xfrm: Honor original L3 slave device in xfrmi policy lookup

If an xfrmi is associated to a vrf layer 3 master device,
xfrm_policy_check() fails after traffic decapsulation. The input
interface is replaced by the layer 3 master device, and hence
xfrmi_decode_session() can't match the xfrmi anymore to satisfy
policy checking.

Extend ingress xfrmi lookup to honor the original layer 3 slave
device, allowing xfrm interfaces to operate within a vrf domain.

Fixes: f203b76d78 ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -295,7 +295,8 @@ struct xfrm_replay {
 };
 
 struct xfrm_if_cb {
-	struct xfrm_if	*(*decode_session)(struct sk_buff *skb);
+	struct xfrm_if	*(*decode_session)(struct sk_buff *skb,
+					   unsigned short family);
 };
 
 void xfrm_if_register_cb(const struct xfrm_if_cb *ifcb);

net/xfrm/xfrm_interface.c

@@ -70,17 +70,28 @@ static struct xfrm_if *xfrmi_lookup(struct net *net, struct xfrm_state *x)
 	return NULL;
 }
 
-static struct xfrm_if *xfrmi_decode_session(struct sk_buff *skb)
+static struct xfrm_if *xfrmi_decode_session(struct sk_buff *skb,
+					    unsigned short family)
 {
 	struct xfrmi_net *xfrmn;
-	int ifindex;
 	struct xfrm_if *xi;
+	int ifindex = 0;
 
 	if (!secpath_exists(skb) || !skb->dev)
 		return NULL;
 
+	switch (family) {
+	case AF_INET6:
+		ifindex = inet6_sdif(skb);
+		break;
+	case AF_INET:
+		ifindex = inet_sdif(skb);
+		break;
+	}
+	if (!ifindex)
+		ifindex = skb->dev->ifindex;
+
 	xfrmn = net_generic(xs_net(xfrm_input_state(skb)), xfrmi_net_id);
-	ifindex = skb->dev->ifindex;
 
 	for_each_xfrmi_rcu(xfrmn->xfrmi[0], xi) {
 		if (ifindex == xi->dev->ifindex &&

net/xfrm/xfrm_policy.c

@@ -3313,7 +3313,7 @@ int __xfrm_policy_check(struct sock *sk, int dir, struct sk_buff *skb,
 	ifcb = xfrm_if_get_cb();
 
 	if (ifcb) {
-		xi = ifcb->decode_session(skb);
+		xi = ifcb->decode_session(skb, family);
 		if (xi) {
 			if_id = xi->p.if_id;
 			net = xi->net;