ipv4 tunnels: fix an oops when using ipip/sit with IPsec

Since commit 3d7b46cd (ip_tunnel: push generic protocol handling to
ip_tunnel module.), an Oops is triggered when an xfrm policy is configured on
an IPv4 over IPv4 tunnel.

xfrm4_policy_check() calls __xfrm_policy_check2(), which uses skb_dst(skb). But
this field is NULL because iptunnel_pull_header() calls skb_dst_drop(skb).
Signed-off-by: Li Hongjun <hongjun.li@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipip.c

@@ -190,15 +190,14 @@ static int ipip_rcv(struct sk_buff *skb)
 	struct ip_tunnel *tunnel;
 	const struct iphdr *iph;
 
-	if (iptunnel_pull_header(skb, 0, tpi.proto))
-		goto drop;
-
 	iph = ip_hdr(skb);
 	tunnel = ip_tunnel_lookup(itn, skb->dev->ifindex, TUNNEL_NO_KEY,
 			iph->saddr, iph->daddr, 0);
 	if (tunnel) {
 		if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 			goto drop;
+		if (iptunnel_pull_header(skb, 0, tpi.proto))
+			goto drop;
 		return ip_tunnel_rcv(tunnel, skb, &tpi, log_ecn_error);
 	}
 

net/ipv6/sit.c

@@ -645,11 +645,7 @@ static int ipip_rcv(struct sk_buff *skb)
 	const struct iphdr *iph;
 	struct ip_tunnel *tunnel;
 
-	if (iptunnel_pull_header(skb, 0, tpi.proto))
-		goto drop;
-
 	iph = ip_hdr(skb);
-
 	tunnel = ipip6_tunnel_lookup(dev_net(skb->dev), skb->dev,
 				     iph->saddr, iph->daddr);
 	if (tunnel != NULL) {
@@ -659,6 +655,8 @@ static int ipip_rcv(struct sk_buff *skb)
 
 		if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 			goto drop;
+		if (iptunnel_pull_header(skb, 0, tpi.proto))
+			goto drop;
 		return ip_tunnel_rcv(tunnel, skb, &tpi, log_ecn_error);
 	}