>>
> >>>> If the request is freed before running queue, the request queue could
> >>>> be released and the hctx may be freed.
> >>>
> >>> No, we still hold a queue enter ref.
> >>
> >> But that one is released when rq is freed since ac7c5675fa45 ("blk-mq: allow
> >> blk_mq_make_request to consume the q_usage_counter reference"), isn't
> >> it?
> > 
> > With commit ac7c5675fa45, any reference to hctx after queuing request could
> > lead to UAF in the code path of blk_mq_submit_bio(). Maybe we need to grab
> > two ref in queue enter, and release one after the bio is submitted.
> 
> I'd rather audit and see if there are any, because extra get+put isn't
> exactly free.

Only direct issue needn't that, looks others do need grab one extra
ref if the request has to be queued somewhere before dispatch.


Thanks,
Ming