netfilter: nf_tables: restore IP sanity checks for netdev/egress

Subtract network offset to skb->len before performing IPv4 header sanity
checks, then adjust transport offset from offset from mac header.

Jorge Ortiz says:

When small UDP packets (< 4 bytes payload) are sent from eth0,
`meta l4proto udp` condition is not met because `NFT_PKTINFO_L4PROTO` is
not set. This happens because there is a comparison that checks if the
transport header offset exceeds the total length.  This comparison does
not take into account the fact that the skb network offset might be
non-zero in egress mode (e.g., 14 bytes for Ethernet header).

Fixes: 0ae8e4cc ("netfilter: nf_tables: set transport offset from mac header for netdev/egress")
Reported-by: Jorge Ortiz <jorge.ortiz.escribano@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables_ipv4.h

@@ -19,7 +19,7 @@ static inline void nft_set_pktinfo_ipv4(struct nft_pktinfo *pkt)
 static inline int __nft_set_pktinfo_ipv4_validate(struct nft_pktinfo *pkt)
 {
 	struct iphdr *iph, _iph;
-	u32 len, thoff;
+	u32 len, thoff, skb_len;
 
 	iph = skb_header_pointer(pkt->skb, skb_network_offset(pkt->skb),
 				 sizeof(*iph), &_iph);
@@ -30,8 +30,10 @@ static inline int __nft_set_pktinfo_ipv4_validate(struct nft_pktinfo *pkt)
 		return -1;
 
 	len = iph_totlen(pkt->skb, iph);
-	thoff = skb_network_offset(pkt->skb) + (iph->ihl * 4);
-	if (pkt->skb->len < len)
+	thoff = iph->ihl * 4;
+	skb_len = pkt->skb->len - skb_network_offset(pkt->skb);
+
+	if (skb_len < len)
 		return -1;
 	else if (len < thoff)
 		return -1;
@@ -40,7 +42,7 @@ static inline int __nft_set_pktinfo_ipv4_validate(struct nft_pktinfo *pkt)
 
 	pkt->flags = NFT_PKTINFO_L4PROTO;
 	pkt->tprot = iph->protocol;
-	pkt->thoff = thoff;
+	pkt->thoff = skb_network_offset(pkt->skb) + thoff;
 	pkt->fragoff = ntohs(iph->frag_off) & IP_OFFSET;
 
 	return 0;