Subject: batman-adv: Request iflink once in batadv-on-batadv check
From: Sven Eckelmann <[email-masked]>
Date: Wed, 2 Mar 2022 19:49:44 +0100

There is no need to call dev_get_iflink multiple times for the same
net_device in batadv_is_on_batman_iface. And since some of the
.ndo_get_iflink callbacks are dynamic (for example via RCUs like in
vxcan_get_iflink), it could easily happen that the returned values are not
stable. The pre-checks before __dev_get_by_index are then of course bogus.

Fixes: 3d48811b27f5 ("batman-adv: prevent using any virtual device created on batman-adv as hard-interface")
Signed-off-by: Sven Eckelmann <[email-masked]>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/f6c0c45356fe3ab9c5e01b81d060f8a436658037

--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -148,22 +148,23 @@ static bool batadv_is_on_batman_iface(co
 	struct net *net = dev_net(net_dev);
 	struct net_device *parent_dev;
 	struct net *parent_net;
+	int iflink;
 	bool ret;
 
 	/* check if this is a batman-adv mesh interface */
 	if (batadv_softif_is_valid(net_dev))
 		return true;
 
+	iflink = dev_get_iflink(net_dev);
+
 	/* no more parents..stop recursion */
-	if (dev_get_iflink(net_dev) == 0 ||
-	    dev_get_iflink(net_dev) == net_dev->ifindex)
+	if (iflink == 0 || iflink == net_dev->ifindex)
 		return false;
 
 	parent_net = batadv_getlink_net(net_dev, net);
 
 	/* recurse over the parent device */
-	parent_dev = __dev_get_by_index((struct net *)parent_net,
-					dev_get_iflink(net_dev));
+	parent_dev = __dev_get_by_index((struct net *)parent_net, iflink);
 	/* if we got a NULL parent_dev there is something broken.. */
 	if (!parent_dev) {
 		pr_err("Cannot find parent device\n");