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

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

On Thu, Sep 16, 2021 at 04:47:08PM +0800, yukuai (C) wrote:
> 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@huawei.com>
> > > ---
> > >   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 ?

How about the following words?

/*
 * 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
 */

Thanks,
Ming