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nbd: fix uaf in nbd_handle_reply()

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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>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210916141810.2325276-1-yukuai3@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Yu Kuai 2021-09-16 22:18:10 +08:00 committed by Jens Axboe
parent 3fe1db626a
commit 8663b210f8
1 changed files with 18 additions and 2 deletions
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20
drivers/block/nbd.c
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@ -830,6 +830,7 @@ 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;
@ -840,13 +841,28 @@ static void recv_work(struct work_struct *work)
if (nbd_read_reply(nbd, args->index, &reply))
break;
cmd = nbd_handle_reply(nbd, args->index, &reply);
if (IS_ERR(cmd))
/*
* 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);
if (IS_ERR(cmd)) {
percpu_ref_put(&q->q_usage_counter);
break;
}
rq = blk_mq_rq_from_pdu(cmd);
if (likely(!blk_should_fake_timeout(rq->q)))
blk_mq_complete_request(rq);
percpu_ref_put(&q->q_usage_counter);
}
nsock = config->socks[args->index];