Re: [RFC PATCH] blk-mq: Clean up references when freeing rqs

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

On Tue, Dec 01, 2020 at 09:02:18PM +0800, John Garry wrote:
> It has been reported many times that a use-after-free can be intermittently
> found when iterating busy requests:
> 
> - https://lore.kernel.org/linux-block/8376443a-ec1b-0cef-8244-ed584b96fa96@huawei.com/
> - https://lore.kernel.org/linux-block/5c3ac5af-ed81-11e4-fee3-f92175f14daf@acm.org/T/#m6c1ac11540522716f645d004e2a5a13c9f218908
> - https://lore.kernel.org/linux-block/04e2f9e8-79fa-f1cb-ab23-4a15bf3f64cc@kernel.dk/
> 
> The issue is that when we switch scheduler or change queue nr_requests,
> the driver tagset may keep references to the stale requests.
> 
> As a solution, clean up any references to those requests in the driver
> tagset when freeing. This is done with a cmpxchg to make safe any race
> with setting the driver tagset request from another queue.
> 
> Signed-off-by: John Garry <john.garry@huawei.com>
> --
> Set as RFC as I need to test more. And not sure on solution method, as
> Bart had another idea.
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index d1eafe2c045c..9b042c7036b3 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -621,7 +621,7 @@ void blk_mq_sched_free_requests(struct request_queue *q)
>  
>  	queue_for_each_hw_ctx(q, hctx, i) {
>  		if (hctx->sched_tags)
> -			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
> +			blk_mq_free_rqs_ext(q->tag_set, hctx->sched_tags, i, hctx->tags);
>  	}
>  }
>  
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 9c92053e704d..562db72e7d79 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -576,7 +576,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>  			return -ENOMEM;
>  		}
>  
> -		blk_mq_free_rqs(set, *tagsptr, hctx->queue_num);
> +		blk_mq_free_rqs_ext(set, *tagsptr, hctx->queue_num, hctx->tags);
>  		blk_mq_free_rq_map(*tagsptr, flags);
>  		*tagsptr = new;
>  	} else {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 55bcee5dc032..f3aad695cd25 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2271,8 +2271,8 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
>  	return BLK_QC_T_NONE;
>  }
>  
> -void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> -		     unsigned int hctx_idx)
> +void blk_mq_free_rqs_ext(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
> +		     unsigned int hctx_idx, struct blk_mq_tags *references)
>  {
>  	struct page *page;
>  
> @@ -2281,10 +2281,13 @@ void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
>  
>  		for (i = 0; i < tags->nr_tags; i++) {
>  			struct request *rq = tags->static_rqs[i];
> +			int j;
>  
>  			if (!rq)
>  				continue;
>  			set->ops->exit_request(set, rq, hctx_idx);
> +			for (j = 0; references && j < references->nr_tags; j++)
> +				cmpxchg(&references->rqs[j], rq, 0);

Seems you didn't address the comment in the following link:

	https://lore.kernel.org/linux-block/10331543-9e45-ae63-8cdb-17e5a2a3b7ef@huawei.com/

The request to be freed may still be refered in another path, such as blk_mq_queue_tag_busy_iter
or blk_mq_tagset_busy_iter(), and cmpxchg() doesn't drain/wait for other refers.

Thanks,
Ming