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Subject: Re: [PATCH V1] block: Fix use-after-free while iterating over
 requests
To:     Pradeep P V K <ppvk@codeaurora.org>, axboe@kernel.dk,
        linux-block@vger.kernel.org
Cc:     stummala@codeaurora.org, linux-kernel@vger.kernel.org
References: <1606402925-24420-1-git-send-email-ppvk@codeaurora.org>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <c94fcada-7f6d-a1e3-4c88-d225af1a676e@acm.org>
Date:   Thu, 26 Nov 2020 08:27:45 -0800
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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@codeaurora.org>
> ---
>  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.

Bart.