Re: [PATCH v7 6/6] nbd: fix uaf in nbd_handle_reply()

[Ming Lei]

On Wed, Sep 15, 2021 at 05:20:10PM +0800, Yu Kuai wrote:
> There is a problem that nbd_handle_reply() might access freed request:
> 
> 1) At first, a normal io is submitted and completed with scheduler:
> 
> internel_tag = blk_mq_get_tag -> get tag from sched_tags
>  blk_mq_rq_ctx_init
>   sched_tags->rq[internel_tag] = sched_tag->static_rq[internel_tag]
> ...
> blk_mq_get_driver_tag
>  __blk_mq_get_driver_tag -> get tag from tags
>  tags->rq[tag] = sched_tag->static_rq[internel_tag]
> 
> So, both tags->rq[tag] and sched_tags->rq[internel_tag] are pointing
> to the request: sched_tags->static_rq[internal_tag]. Even if the
> io is finished.
> 
> 2) nbd server send a reply with random tag directly:
> 
> recv_work
>  nbd_handle_reply
>   blk_mq_tag_to_rq(tags, tag)
>    rq = tags->rq[tag]
> 
> 3) if the sched_tags->static_rq is freed:
> 
> blk_mq_sched_free_requests
>  blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i)
>   -> step 2) access rq before clearing rq mapping
>   blk_mq_clear_rq_mapping(set, tags, hctx_idx);
>   __free_pages() -> rq is freed here
> 
> 4) Then, nbd continue to use the freed request in nbd_handle_reply
> 
> Fix the problem by get 'q_usage_counter' before blk_mq_tag_to_rq(),
> thus request is ensured not to be freed because 'q_usage_counter' is
> not zero.
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/block/nbd.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 9a7bbf8ebe74..3e8b70b5d4f9 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -824,6 +824,7 @@ static void recv_work(struct work_struct *work)
>  						     work);
>  	struct nbd_device *nbd = args->nbd;
>  	struct nbd_config *config = nbd->config;
> +	struct request_queue *q = nbd->disk->queue;
>  	struct nbd_sock *nsock;
>  	struct nbd_cmd *cmd;
>  	struct request *rq;
> @@ -834,7 +835,24 @@ static void recv_work(struct work_struct *work)
>  		if (nbd_read_reply(nbd, args->index, &reply))
>  			break;
>  
> +		/*
> +		 * Grab ref of q_usage_counter can prevent request being freed
> +		 * during nbd_handle_reply(). If q_usage_counter is zero, then
> +		 * no request is inflight, which means something is wrong since
> +		 * we expect to find a request to complete here.
> +		 */

The above comment is wrong, the purpose is simply for avoiding request
pool freed, such as elevator switching won't happen once
->q_usage_counter is grabbed. So no any request UAF can be triggered
when calling into nbd_handle_reply().

> +		if (!percpu_ref_tryget(&q->q_usage_counter)) {
> +			dev_err(disk_to_dev(nbd->disk), "%s: no io inflight\n",
> +				__func__);
> +			break;
> +		}
> +
>  		cmd = nbd_handle_reply(nbd, args->index, &reply);
> +		/*
> +		 * It's safe to drop ref before request completion, inflight
> +		 * request will ensure q_usage_counter won't be zero.
> +		 */

The above comment is useless actually.

-- 
Ming