Re: [PATCH V6 0/4] blk-mq: fix request UAF related with iterating over tagset requests

Shinichiro Kawasaki
2021-05-11T12:23:29Z

On May 11, 2021 / 14:36, Ming Lei wrote:
> Hello Shinichiro,
>
> Thanks for your test!
>
> On Tue, May 11, 2021 at 05:05:52AM +0000, Shinichiro Kawasaki wrote:
> > On May 07, 2021 / 22:42, Ming Lei wrote:
> > > Hi Jens,
> > >
> > > This patchset fixes the request UAF issue by one simple approach,
> > > without clearing ->rqs[] in fast path, please consider it for 5.13.
> > >
> > > 1) grab request's ref before calling ->fn in blk_mq_tagset_busy_iter,
> > > and release it after calling ->fn, so ->fn won't be called for one
> > > request if its queue is frozen, done in 2st patch
> > >
> > > 2) clearing any stale request referred in ->rqs[] before freeing the
> > > request pool, one per-tags spinlock is added for protecting
> > > grabbing request ref vs. clearing ->rqs[tag], so UAF by refcount_inc_not_zero
> > > in bt_tags_iter() is avoided, done in 3rd patch.
> >
> > Ming, thank you for your effort to fix the UAF issue. I applied the V6 series to
> > the kernel v5.13-rc1, and confirmed that the series avoids the UAF I had been
> > observing with blktests block/005 and HDD behind HBA. This is good. However, I
> > found that the series triggered block/029 hang. Let me share the kernel message
> > below, which was printed at the hang. KASAN reported null-ptr-deref.
> >
> > [ 2124.489023] run blktests block/029 at 2021-05-11 13:42:22
> > [ 2124.561386] null_blk: module loaded
> > [ 2125.201166] general protection fault, probably for non-canonical address 0xdffffc0000000012: 0000 [#1] SMP KASAN PTI
> > [ 2125.212387] KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097]
>
> It is because this hw queue isn't mapped yet and new added hw queue is
> mapped in blk_mq_map_swqueue(), and the following change can fix it, and
> I will post V7 after careful test.
>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index fcd5ed79011f..691b555c26fa 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2652,6 +2652,10 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
> int i;
> unsigned long flags;
>
> + /* return if hw queue isn't mapped */
> + if (!tags)
> + return;
> +
> WARN_ON_ONCE(refcount_read(&flush_rq->ref) != 0);
>
> for (i = 0; i < queue_depth; i++)

Thank you Ming. I confirmed that this change avoids the hang at block/029.

--
Best Regards,
Shin'ichiro Kawasaki