rfcomm_sock_cleanup_listen() closes unaccepted child sockets through
rfcomm_sock_close(), which takes the child socket lock before
rfcomm_dlc_close() acquires rfcomm_mutex. The RFCOMM worker takes these
locks in reverse order while handling connections and DLC state changes,
so lockdep reports a possible deadlock.

Close dequeued children without taking their socket lock. The accept queue
owns a reference to each child, and bt_accept_dequeue() locks the child
while unlinking it and clearing its parent pointer.

Dropping the child lock makes it important to prevent a concurrent
rfcomm_connect_ind() from enqueueing a new child after cleanup observes an
empty queue. Set a listening socket to BT_CLOSED while its lock is still
held, before dropping the lock and draining the queue. The state check in
rfcomm_connect_ind() then rejects new children once cleanup starts.

Reported-by: syzbot+0cece8fa7d83523f47a3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=0cece8fa7d83523f47a3
Fixes: b7ce436a5d79 ("Bluetooth: switch to lock_sock in RFCOMM")
Signed-off-by: Juan Perdomo <jcperdomo100@gmail.com>
---
Changes in v2:
- Mark the listener BT_CLOSED under its socket lock before accept-queue
  cleanup, preventing new children from racing with teardown.
- Retest with syzbot after its v1 run exposed the accept-queue race. The
  revised patch completed without triggering an issue:
  https://syzkaller.appspot.com/x/bisect.txt?x=156ca925580000

 net/bluetooth/rfcomm/sock.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/bluetooth/rfcomm/sock.c b/net/bluetooth/rfcomm/sock.c
index 958081adb..e2486bc11 100644
--- a/net/bluetooth/rfcomm/sock.c
+++ b/net/bluetooth/rfcomm/sock.c
@@ -242,9 +242,7 @@ static void __rfcomm_sock_close(struct sock *sk)
  */
 static void rfcomm_sock_close(struct sock *sk)
 {
-	lock_sock(sk);
 	__rfcomm_sock_close(sk);
-	release_sock(sk);
 }
 
 static void rfcomm_sock_init(struct sock *sk, struct sock *parent)
@@ -905,6 +903,7 @@ static int rfcomm_sock_compat_ioctl(struct socket *sock, unsigned int cmd, unsig
 static int rfcomm_sock_shutdown(struct socket *sock, int how)
 {
 	struct sock *sk = sock->sk;
+	bool cleanup_listen = false;
 	int err = 0;
 
 	BT_DBG("sock %p, sk %p", sock, sk);
@@ -915,9 +914,17 @@ static int rfcomm_sock_shutdown(struct socket *sock, int how)
 	lock_sock(sk);
 	if (!sk->sk_shutdown) {
 		sk->sk_shutdown = SHUTDOWN_MASK;
+		if (sk->sk_state == BT_LISTEN) {
+			/* Block new children before cleaning up without sk lock. */
+			sk->sk_state = BT_CLOSED;
+			cleanup_listen = true;
+		}
 
 		release_sock(sk);
-		__rfcomm_sock_close(sk);
+		if (cleanup_listen)
+			rfcomm_sock_cleanup_listen(sk);
+		else
+			__rfcomm_sock_close(sk);
 		lock_sock(sk);
 
 		if (sock_flag(sk, SOCK_LINGER) && sk->sk_lingertime &&
-- 
2.50.1 (Apple Git-155)