Re: [PATCH] blk-mq: fix kernel panic during iterating over flush request

[John Garry <john.garry@huawei.com>]

On 11/08/2021 15:26, Ming Lei wrote:
> For fixing use-after-free during iterating over requests, we grabbed
> request's refcount before calling ->fn in commit 2e315dc07df0 ("blk-mq:
> grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter").
> Turns out this way may cause kernel panic when iterating over one flush
> request:
> 
> 1) old flush request's tag is just released, and this tag is reused by
> one new request, but ->rqs[] isn't updated yet
> 
> 2) the flush request can be re-used for submitting one new flush command,
> so blk_rq_init() is called at the same time
> 
> 3) meantime blk_mq_queue_tag_busy_iter() is called, and old flush request
> is retrieved from ->rqs[tag]; when blk_mq_put_rq_ref() is called,
> flush_rq->end_io may not be updated yet, so NULL pointer dereference
> is triggered in blk_mq_put_rq_ref().
> 
> Fix the issue by calling refcount_set(&flush_rq->ref, 1) after
> flush_rq->end_io is set. So far the only other caller of blk_rq_init() is
> scsi_ioctl_reset() in which the request doesn't enter block IO stack and
> the request reference count isn't used, so the change is safe.
> 
> Fixes: 2e315dc07df0 ("blk-mq: grab rq->refcount before calling ->fn in
> blk_mq_tagset_busy_iter")
> Reported-by: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
> Tested-by: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

it looks ok:

Reviewed-by: John Garry <john.garry@huawei.com>

> ---
>   block/blk-core.c  | 1 -
>   block/blk-flush.c | 8 ++++++++
>   2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 0874bc2fcdb4..b5098739f72a 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -121,7 +121,6 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
>   	rq->internal_tag = BLK_MQ_NO_TAG;
>   	rq->start_time_ns = ktime_get_ns();
>   	rq->part = NULL;
> -	refcount_set(&rq->ref, 1);
>   	blk_crypto_rq_set_defaults(rq);
>   }
>   EXPORT_SYMBOL(blk_rq_init);
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 1002f6c58181..4912c8dbb1d8 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -329,6 +329,14 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
>   	flush_rq->rq_flags |= RQF_FLUSH_SEQ;
>   	flush_rq->rq_disk = first_rq->rq_disk;
>   	flush_rq->end_io = flush_end_io;
> +	/*
> +	 * Order WRITE ->end_io and WRITE rq->ref, and its pair is the one
> +	 * implied in refcount_inc_not_zero() called from
> +	 * blk_mq_find_and_get_req(), which orders WRITE/READ flush_rq->ref
> +	 * and READ flush_rq->end_io
> +	 */
> +	smp_wmb();
> +	refcount_set(&flush_rq->ref, 1);