--- a/net/ipv6/fou6.c
+++ b/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);