xfrm interface: fix list corruption for x-netns

dev_net(dev) is the netns of the device and xi->net is the link netns,
where the device has been linked.
changelink() must operate in the link netns to avoid a corruption of
the xfrm lists.

Note that xi->net and dev_net(xi->physdev) are always the same.

Before the patch, the xfrmi lists may be corrupted and can later trigger a
kernel panic.

Fixes: f203b76d ("xfrm: Add virtual xfrm interfaces")
Reported-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_interface.c

@@ -503,7 +503,7 @@ static int xfrmi_change(struct xfrm_if *xi, const struct xfrm_if_parms *p)
 
 static int xfrmi_update(struct xfrm_if *xi, struct xfrm_if_parms *p)
 {
-	struct net *net = dev_net(xi->dev);
+	struct net *net = xi->net;
 	struct xfrmi_net *xfrmn = net_generic(net, xfrmi_net_id);
 	int err;
 
@@ -663,9 +663,9 @@ static int xfrmi_changelink(struct net_device *dev, struct nlattr *tb[],
 			   struct nlattr *data[],
 			   struct netlink_ext_ack *extack)
 {
-	struct net *net = dev_net(dev);
+	struct xfrm_if *xi = netdev_priv(dev);
+	struct net *net = xi->net;
 	struct xfrm_if_parms p;
-	struct xfrm_if *xi;
 
 	xfrmi_netlink_parms(data, &p);
 	xi = xfrmi_locate(net, &p);
@@ -707,7 +707,7 @@ static struct net *xfrmi_get_link_net(const struct net_device *dev)
 {
 	struct xfrm_if *xi = netdev_priv(dev);
 
-	return dev_net(xi->phydev);
+	return xi->net;
 }
 
 static const struct nla_policy xfrmi_policy[IFLA_XFRM_MAX + 1] = {