Bluetooth: RFCOMM - Fix missing msg_namelen update in rfcomm_sock_recvmsg()

If RFCOMM_DEFER_SETUP is set in the flags, rfcomm_sock_recvmsg() returns
early with 0 without updating the possibly set msg_namelen member. This,
in turn, leads to a 128 byte kernel stack leak in net/socket.c.

Fix this by updating msg_namelen in this case. For all other cases it
will be handled in bt_sock_stream_recvmsg().

Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static int rfcomm_sock_recvmsg(struct kiocb *iocb, struct socket *sock,
			       struct msghdr *msg, size_t size, int flags)
{
	struct sock *sk = sock->sk;
	struct rfcomm_dlc *d = rfcomm_pi(sk)->dlc;
	int len;
 
 	if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
 		rfcomm_dlc_accept(d);
		msg->msg_namelen = 0;
 		return 0;
 	}
 
	len = bt_sock_stream_recvmsg(iocb, sock, msg, size, flags);

	lock_sock(sk);
	if (!(flags & MSG_PEEK) && len > 0)
		atomic_sub(len, &sk->sk_rmem_alloc);

	if (atomic_read(&sk->sk_rmem_alloc) <= (sk->sk_rcvbuf >> 2))
		rfcomm_dlc_unthrottle(rfcomm_pi(sk)->dlc);
	release_sock(sk);

	return len;
}

static int rfcomm_sock_recvmsg(struct kiocb *iocb, struct socket *sock,
			       struct msghdr *msg, size_t size, int flags)
{
	struct sock *sk = sock->sk;
	struct rfcomm_dlc *d = rfcomm_pi(sk)->dlc;
	int len;
 
 	if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
 		rfcomm_dlc_accept(d);
 		return 0;
 	}
 
	len = bt_sock_stream_recvmsg(iocb, sock, msg, size, flags);

	lock_sock(sk);
	if (!(flags & MSG_PEEK) && len > 0)
		atomic_sub(len, &sk->sk_rmem_alloc);

	if (atomic_read(&sk->sk_rmem_alloc) <= (sk->sk_rcvbuf >> 2))
		rfcomm_dlc_unthrottle(rfcomm_pi(sk)->dlc);
	release_sock(sk);

	return len;
}
