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

Yu Kuai
2021-09-16T09:24:20Z

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;

--
2.31.1