ip_gre: add validation for csum_start

Validate csum_start in gre_handle_offloads before we call _gre_xmit so
that we do not crash later when the csum_start value is used in the
lco_csum function call.

This patch deals with ipv4 code.

Fixes: c5441932 ("GRE: Refactor GRE tunneling code.")
Reported-by:  <syzbot+ff8e1b9f2f36481e2efc@syzkaller.appspotmail.com>
Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -473,6 +473,8 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
 
 static int gre_handle_offloads(struct sk_buff *skb, bool csum)
 {
+	if (csum && skb_checksum_start(skb) < skb->data)
+		return -EINVAL;
 	return iptunnel_handle_offloads(skb, csum ? SKB_GSO_GRE_CSUM : SKB_GSO_GRE);
 }