netfilter: nf_tables: set transport offset from mac header for netdev/egress

Before this patch, transport offset (pkt->thoff) provides an offset
relative to the network header. This is fine for the inet families
because skb->data points to the network header in such case. However,
from netdev/egress, skb->data points to the mac header (if available),
thus, pkt->thoff is missing the mac header length.

Add skb_network_offset() to the transport offset (pkt->thoff) for
netdev, so transport header mangling works as expected. Adjust payload
fast eval function to use skb->data now that pkt->thoff provides an
absolute offset. This explains why users report that matching on
egress/netdev works but payload mangling does not.

This patch implicitly fixes payload mangling for IPv4 packets in
netdev/egress given skb_store_bits() requires an offset from skb->data
to reach the transport header.

I suspect that nft_exthdr and the trace infra were also broken from
netdev/egress because they also take skb->data as start, and pkt->thoff
was not correct.

Note that IPv6 is fine because ipv6_find_hdr() already provides a
transport offset starting from skb->data, which includes
skb_network_offset().

The bridge family also uses nft_set_pktinfo_ipv4_validate(), but there
skb_network_offset() is zero, so the update in this patch does not alter
the existing behaviour.

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

include/net/netfilter/nf_tables_ipv4.h

@@ -30,7 +30,7 @@ static inline int __nft_set_pktinfo_ipv4_validate(struct nft_pktinfo *pkt)
 		return -1;
 
 	len = iph_totlen(pkt->skb, iph);
-	thoff = iph->ihl * 4;
+	thoff = skb_network_offset(pkt->skb) + (iph->ihl * 4);
 	if (pkt->skb->len < len)
 		return -1;
 	else if (len < thoff)

net/netfilter/nf_tables_core.c

@@ -158,7 +158,7 @@ static bool nft_payload_fast_eval(const struct nft_expr *expr,
 	else {
 		if (!(pkt->flags & NFT_PKTINFO_L4PROTO))
 			return false;
-		ptr = skb_network_header(skb) + nft_thoff(pkt);
+		ptr = skb->data + nft_thoff(pkt);
 	}
 
 	ptr += priv->offset;