[PATCH] Bluetooth: L2CAP: cancel pending_rx_work before taking conn->lock

[Runyu Xiao]

l2cap_conn_del() takes conn->lock and then calls cancel_work_sync() for
pending_rx_work.  process_pending_rx() takes the same mutex, so teardown
can deadlock against the worker it is flushing.

This issue was found by our static analysis tool and then manually
reviewed against the current tree.

The grounded PoC kept the l2cap_conn_ready() -> queue_work(...,
&conn->pending_rx_work) submit path, the l2cap_conn_del() ->
cancel_work_sync(&conn->pending_rx_work) teardown path, and the
process_pending_rx() -> mutex_lock(&conn->lock) worker edge.  Lockdep
reported:

  WARNING: possible circular locking dependency detected
  process_pending_rx+0x21/0x2a [vuln_msv]
  l2cap_conn_del.constprop.0+0x3f/0x4e [vuln_msv]
  *** DEADLOCK ***

Cancel pending_rx_work before taking conn->lock, matching the existing
lock-before-drain ordering used for the two delayed works in the same
teardown path.  The pending_rx queue is still purged after the work has
been cancelled and conn->lock has been acquired.

Fixes: 7ab56c3a6ecc ("Bluetooth: Fix deadlock in l2cap_conn_del()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Runyu Xiao <runyu.xiao@xxxxxxxxxx>
---
 net/bluetooth/l2cap_core.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 29e23f20dc43..0dad72716cca 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1774,19 +1774,13 @@ static void l2cap_conn_del(struct hci_conn *hcon, int err)
 	disable_delayed_work_sync(&conn->info_timer);
 	disable_delayed_work_sync(&conn->id_addr_timer);
 
+	cancel_work_sync(&conn->pending_rx_work);
+
 	mutex_lock(&conn->lock);
 
 	kfree_skb(conn->rx_skb);
 
 	skb_queue_purge(&conn->pending_rx);
-
-	/* We can not call flush_work(&conn->pending_rx_work) here since we
-	 * might block if we are running on a worker from the same workqueue
-	 * pending_rx_work is waiting on.
-	 */
-	if (work_pending(&conn->pending_rx_work))
-		cancel_work_sync(&conn->pending_rx_work);
-
 	ida_destroy(&conn->tx_ida);
 
 	l2cap_unregister_all_users(conn);
-- 
2.34.1