On Wed, Nov 6, 2024 at 11:52 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> 在 2024/11/06 11:36, Ming Lei 写道:
> > On Wed, Nov 06, 2024 at 11:25:07AM +0800, Yu Kuai wrote:
> >> Hi,
> >>
> >> 在 2024/11/06 11:11, Ming Lei 写道:
> >>> On Wed, Nov 06, 2024 at 10:58:40AM +0800, Yu Kuai wrote:
> >>>> Hi,Ming and Christoph
> >>>>
> >>>> 在 2024/11/05 19:19, Christoph Hellwig 写道:
> >>>>> On Mon, Nov 04, 2024 at 07:00:05PM +0800, Yu Kuai wrote:
> >>>>>> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >>>>>>
> >>>>>> blk_mq_clear_flush_rq_mapping() is not called during scsi probe, by
> >>>>>> checking blk_queue_init_done(). However, QUEUE_FLAG_INIT_DONE is cleared
> >>>>>> in del_gendisk by commit aec89dc5d421 ("block: keep q_usage_counter in
> >>>>>> atomic mode after del_gendisk"), hence for disk like scsi, following
> >>>>>> blk_mq_destroy_queue() will not clear flush rq from tags->rqs[] as well,
> >>>>>> cause following uaf that is found by our syzkaller for v6.6:
> >>>>>
> >>>>> Which means we leave the flush request lingering after del_gendisk,
> >>>>> which sounds like the real bug. I suspect we just need to move the
> >>>>> call to blk_mq_clear_flush_rq_mapping so that it is called from
> >>>>> del_gendisk and doesn't leave the flush tag lingering around.
> >>>>>
> >>>>
> >>>> This remind me that del_gendisk is still too late to do that. Noted that
> >>>> flush_rq can acquire different tags, so if the multiple flush_rq is done
> >>>> and those tags are not reused, the flush_rq can exist in multiple
> >>>> entries in tags->rqs[]. The consequence I can think of is that iterating
> >>>> tags can found the same flush_rq multiple times, and the flush_rq can be
> >>>> inflight.
> >>>
> >>> How can that be one problem?
> >>>
> >>> Please look at
> >>>
> >>> commit 364b61818f65 ("blk-mq: clearing flush request reference in tags->rqs[]")
> >>> commit bd63141d585b ("blk-mq: clear stale request in tags->rq[] before freeing one request pool")
> >>>
> >>> and understand the motivation.
> >>>
> >>> That also means it is just fine to delay blk_mq_clear_flush_rq_mapping()
> >>> after disk is deleted.
> >>
> >> I do understand what you mean. Let's see if you want this to be avoided,
> >> for example(no disk is deleted):
> >
> > It is definitely another issue, and not supposed to be covered by
> > blk_mq_clear_flush_rq_mapping().
> >
> >>
> >> 1) issue a flush, and tag 0 is used, after the flush is done,
> >> tags->rqs[0] = flush_rq
> >> 2) issue another flush, and tag 1 is used, after the flush is done,
> >> tags->rqs[1] = flush_rq
> >> 3) issue a flush again, and tag 2 is used, and the flush_rq is
> >> dispatched to disk;
> >
> > Yes, this kind of thing exists since blk-mq begins, and you can't expect
> > blk_mq_in_flight_rw() to get accurate inflight requests.
> >
> >> 4) Then in this case, blk_mq_in_flight_rw() will found the same flush_rq
> >> 3 times and think there are 3 inflight request, same for
> >> hctx_busy_show()...
> >
> > But we have tried to avoid it, such as in blk_mq_find_and_get_req()
> > req->tag is checked against the current 'bitnr' of sbitmap when walking
> > over tags. Then the same flush_rq won't be counted 3 times.
>
> Yes, I missed that req->tag is checked. Looks like there is no porblem
> now. :)
>
> Just to make sure, we're still on the same page for this patch, right?
Yeah, my reviewed-by still stands, and it is fine to delay to clear flush req
wrt. the original motivation, what matters is that it is cleared before the
flush req is freed.
Thanks,
