gre6: Fix reception with IP6_TNL_F_RCV_DSCP_COPY

When receiving an IPv4 packet inside an IPv6 GRE packet, and the
IP6_TNL_F_RCV_DSCP_COPY flag is set on the tunnel, the IPv4 header would
get corrupted. This is due to the common ip6_tnl_rcv() function assuming
that the inner header is always IPv6. This patch checks the tunnel
protocol for IPv4 inner packets, but still defaults to IPv6.

Fixes: 308edfdf ("gre6: Cleanup GREv6 receive path, call common GRE functions")
Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -915,7 +915,15 @@ int ip6_tnl_rcv(struct ip6_tnl *t, struct sk_buff *skb,
 		struct metadata_dst *tun_dst,
 		bool log_ecn_err)
 {
-	return __ip6_tnl_rcv(t, skb, tpi, tun_dst, ip6ip6_dscp_ecn_decapsulate,
+	int (*dscp_ecn_decapsulate)(const struct ip6_tnl *t,
+				    const struct ipv6hdr *ipv6h,
+				    struct sk_buff *skb);
+
+	dscp_ecn_decapsulate = ip6ip6_dscp_ecn_decapsulate;
+	if (tpi->proto == htons(ETH_P_IP))
+		dscp_ecn_decapsulate = ip4ip6_dscp_ecn_decapsulate;
+
+	return __ip6_tnl_rcv(t, skb, tpi, tun_dst, dscp_ecn_decapsulate,
 			     log_ecn_err);
 }
 EXPORT_SYMBOL(ip6_tnl_rcv);