Bluetooth: Fix possible deadlock in rfcomm_sk_state_change

syzbot reports a possible deadlock in rfcomm_sk_state_change [1].
While rfcomm_sock_connect acquires the sk lock and waits for
the rfcomm lock, rfcomm_sock_release could have the rfcomm
lock and hit a deadlock for acquiring the sk lock.
Here's a simplified flow:

rfcomm_sock_connect:
  lock_sock(sk)
  rfcomm_dlc_open:
    rfcomm_lock()

rfcomm_sock_release:
  rfcomm_sock_shutdown:
    rfcomm_lock()
    __rfcomm_dlc_close:
        rfcomm_k_state_change:
	  lock_sock(sk)

This patch drops the sk lock before calling rfcomm_dlc_open to
avoid the possible deadlock and holds sk's reference count to
prevent use-after-free after rfcomm_dlc_open completes.

Reported-by: syzbot+d7ce59...@syzkaller.appspotmail.com
Fixes: 1804fdf6e4 ("Bluetooth: btintel: Combine setting up MSFT extension")
Link: https://syzkaller.appspot.com/bug?extid=d7ce59b06b3eb14fd218 [1]

Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/rfcomm/sock.c

@@ -391,6 +391,7 @@ static int rfcomm_sock_connect(struct socket *sock, struct sockaddr *addr, int a
 	    addr->sa_family != AF_BLUETOOTH)
 		return -EINVAL;
 
+	sock_hold(sk);
 	lock_sock(sk);
 
 	if (sk->sk_state != BT_OPEN && sk->sk_state != BT_BOUND) {
@@ -410,14 +411,18 @@ static int rfcomm_sock_connect(struct socket *sock, struct sockaddr *addr, int a
 	d->sec_level = rfcomm_pi(sk)->sec_level;
 	d->role_switch = rfcomm_pi(sk)->role_switch;
 
+	/* Drop sock lock to avoid potential deadlock with the RFCOMM lock */
+	release_sock(sk);
 	err = rfcomm_dlc_open(d, &rfcomm_pi(sk)->src, &sa->rc_bdaddr,
 			      sa->rc_channel);
-	if (!err)
+	lock_sock(sk);
+	if (!err && !sock_flag(sk, SOCK_ZAPPED))
 		err = bt_sock_wait_state(sk, BT_CONNECTED,
 				sock_sndtimeo(sk, flags & O_NONBLOCK));
 
 done:
 	release_sock(sk);
+	sock_put(sk);
 	return err;
 }