netfilter: nft_payload: add C-VLAN support

If the encapsulated ethertype announces another inner VLAN header and
the offset falls within the boundaries of the inner VLAN header, then
adjust arithmetics to include the extra VLAN header length and fetch the
bytes from the vlan header in the skbuff data area that represents this
inner VLAN header.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_payload.c

@@ -43,27 +43,36 @@ nft_payload_copy_vlan(u32 *d, const struct sk_buff *skb, u8 offset, u8 len)
 	int mac_off = skb_mac_header(skb) - skb->data;
 	u8 *vlanh, *dst_u8 = (u8 *) d;
 	struct vlan_ethhdr veth;
+	u8 vlan_hlen = 0;
+
+	if ((skb->protocol == htons(ETH_P_8021AD) ||
+	     skb->protocol == htons(ETH_P_8021Q)) &&
+	    offset >= VLAN_ETH_HLEN && offset < VLAN_ETH_HLEN + VLAN_HLEN)
+		vlan_hlen += VLAN_HLEN;
 
 	vlanh = (u8 *) &veth;
-	if (offset < VLAN_ETH_HLEN) {
+	if (offset < VLAN_ETH_HLEN + vlan_hlen) {
 		u8 ethlen = len;
 
-		if (!nft_payload_rebuild_vlan_hdr(skb, mac_off, &veth))
+		if (vlan_hlen &&
+		    skb_copy_bits(skb, mac_off, &veth, VLAN_ETH_HLEN) < 0)
+			return false;
+		else if (!nft_payload_rebuild_vlan_hdr(skb, mac_off, &veth))
 			return false;
 
-		if (offset + len > VLAN_ETH_HLEN)
-			ethlen -= offset + len - VLAN_ETH_HLEN;
+		if (offset + len > VLAN_ETH_HLEN + vlan_hlen)
+			ethlen -= offset + len - VLAN_ETH_HLEN + vlan_hlen;
 
-		memcpy(dst_u8, vlanh + offset, ethlen);
+		memcpy(dst_u8, vlanh + offset - vlan_hlen, ethlen);
 
 		len -= ethlen;
 		if (len == 0)
 			return true;
 
 		dst_u8 += ethlen;
-		offset = ETH_HLEN;
+		offset = ETH_HLEN + vlan_hlen;
 	} else {
-		offset -= VLAN_HLEN;
+		offset -= VLAN_HLEN + vlan_hlen;
 	}
 
 	return skb_copy_bits(skb, offset + mac_off, dst_u8, len) == 0;