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

[yukuai (C)]

On 2021/09/16 16:04, Ming Lei wrote:
> 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().

Do you mean the comment about q_usage_counter is zero is wrong ?
> > +		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.

Will remove the comments.

Thanks,
Kuai
>