Subject: [PATCH net 2/5] net: check for altname conflicts when changing netdev's netns

From: Jakub Kicinski <[email-masked]>
Date: Mon, 16 Oct 2023 13:16:54 -0700

It's currently possible to create an altname conflicting
with an altname or real name of another device by creating
it in another netns and moving it over:

 [ ~]$ ip link add dev eth0 type dummy

 [ ~]$ ip netns add test
 [ ~]$ ip -netns test link add dev ethX netns test type dummy
 [ ~]$ ip -netns test link property add dev ethX altname eth0
 [ ~]$ ip -netns test link set dev ethX netns 1

 [ ~]$ ip link
 ...
 3: eth0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
     link/ether 02:40:88:62:ec:b8 brd ff:ff:ff:ff:ff:ff
 ...
 5: ethX: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
     link/ether 26:b7:28:78:38:0f brd ff:ff:ff:ff:ff:ff
     altname eth0

Create a macro for walking the altnames, this hopefully makes
it clearer that the list we walk contains only altnames.
Which is otherwise not entirely intuitive.

Fixes: 36fbf1e52bd3 ("net: rtnetlink: add linkprop commands to add and delete alternative ifnames")
Signed-off-by: Jakub Kicinski <[email-masked]>
---
CC: [email-masked]
CC: [email-masked]
CC: [email-masked]
CC: [email-masked]
---
 net/core/dev.c | 9 ++++++++-
 net/core/dev.h | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index b08031957ffe..f4fa2692cf6d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1086,7 +1086,8 @@ static int __dev_alloc_name(struct net *net, const char *name, char *buf)
 
 		for_each_netdev(net, d) {
 			struct netdev_name_node *name_node;
-			list_for_each_entry(name_node, &d->name_node->list, list) {
+
+			netdev_for_each_altname(d, name_node) {
 				if (!sscanf(name_node->name, name, &i))
 					continue;
 				if (i < 0 || i >= max_netdevices)
@@ -11047,6 +11048,7 @@ EXPORT_SYMBOL(unregister_netdev);
 int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 			       const char *pat, int new_ifindex)
 {
+	struct netdev_name_node *name_node;
 	struct net *net_old = dev_net(dev);
 	char new_name[IFNAMSIZ] = {};
 	int err, new_nsid;
@@ -11079,6 +11081,11 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 		if (err < 0)
 			goto out;
 	}
+	/* Check that none of the altnames conflicts. */
+	err = -EEXIST;
+	netdev_for_each_altname(dev, name_node)
+		if (netdev_name_in_use(net, name_node->name))
+			goto out;
 
 	/* Check that new_ifindex isn't used yet. */
 	if (new_ifindex) {
diff --git a/net/core/dev.h b/net/core/dev.h
index e075e198092c..d093be175bd0 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -62,6 +62,9 @@ struct netdev_name_node {
 int netdev_get_name(struct net *net, char *name, int ifindex);
 int dev_change_name(struct net_device *dev, const char *newname);
 
+#define netdev_for_each_altname(dev, name_node)				\
+	list_for_each_entry((name_node), &(dev)->name_node->list, list)
+
 int netdev_name_node_alt_create(struct net_device *dev, const char *name);
 int netdev_name_node_alt_destroy(struct net_device *dev, const char *name);
 
-- 
2.41.0