Re: [PATCH v5 5/6] nbd: convert to use blk_mq_find_and_get_req()
From: yukuai (C)
Date: Tue Sep 14 2021 - 05:08:05 EST
On 2021/09/14 15:46, Ming Lei wrote:
On Tue, Sep 14, 2021 at 03:13:38PM +0800, yukuai (C) wrote:
On 2021/09/14 14:44, Ming Lei wrote:
On Tue, Sep 14, 2021 at 11:11:06AM +0800, yukuai (C) wrote:
On 2021/09/14 9:11, Ming Lei wrote:
On Thu, Sep 09, 2021 at 10:12:55PM +0800, Yu Kuai wrote:
blk_mq_tag_to_rq() can only ensure to return valid request in
following situation:
1) client send request message to server first
submit_bio
...
blk_mq_get_tag
...
blk_mq_get_driver_tag
...
nbd_queue_rq
nbd_handle_cmd
nbd_send_cmd
2) client receive respond message from server
recv_work
nbd_read_stat
blk_mq_tag_to_rq
If step 1) is missing, blk_mq_tag_to_rq() will return a stale
request, which might be freed. Thus convert to use
blk_mq_find_and_get_req() to make sure the returned request is not
freed.
But NBD_CMD_INFLIGHT has been added for checking if the reply is
expected, do we still need blk_mq_find_and_get_req() for covering
this issue? BTW, request and its payload is pre-allocated, so there
isn't real use-after-free.
Hi, Ming
Checking NBD_CMD_INFLIGHT relied on the request founded by tag is valid,
not the other way round.
nbd_read_stat
req = blk_mq_tag_to_rq()
cmd = blk_mq_rq_to_pdu(req)
mutex_lock(cmd->lock)
checking NBD_CMD_INFLIGHT
Request and its payload is pre-allocated, and either req->ref or cmd->lock can
serve the same purpose here. Once cmd->lock is held, you can check if the cmd is
inflight or not. If it isn't inflight, just return -ENOENT. Is there any
problem to handle in this way?
Hi, Ming
in nbd_read_stat:
1) get a request by tag first
2) get nbd_cmd by the request
3) hold cmd->lock and check if cmd is inflight
If we want to check if the cmd is inflight in step 3), we have to do
setp 1) and 2) first. As I explained in patch 0, blk_mq_tag_to_rq()
can't make sure the returned request is not freed:
nbd_read_stat
blk_mq_sched_free_requests
blk_mq_free_rqs
blk_mq_tag_to_rq
-> get rq before clear mapping
blk_mq_clear_rq_mapping
__free_pages -> rq is freed
blk_mq_request_started -> UAF
If the above can happen, blk_mq_find_and_get_req() may not fix it too, just
Hi, Ming
Why can't blk_mq_find_and_get_req() fix it? I can't think of any
scenario that might have problem currently.
wondering why not take the following simpler way for avoiding the UAF?
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 5170a630778d..dfa5cce71f66 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -795,9 +795,13 @@ 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_cmd *cmd;
struct request *rq;
+ if (!percpu_ref_tryget(&q->q_usage_counter))
+ return;
+
We can't make sure freeze_queue is called before this, thus this approch
can't fix the problem, right?
nbd_read_stat
blk_mq_tag_to_rq
elevator_switch
blk_mq_freeze_queue(q);
elevator_switch_mq
elevator_exit
blk_mq_sched_free_requests
blk_mq_request_started -> UAF
Thanks,
Kuai
while (1) {
cmd = nbd_read_stat(nbd, args->index);
if (IS_ERR(cmd)) {
@@ -813,6 +817,7 @@ static void recv_work(struct work_struct *work)
if (likely(!blk_should_fake_timeout(rq->q)))
blk_mq_complete_request(rq);
}
+ blk_queue_exit(q);
nbd_config_put(nbd);
atomic_dec(&config->recv_threads);
wake_up(&config->recv_wq);
Thanks,
Ming
.