netfilter: nf_tables_ipv6: consider network offset in netdev/egress validation

From netdev/egress, skb->len can include the ethernet header, therefore,
subtract network offset from skb->len when validating IPv6 packet length.

Fixes: 42df6e1d ("netfilter: Introduce egress hook")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables_ipv6.h

@@ -31,8 +31,8 @@ static inline int __nft_set_pktinfo_ipv6_validate(struct nft_pktinfo *pkt)
 	struct ipv6hdr *ip6h, _ip6h;
 	unsigned int thoff = 0;
 	unsigned short frag_off;
+	u32 pkt_len, skb_len;
 	int protohdr;
-	u32 pkt_len;
 
 	ip6h = skb_header_pointer(pkt->skb, skb_network_offset(pkt->skb),
 				  sizeof(*ip6h), &_ip6h);
@@ -43,7 +43,8 @@ static inline int __nft_set_pktinfo_ipv6_validate(struct nft_pktinfo *pkt)
 		return -1;
 
 	pkt_len = ntohs(ip6h->payload_len);
-	if (pkt_len + sizeof(*ip6h) > pkt->skb->len)
+	skb_len = pkt->skb->len - skb_network_offset(pkt->skb);
+	if (pkt_len + sizeof(*ip6h) > skb_len)
 		return -1;
 
 	protohdr = ipv6_find_hdr(pkt->skb, &thoff, -1, &frag_off, &flags);