Re: [PATCH V1] block: Fix use-after-free while iterating over requests

[John Garry]

On 26/11/2020 16:27, Bart Van Assche wrote:
> On 11/26/20 7:02 AM, Pradeep P V K wrote:
> > Observes below crash while accessing (use-after-free) request queue
> > member of struct request.
> >
> > 191.784789:   <2> Unable to handle kernel paging request at virtual
> > address ffffff81429a4440
> > ...
> > 191.786174:   <2> CPU: 3 PID: 213 Comm: kworker/3:1H Tainted: G S
> > O      5.4.61-qgki-debug-ge45de39 #1
> > ...
> > 191.786226:   <2> Workqueue: kblockd blk_mq_timeout_work
> > 191.786242:   <2> pstate: 20c00005 (nzCv daif +PAN +UAO)
> > 191.786261:   <2> pc : bt_for_each+0x114/0x1a4
> > 191.786274:   <2> lr : bt_for_each+0xe0/0x1a4
> > ...
> > 191.786494:   <2> Call trace:
> > 191.786507:   <2>  bt_for_each+0x114/0x1a4
> > 191.786519:   <2>  blk_mq_queue_tag_busy_iter+0x60/0xd4
> > 191.786532:   <2>  blk_mq_timeout_work+0x54/0xe8
> > 191.786549:   <2>  process_one_work+0x2cc/0x568
> > 191.786562:   <2>  worker_thread+0x28c/0x518
> > 191.786577:   <2>  kthread+0x160/0x170
> > 191.786594:   <2>  ret_from_fork+0x10/0x18
> > 191.786615:   <2> Code: 0b080148 f9404929 f8685921 b4fffe01 (f9400028)
> > 191.786630:   <2> ---[ end trace 0f1f51d79ab3f955 ]---
> > 191.786643:   <2> Kernel panic - not syncing: Fatal exception
> >
> > Fix this by updating the freed request with NULL.
> > This could avoid accessing the already free request from other
> > contexts while iterating over the requests.
> >
> > Signed-off-by: Pradeep P V K <ppvk@xxxxxxxxxxxxxx>
> > ---
> >   block/blk-mq.c | 1 +
> >   block/blk-mq.h | 1 +
> >   2 files changed, 2 insertions(+)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 55bcee5..9996cb1 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -492,6 +492,7 @@ static void __blk_mq_free_request(struct request *rq)
> >   
> >   	blk_crypto_free_request(rq);
> >   	blk_pm_mark_last_busy(rq);
> > +	hctx->tags->rqs[rq->tag] = NULL;
> >   	rq->mq_hctx = NULL;
> >   	if (rq->tag != BLK_MQ_NO_TAG)
> >   		blk_mq_put_tag(hctx->tags, ctx, rq->tag);
> > diff --git a/block/blk-mq.h b/block/blk-mq.h
> > index a52703c..8747bf1 100644
> > --- a/block/blk-mq.h
> > +++ b/block/blk-mq.h
> > @@ -224,6 +224,7 @@ static inline int __blk_mq_active_requests(struct blk_mq_hw_ctx *hctx)
> >   static inline void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
> >   					   struct request *rq)
> >   {
> > +	hctx->tags->rqs[rq->tag] = NULL;
> >   	blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag);
> >   	rq->tag = BLK_MQ_NO_TAG;
>
> Is this perhaps a block driver bug instead of a block layer core bug? If
> this would be a block layer core bug, it would have been reported before.

Isn't this the same issue which as been reported many times:

https://lore.kernel.org/linux-block/20200820180335.3109216-1-ming.lei@xxxxxxxxxx/

https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@xxxxxxxxxx/

But I never saw a crash, just kasan report.

Thanks,
John