net: validate untrusted gso packets without csum offload

Syzkaller again found a path to a kernel crash through bad gso input.
By building an excessively large packet to cause an skb field to wrap.

If VIRTIO_NET_HDR_F_NEEDS_CSUM was set this would have been dropped in
skb_partial_csum_set.

GSO packets that do not set checksum offload are suspicious and rare.
Most callers of virtio_net_hdr_to_skb already pass them to
skb_probe_transport_header.

Move that test forward, change it to detect parse failure and drop
packets on failure as those cleary are not one of the legitimate
VIRTIO_NET_HDR_GSO types.

Fixes: bfd5f4a ("packet: Add GSO/csum offload support.")
Fixes: f43798c ("tun: Allow GSO using virtio_net_hdr")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2434,7 +2434,7 @@ static inline void skb_probe_transport_header(struct sk_buff *skb,
 
 	if (skb_flow_dissect_flow_keys_basic(skb, &keys, NULL, 0, 0, 0, 0))
 		skb_set_transport_header(skb, keys.control.thoff);
-	else
+	else if (offset_hint >= 0)
 		skb_set_transport_header(skb, offset_hint);
 }
 

include/linux/virtio_net.h

@@ -57,6 +57,15 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 
 		if (!skb_partial_csum_set(skb, start, off))
 			return -EINVAL;
+	} else {
+		/* 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) {
+			skb_probe_transport_header(skb, -1);
+			if (!skb_transport_header_was_set(skb))
+				return -EINVAL;
+		}
 	}
 
 	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {