net: add BUG_ON if kernel advertises msg_namelen > sizeof(struct sockaddr_storage)

In that case it is probable that kernel code overwrote part of the
stack. So we should bail out loudly here.

The BUG_ON may be removed in future if we are sure all protocols are
conformant.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -221,12 +221,13 @@ static int move_addr_to_user(struct sockaddr_storage *kaddr, int klen,
 	int err;
 	int len;
 
+	BUG_ON(klen > sizeof(struct sockaddr_storage));
 	err = get_user(len, ulen);
 	if (err)
 		return err;
 	if (len > klen)
 		len = klen;
-	if (len < 0 || len > sizeof(struct sockaddr_storage))
+	if (len < 0)
 		return -EINVAL;
 	if (len) {
 		if (audit_sockaddr(klen, kaddr))