netfilter: nft_payload: rebuild vlan header on h_proto access

nft can perform merging of adjacent payload requests.
This means that:

ether saddr 00:11 ... ether type 8021ad ...

is a single payload expression, for 8 bytes, starting at the
ethernet source offset.

Check that offset+length is fully within the source/destination mac
addersses.

This bug prevents 'ether type' from matching the correct h_proto in case
vlan tag got stripped.

Fixes: de6843b ("netfilter: nft_payload: rebuild vlan header when needed")
Reported-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nft_payload.c

@@ -154,6 +154,17 @@ int nft_payload_inner_offset(const struct nft_pktinfo *pkt)
 	return pkt->inneroff;
 }
 
+static bool nft_payload_need_vlan_copy(const struct nft_payload *priv)
+{
+	unsigned int len = priv->offset + priv->len;
+
+	/* data past ether src/dst requested, copy needed */
+	if (len > offsetof(struct ethhdr, h_proto))
+		return true;
+
+	return false;
+}
+
 void nft_payload_eval(const struct nft_expr *expr,
 		      struct nft_regs *regs,
 		      const struct nft_pktinfo *pkt)
@@ -172,7 +183,7 @@ void nft_payload_eval(const struct nft_expr *expr,
 			goto err;
 
 		if (skb_vlan_tag_present(skb) &&
-		    priv->offset >= offsetof(struct ethhdr, h_proto)) {
+		    nft_payload_need_vlan_copy(priv)) {
 			if (!nft_payload_copy_vlan(dest, skb,
 						   priv->offset, priv->len))
 				goto err;