Re: [PATCH] block: fix null pointer dereference in blk_mq_rq_timed_out()

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

On Sat, Sep 07, 2019 at 06:24:50PM +0800, Yufen Yu wrote:
> There is a race condition between timeout check and completion for
> flush request as follow:
> 
> timeout_work    issue flush      issue flush
>                 blk_insert_flush
>                                  blk_insert_flush
> blk_mq_timeout_work
>                 blk_kick_flush
> 
> blk_mq_queue_tag_busy_iter
> blk_mq_check_expired(flush_rq)
> 
>                 __blk_mq_end_request
>                flush_end_io
>                blk_kick_flush
>                blk_rq_init(flush_rq)
>                memset(flush_rq, 0)

Not see there is memset(flush_rq, 0) in block/blk-flush.c

> 
> blk_mq_timed_out
> BUG_ON flush_rq->q->mq_ops

flush_rq->q won't be changed by blk_rq_init(), and either READ or WRITE
on variable with machine WORD length are atomic, so how can the BUG_ON()
be triggered? Do you have the actual BUG log?

Also now it is driver's responsibility for avoiding race between normal
completion and timeout.

> 
> For normal request, we need to get a tag and then allocate corresponding request.
> Thus, the request cannot be reallocated before the tag have been free.
> Commit 1d9bd5161ba ("blk-mq: replace timeout synchronization with a RCU and
> generation based scheme") and commit 12f5b93145 ("blk-mq: Remove generation
> seqeunce") can guarantee the consistency of timeout handle and completion.
> 
> However, 'flush_rq' have been forgotten. 'flush_rq' allocation management
> dependents on flush implemention mechanism. Each hctx has only one 'flush_rq'.
> When a flush request have completed, the next flush request will hold the 'flush_rq'.
> In the end, timeout handle may access the cleared 'flush_rq'.
> 
> We fix this problem by checking request refcount 'rq->ref', as normal request.
> If the refcount is not zero, flush_end_io() return and wait the last holder
> recall it. To record the request status, we add a new entry 'rq_status',
> which will be used in flush_end_io().
> 
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>
> ---
>  block/blk-flush.c      | 8 ++++++++
>  block/blk-mq.c         | 7 +++++--
>  block/blk.h            | 5 +++++
>  include/linux/blkdev.h | 2 ++
>  4 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index aedd9320e605..359a7e1a0925 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -212,6 +212,14 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
>  	struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);
>  	struct blk_mq_hw_ctx *hctx;
>  
> +	if (!refcount_dec_and_test(&flush_rq->ref)) {
> +		flush_rq->rq_status = error;
> +		return;
> +	}
> +
> +	if (flush_rq->rq_status != BLK_STS_OK)
> +		error = flush_rq->rq_status;
> +
>  	/* release the tag's ownership to the req cloned from */
>  	spin_lock_irqsave(&fq->mq_flush_lock, flags);
>  	hctx = flush_rq->mq_hctx;
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 0835f4d8d42e..3d2b2c2e9cdf 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -905,9 +905,12 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
>  	 */
>  	if (blk_mq_req_expired(rq, next))
>  		blk_mq_rq_timed_out(rq, reserved);
> -	if (refcount_dec_and_test(&rq->ref))
> -		__blk_mq_free_request(rq);
>  
> +	if (is_flush_rq(rq, hctx)) {
> +		rq->end_io(rq, 0);
> +	} else if (refcount_dec_and_test(&rq->ref)) {
> +		__blk_mq_free_request(rq);
> +	}
>  	return true;
>  }
>  
> diff --git a/block/blk.h b/block/blk.h
> index de6b2e146d6e..f503ef9ad3e6 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -47,6 +47,11 @@ static inline void __blk_get_queue(struct request_queue *q)
>  	kobject_get(&q->kobj);
>  }
>  
> +static inline bool
> +is_flush_rq(struct request *req, struct blk_mq_hw_ctx *hctx) {
> +	return hctx->fq->flush_rq == req;
> +}
> +
>  struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
>  		int node, int cmd_size, gfp_t flags);
>  void blk_free_flush_queue(struct blk_flush_queue *q);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 1ef375dafb1c..b1d05077e03f 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -237,6 +237,8 @@ struct request {
>  	 */
>  	rq_end_io_fn *end_io;
>  	void *end_io_data;
> +
> +	blk_status_t rq_status;
>  };

'rq_status' is only for flush request, so it may be added to 'struct
blk_flush_queue' instead of 'struct request'.


Thanks,
Ming