linux-kernel - [PATCH] nbd: defer config put in recv

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Message-Id: <20251108070202.1816004-1-zhengqixing@huaweicloud.com>
Date: Sat,  8 Nov 2025 15:02:02 +0800
From: Zheng Qixing <zhengqixing@...weicloud.com>
To: josef@...icpanda.com,
	axboe@...nel.dk,
	xiubli@...hat.com
Cc: linux-block@...r.kernel.org,
	nbd@...er.debian.org,
	linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	houtao1@...wei.com,
	linan122@...artners.com,
	zhengqixing@...wei.com
Subject: [PATCH] nbd: defer config put in recv_work

From: Zheng Qixing <zhengqixing@...wei.com>

There is one uaf issue in recv_work when running NBD_CLEAR_SOCK and
NBD_CMD_RECONFIGURE:
  nbd_genl_connect     // conf_ref=2 (connect and recv_work A)
  nbd_open	       // conf_ref=3
  recv_work A done     // conf_ref=2
  NBD_CLEAR_SOCK       // conf_ref=1
  nbd_genl_reconfigure // conf_ref=2 (trigger recv_work B)
  close nbd	       // conf_ref=1
  recv_work B
    config_put         // conf_ref=0
    atomic_dec(&config->recv_threads); -> UAF

Or only running NBD_CLEAR_SOCK:
  nbd_genl_connect   // conf_ref=2
  nbd_open 	     // conf_ref=3
  NBD_CLEAR_SOCK     // conf_ref=2
  close nbd
    nbd_release
      config_put     // conf_ref=1
  recv_work
    config_put 	     // conf_ref=0
    atomic_dec(&config->recv_threads); -> UAF

Commit 87aac3a80af5 ("nbd: call nbd_config_put() before notifying the
waiter") moved nbd_config_put() to run before waking up the waiter in
recv_work, in order to ensure that nbd_start_device_ioctl() would not
be woken up while nbd->task_recv was still uncleared.

However, in nbd_start_device_ioctl(), after being woken up it explicitly
calls flush_workqueue() to make sure all current works are finished.
Therefore, there is no need to move the config put ahead of the wakeup.

Move nbd_config_put() to the end of recv_work, so that the reference is
held for the whole lifetime of the worker thread. This makes sure the
config cannot be freed while recv_work is still running, even if clear
+ reconfigure interleave.

In addition, we don't need to worry about recv_work dropping the last
nbd_put (which causes deadlock):

path A (netlink with NBD_CFLAG_DESTROY_ON_DISCONNECT):
  connect  // nbd_refs=1 (trigger recv_work)
  open nbd // nbd_refs=2
  NBD_CLEAR_SOCK
  close nbd
    nbd_release
      nbd_disconnect_and_put
        flush_workqueue // recv_work done
      nbd_config_put
        nbd_put // nbd_refs=1
      nbd_put // nbd_refs=0
        queue_work

path B (netlink without NBD_CFLAG_DESTROY_ON_DISCONNECT):
  connect  // nbd_refs=2 (trigger recv_work)
  open nbd // nbd_refs=3
  NBD_CLEAR_SOCK // conf_refs=2
  close nbd
    nbd_release
      nbd_config_put // conf_refs=1
      nbd_put // nbd_refs=2
  recv_work done // conf_refs=0, nbd_refs=1
  rmmod // nbd_refs=0

Reported-by: syzbot+56fbf4c7ddf65e95c7cc@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/6907edce.a70a0220.37351b.0014.GAE@google.com/T/
Fixes: 87aac3a80af5 ("nbd: make the config put is called before the notifying the waiter")
Depends-on: e2daec488c57 ("nbd: Fix hungtask when nbd_config_put")
Signed-off-by: Zheng Qixing <zhengqixing@...wei.com>
---
 drivers/block/nbd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index a853c65ac65d..215fc18115b7 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1024,9 +1024,9 @@ static void recv_work(struct work_struct *work)
 	nbd_mark_nsock_dead(nbd, nsock, 1);
 	mutex_unlock(&nsock->tx_lock);
 
-	nbd_config_put(nbd);
 	atomic_dec(&config->recv_threads);
 	wake_up(&config->recv_wq);
+	nbd_config_put(nbd);
 	kfree(args);
 }
 
-- 
2.39.2