net: avoid false positives in untrusted gso validation

GSO packets with vnet_hdr must conform to a small set of gso_types.
The below commit uses flow dissection to drop packets that do not.

But it has false positives when the skb is not fully initialized.
Dissection needs skb->protocol and skb->network_header.

Infer skb->protocol from gso_type as the two must agree.
SKB_GSO_UDP can use both ipv4 and ipv6, so try both.

Exclude callers for which network header offset is not known.

Fixes: d5be7f63 ("net: validate untrusted gso packets without csum offload")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/virtio_net.h

@@ -61,12 +61,22 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		/* gso packets without NEEDS_CSUM do not set transport_offset.
 		 * probe and drop if does not match one of the above types.
 		 */
-		if (gso_type) {
+		if (gso_type && skb->network_header) {
+			if (!skb->protocol)
+				virtio_net_hdr_set_proto(skb, hdr);
+retry:
 			skb_probe_transport_header(skb, -1);
-			if (!skb_transport_header_was_set(skb))
+			if (!skb_transport_header_was_set(skb)) {
+				/* UFO does not specify ipv4 or 6: try both */
+				if (gso_type & SKB_GSO_UDP &&
+				    skb->protocol == htons(ETH_P_IP)) {
+					skb->protocol = htons(ETH_P_IPV6);
+					goto retry;
+				}
 				return -EINVAL;
 			}
 		}
+	}
 
 	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
 		u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);