net: add rcu safety to rtnl_prop_list_size()

rtnl_prop_list_size() can be called while alternative names
are added or removed concurrently.

if_nlmsg_size() / rtnl_calcit() can indeed be called
without RTNL held.

Use explicit RCU protection to avoid UAF.

Fixes: 88f4fb0c ("net: rtnetlink: put alternative names to getlink message")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240209181248.96637-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -336,7 +336,7 @@ int netdev_name_node_alt_create(struct net_device *dev, const char *name)
 		return -ENOMEM;
 	netdev_name_node_add(net, name_node);
 	/* The node that holds dev->name acts as a head of per-device list. */
-	list_add_tail(&name_node->list, &dev->name_node->list);
+	list_add_tail_rcu(&name_node->list, &dev->name_node->list);
 
 	return 0;
 }

net/core/rtnetlink.c

@@ -1020,14 +1020,17 @@ static size_t rtnl_xdp_size(void)
 static size_t rtnl_prop_list_size(const struct net_device *dev)
 {
 	struct netdev_name_node *name_node;
-	size_t size;
+	unsigned int cnt = 0;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(name_node, &dev->name_node->list, list)
+		cnt++;
+	rcu_read_unlock();
 
-	if (list_empty(&dev->name_node->list))
+	if (!cnt)
 		return 0;
-	size = nla_total_size(0);
-	list_for_each_entry(name_node, &dev->name_node->list, list)
-		size += nla_total_size(ALTIFNAMSIZ);
-	return size;
+
+	return nla_total_size(0) + cnt * nla_total_size(ALTIFNAMSIZ);
 }
 
 static size_t rtnl_proto_down_size(const struct net_device *dev)