Re: [patch v8 7/7] nbd: fix uaf in nbd_handle_reply()

Ming Lei
2021-09-16T12:58:26Z

On Thu, Sep 16, 2021 at 05:33:50PM +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@huawei.com>
> ---
> drivers/block/nbd.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 69dc5eac9ad3..b3a47fc6237f 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -825,6 +825,7 @@ 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;
> @@ -835,7 +836,20 @@ static void recv_work(struct work_struct *work)
> if (nbd_read_reply(nbd, args->index, &reply))
> break;
>
> + /*
> + * Grab .q_usage_counter so request pool won't go away, then no
> + * request use-after-free is possible during nbd_handle_reply().
> + * If queue is frozen, there won't be any inflight requests, we
> + * needn't to handle the incoming garbage message.
> + */
> + 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);
> + percpu_ref_put(&q->q_usage_counter);
> if (IS_ERR(cmd))
> break;

The refcount needs to be grabbed when completing the request because
the request may be completed from other code path, then the request pool
will be freed from that code path when the request is referred.

Thanks,
Ming