[PATCH] batman-adv: Release references to inactive interfaces

[Sven Eckelmann <sven@narfation.org>]

Trying to dump the originators or the neighbors via netlink for a meshif
with an inactive primary interface is not allowed. The dump functions were
checking this correctly but they didn't handle non-existing primary
interfaces and existing _inactive_ interfaces differently.

(Primary) batadv_hard_ifaces hold a references to a net_device. And
accessing them is only allowed when either being in a RCU/spinlock
protected section or when holding a valid reference to them. The netlink
dump functions use the latter.

But because the missing specific error handling for inactive primary
interfaces, the reference was never dropped. This reference counting error
was only detected when the interface should have been removed from the
system:

  unregister_netdevice: waiting for batadv_slave_0 to become free. Usage count = 2

Fixes: 50eddf397ac3 ("batman-adv: netlink: reduce duplicate code by returning interfaces")
Reported-by: syzbot+881d65229ca4f9ae8c84@syzkaller.appspotmail.com
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 net/batman-adv/originator.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/batman-adv/originator.c b/net/batman-adv/originator.c
index c84420cb410d5d948c6612cf27e320e7e0af017e..a662408ad8673c3f5532201bb6e47caa5779b627 100644
--- a/net/batman-adv/originator.c
+++ b/net/batman-adv/originator.c
@@ -763,11 +763,16 @@ int batadv_hardif_neigh_dump(struct sk_buff *msg, struct netlink_callback *cb)
 	bat_priv = netdev_priv(mesh_iface);
 
 	primary_if = batadv_primary_if_get_selected(bat_priv);
-	if (!primary_if || primary_if->if_status != BATADV_IF_ACTIVE) {
+	if (!primary_if) {
 		ret = -ENOENT;
 		goto out_put_mesh_iface;
 	}
 
+	if (primary_if->if_status != BATADV_IF_ACTIVE) {
+		ret = -ENOENT;
+		goto out_put_primary_if;
+	}
+
 	hard_iface = batadv_netlink_get_hardif(bat_priv, cb);
 	if (IS_ERR(hard_iface) && PTR_ERR(hard_iface) != -ENONET) {
 		ret = PTR_ERR(hard_iface);
@@ -1327,11 +1332,16 @@ int batadv_orig_dump(struct sk_buff *msg, struct netlink_callback *cb)
 	bat_priv = netdev_priv(mesh_iface);
 
 	primary_if = batadv_primary_if_get_selected(bat_priv);
-	if (!primary_if || primary_if->if_status != BATADV_IF_ACTIVE) {
+	if (!primary_if) {
 		ret = -ENOENT;
 		goto out_put_mesh_iface;
 	}
 
+	if (primary_if->if_status != BATADV_IF_ACTIVE) {
+		ret = -ENOENT;
+		goto out_put_primary_if;
+	}
+
 	hard_iface = batadv_netlink_get_hardif(bat_priv, cb);
 	if (IS_ERR(hard_iface) && PTR_ERR(hard_iface) != -ENONET) {
 		ret = PTR_ERR(hard_iface);

---
base-commit: def64eeace3150b88d5823fcb733dadd79d7562a
change-id: 20250927-netlink-free-inactive-if-121d06a4b616

Best regards,
-- 
Sven Eckelmann <sven@narfation.org>