net: check untrusted gso_size at kernel entry

Syzkaller again found a path to a kernel crash through bad gso input:
a packet with gso size exceeding len.

These packets are dropped in tcp_gso_segment and udp[46]_ufo_fragment.
But they may affect gso size calculations earlier in the path.

Now that we have thlen as of commit 9274124f ("net: stricter
validation of untrusted gso packets"), check gso_size at entry too.

Fixes: bfd5f4a3 ("packet: Add GSO/csum offload support.")
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/virtio_net.h

@@ -31,6 +31,7 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 {
 	unsigned int gso_type = 0;
 	unsigned int thlen = 0;
+	unsigned int p_off = 0;
 	unsigned int ip_proto;
 
 	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
@@ -68,7 +69,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		if (!skb_partial_csum_set(skb, start, off))
 			return -EINVAL;
 
-		if (skb_transport_offset(skb) + thlen > skb_headlen(skb))
+		p_off = skb_transport_offset(skb) + thlen;
+		if (p_off > skb_headlen(skb))
 			return -EINVAL;
 	} else {
 		/* gso packets without NEEDS_CSUM do not set transport_offset.
@@ -92,17 +94,25 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 				return -EINVAL;
 			}
 
-			if (keys.control.thoff + thlen > skb_headlen(skb) ||
+			p_off = keys.control.thoff + thlen;
+			if (p_off > skb_headlen(skb) ||
 			    keys.basic.ip_proto != ip_proto)
 				return -EINVAL;
 
 			skb_set_transport_header(skb, keys.control.thoff);
+		} else if (gso_type) {
+			p_off = thlen;
+			if (p_off > skb_headlen(skb))
+				return -EINVAL;
 		}
 	}
 
 	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
 		u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
 
+		if (skb->len - p_off <= gso_size)
+			return -EINVAL;
+
 		skb_shinfo(skb)->gso_size = gso_size;
 		skb_shinfo(skb)->gso_type = gso_type;