[PATCH net] net: fix removing a namespace with conflicting altnames

[Jakub Kicinski <kuba@kernel.org>]

Mark reports a BUG() when a net namespace is removed.

    kernel BUG at net/core/dev.c:11520!

Physical interfaces moved outside of init_net get "refunded"
to init_net when that namespace disappears. The main interface
name may get overwritten in the process if it would have
conflicted. We need to also discard all conflicting altnames.
Recent fixes addressed ensuring that altnames get moved
with the main interface, which surfaced this problem.

Reported-by: Марк Коренберг <socketpair@gmail.com>
Link: https://lore.kernel.org/all/CAEmTpZFZ4Sv3KwqFOY2WKDHeZYdi0O7N5H1nTvcGp=SAEavtDg@mail.gmail.com/
Fixes: 7663d522099e ("net: check for altname conflicts when changing netdev's netns")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: daniel@iogearbox.net
CC: jiri@resnulli.us
CC: lucien.xin@gmail.com
CC: johannes.berg@intel.com

I'll follow up with a conversion to RCU freeing in -next.
---
 net/core/dev.c | 9 +++++++++
 net/core/dev.h | 3 +++
 2 files changed, 12 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index f01a9b858347..cb2dab0feee0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11551,6 +11551,7 @@ static struct pernet_operations __net_initdata netdev_net_ops = {
 
 static void __net_exit default_device_exit_net(struct net *net)
 {
+	struct netdev_name_node *name_node, *tmp;
 	struct net_device *dev, *aux;
 	/*
 	 * Push all migratable network devices back to the
@@ -11573,6 +11574,14 @@ static void __net_exit default_device_exit_net(struct net *net)
 		snprintf(fb_name, IFNAMSIZ, "dev%d", dev->ifindex);
 		if (netdev_name_in_use(&init_net, fb_name))
 			snprintf(fb_name, IFNAMSIZ, "dev%%d");
+
+		netdev_for_each_altname_safe(dev, name_node, tmp)
+			if (netdev_name_in_use(&init_net, name_node->name)) {
+				netdev_name_node_del(name_node);
+				synchronize_rcu();
+				__netdev_name_node_alt_destroy(name_node);
+			}
+
 		err = dev_change_net_namespace(dev, &init_net, fb_name);
 		if (err) {
 			pr_emerg("%s: failed to move %s to init_net: %d\n",
diff --git a/net/core/dev.h b/net/core/dev.h
index cf93e188785b..7480b4c84298 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -63,6 +63,9 @@ int dev_change_name(struct net_device *dev, const char *newname);
 
 #define netdev_for_each_altname(dev, namenode)				\
 	list_for_each_entry((namenode), &(dev)->name_node->list, list)
+#define netdev_for_each_altname_safe(dev, namenode, next)		\
+	list_for_each_entry_safe((namenode), (next), &(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);
-- 
2.43.0