xfrm: Fix local error reporting crash with interfamily tunnels

We set the outer mode protocol too early. As a result, the
local error handler might dispatch to the wrong address family
and report the error to a wrong socket type. We fix this by
setting the outer protocol to the skb after we accessed the
inner mode for the last time, right before we do the atcual
encapsulation where we switch finally to the outer mode.
Reported-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Tested-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_output.c

@@ -63,6 +63,7 @@ int xfrm4_prepare_output(struct xfrm_state *x, struct sk_buff *skb)
 		return err;
 
 	IPCB(skb)->flags |= IPSKB_XFRM_TUNNEL_SIZE;
+	skb->protocol = htons(ETH_P_IP);
 
 	return x->outer_mode->output2(x, skb);
 }
@@ -71,7 +72,6 @@ EXPORT_SYMBOL(xfrm4_prepare_output);
 int xfrm4_output_finish(struct sk_buff *skb)
 {
 	memset(IPCB(skb), 0, sizeof(*IPCB(skb)));
-	skb->protocol = htons(ETH_P_IP);
 
 #ifdef CONFIG_NETFILTER
 	IPCB(skb)->flags |= IPSKB_XFRM_TRANSFORMED;

net/ipv6/xfrm6_output.c

@@ -114,6 +114,7 @@ int xfrm6_prepare_output(struct xfrm_state *x, struct sk_buff *skb)
 		return err;
 
 	skb->ignore_df = 1;
+	skb->protocol = htons(ETH_P_IPV6);
 
 	return x->outer_mode->output2(x, skb);
 }
@@ -122,7 +123,6 @@ EXPORT_SYMBOL(xfrm6_prepare_output);
 int xfrm6_output_finish(struct sk_buff *skb)
 {
 	memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
-	skb->protocol = htons(ETH_P_IPV6);
 
 #ifdef CONFIG_NETFILTER
 	IP6CB(skb)->flags |= IP6SKB_XFRM_TRANSFORMED;