Bluetooth: fix UAF in bt_accept_dequeue()

commit 4bd0b27 upstream.

bt_accept_get() takes a temporary reference before dropping the accept
queue lock. bt_accept_dequeue() currently drops that reference before
bt_accept_unlink(), leaving only the queue reference.

bt_accept_unlink() drops the queue reference. The subsequent
sock_hold() therefore accesses freed memory if it was the final
reference, as observed by KASAN during listening L2CAP socket cleanup.

Retain the temporary queue-walk reference through unlink and hand it to
the caller on success. Drop it explicitly on the closed and
not-yet-connected paths.

Fixes: ab15135 ("Bluetooth: fix UAF in l2cap_sock_cleanup_listen() vs l2cap_conn_del()")
Reported-by: syzbot+674ff7e4d7fdfd572afc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=674ff7e4d7fdfd572afc
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Yousef13710

net/bluetooth/af_bluetooth.c

@@ -305,7 +305,7 @@ struct sock *bt_accept_dequeue(struct sock *parent, struct socket *newsock)
 
 restart:
 	for (sk = bt_accept_get(parent, NULL); sk; sk = next) {
-		/* Prevent early freeing of sk due to unlink and sock_kill */
+		/* The reference from bt_accept_get() keeps sk alive. */
 		lock_sock(sk);
 
 		/* Check sk has not already been unlinked via
@@ -321,13 +321,11 @@ struct sock *bt_accept_dequeue(struct sock *parent, struct socket *newsock)
 
 		next = bt_accept_get(parent, sk);
 
-		/* sk is safely in the parent list so reduce reference count */
-		sock_put(sk);
-
 		/* FIXME: Is this check still needed */
 		if (sk->sk_state == BT_CLOSED) {
 			bt_accept_unlink(sk);
 			release_sock(sk);
+			sock_put(sk);
 			continue;
 		}
 
@@ -337,23 +335,14 @@ struct sock *bt_accept_dequeue(struct sock *parent, struct socket *newsock)
 			if (newsock)
 				sock_graft(sk, newsock);
 
-			/* Hand the caller a reference taken while sk is
-			 * still locked.  bt_accept_unlink() just dropped
-			 * the accept-queue reference; without this hold a
-			 * concurrent teardown (e.g. l2cap_conn_del() ->
-			 * l2cap_sock_kill()) could free sk between
-			 * release_sock() and the caller using it.  Every
-			 * caller drops this with sock_put() when done.
-			 */
-			sock_hold(sk);
-
 			release_sock(sk);
 			if (next)
 				sock_put(next);
 			return sk;
 		}
 
 		release_sock(sk);
+		sock_put(sk);
 	}
 
 	return NULL;

net/bluetooth/l2cap_sock.c

@@ -1448,8 +1448,8 @@ static void l2cap_sock_cleanup_listen(struct sock *parent)
 
 	/* Close not yet accepted channels.
 	 *
-	 * bt_accept_dequeue() now returns sk with an extra reference held
-	 * (taken while sk was still locked) so a concurrent l2cap_conn_del()
+	 * bt_accept_dequeue() returns sk with its temporary queue-walk
+	 * reference held, so a concurrent l2cap_conn_del()
 	 * -> l2cap_sock_kill() cannot free sk under us.
 	 *
 	 * cleanup_listen() runs under the parent sk lock, so unlike