ip_vti: receive ipip packet by calling ip_tunnel_rcv

In Commit dd9ee34440 ("vti4: Fix a ipip packet processing bug in
'IPCOMP' virtual tunnel"), it tries to receive IPIP packets in vti
by calling xfrm_input(). This case happens when a small packet or
frag sent by peer is too small to get compressed.

However, xfrm_input() will still get to the IPCOMP path where skb
sec_path is set, but never dropped while it should have been done
in vti_ipcomp4_protocol.cb_handler(vti_rcv_cb), as it's not an
ipcomp4 packet. This will cause that the packet can never pass
xfrm4_policy_check() in the upper protocol rcv functions.

So this patch is to call ip_tunnel_rcv() to process IPIP packets
instead.

Fixes: dd9ee34440 ("vti4: Fix a ipip packet processing bug in 'IPCOMP' virtual tunnel")
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/ip_vti.c

@@ -93,7 +93,28 @@ static int vti_rcv_proto(struct sk_buff *skb)
 
 static int vti_rcv_tunnel(struct sk_buff *skb)
 {
-	return vti_rcv(skb, ip_hdr(skb)->saddr, true);
+	struct ip_tunnel_net *itn = net_generic(dev_net(skb->dev), vti_net_id);
+	const struct iphdr *iph = ip_hdr(skb);
+	struct ip_tunnel *tunnel;
+
+	tunnel = ip_tunnel_lookup(itn, skb->dev->ifindex, TUNNEL_NO_KEY,
+				  iph->saddr, iph->daddr, 0);
+	if (tunnel) {
+		struct tnl_ptk_info tpi = {
+			.proto = htons(ETH_P_IP),
+		};
+
+		if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
+			goto drop;
+		if (iptunnel_pull_header(skb, 0, tpi.proto, false))
+			goto drop;
+		return ip_tunnel_rcv(tunnel, skb, &tpi, NULL, false);
+	}
+
+	return -EINVAL;
+drop:
+	kfree_skb(skb);
+	return 0;
 }
 
 static int vti_rcv_cb(struct sk_buff *skb, int err)