fou6: Prevent unbounded recursion in GUE error handler

I forgot to deal with IPv6 in commit 11789039 ("fou: Prevent unbounded
recursion in GUE error handler").

Now syzbot reported what might be the same type of issue, caused by
gue6_err(), that is, handling exceptions for direct UDP encapsulation in
GUE (UDP-in-UDP) leads to unbounded recursion in the GUE exception
handler.

As it probably doesn't make sense to set up GUE this way, and it's
currently not even possible to configure this, skip exception handling for
UDP (or UDP-Lite) packets encapsulated in UDP (or UDP-Lite) packets with
GUE on IPv6.

Reported-by: syzbot+4ad25edc7a33e4ab91e0@syzkaller.appspotmail.com
Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: b8a51b38 ("fou, fou6: ICMP error handlers for FoU and GUE")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/fou6.c

@@ -131,6 +131,14 @@ static int gue6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	if (validate_gue_flags(guehdr, optlen))
 		return -EINVAL;
 
+	/* Handling exceptions for direct UDP encapsulation in GUE would lead to
+	 * recursion. Besides, this kind of encapsulation can't even be
+	 * configured currently. Discard this.
+	 */
+	if (guehdr->proto_ctype == IPPROTO_UDP ||
+	    guehdr->proto_ctype == IPPROTO_UDPLITE)
+		return -EOPNOTSUPP;
+
 	skb_set_transport_header(skb, -(int)sizeof(struct icmp6hdr));
 	ret = gue6_err_proto_handler(guehdr->proto_ctype, skb,
 				     opt, type, code, offset, info);