xfrm: Verify MAC header exists before overwriting eth_hdr(skb)->h_proto

Artem Savkov reported that commit 5efec5c655dd leads to a packet loss under
IPSec configuration. It appears that his setup consists of a TUN device,
which does not have a MAC header.

Make sure MAC header exists.

Note: TUN device sets a MAC header pointer, although it does not have one.

Fixes: 5efec5c655dd ("xfrm: Fix eth_hdr(skb)->h_proto to reflect inner IP version")
Reported-by: Artem Savkov <artem.savkov@gmail.com>
Tested-by: Artem Savkov <artem.savkov@gmail.com>
Signed-off-by: Yossi Kuperman <yossiku@mellanox.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_mode_tunnel.c

@@ -92,7 +92,8 @@ static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 
 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);
-	eth_hdr(skb)->h_proto = skb->protocol;
+	if (skb->mac_len)
+		eth_hdr(skb)->h_proto = skb->protocol;
 
 	err = 0;
 

net/ipv6/xfrm6_mode_tunnel.c

@@ -92,7 +92,8 @@ static int xfrm6_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 
 	skb_reset_network_header(skb);
 	skb_mac_header_rebuild(skb);
-	eth_hdr(skb)->h_proto = skb->protocol;
+	if (skb->mac_len)
+		eth_hdr(skb)->h_proto = skb->protocol;
 
 	err = 0;