ip6_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 ipv6 code.

Fixes: Fixes: b05229f4 ("gre6: Cleanup GREv6 transmit path, call common
GRE functions")
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/ipv6/ip6_gre.c

@@ -629,6 +629,8 @@ static int gre_rcv(struct sk_buff *skb)
 
 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);
 }