[PATCH] wireguard: device: use exit_rtnl callback instead of manual rtnl_lock in pre_exit

[Shardul Bankar]

wg_netns_pre_exit() manually acquires rtnl_lock() inside the
pernet .pre_exit callback.  This causes a hung task when another
thread holds rtnl_mutex - the cleanup_net workqueue (or the
setup_net failure rollback path) blocks indefinitely in
wg_netns_pre_exit() waiting to acquire the lock.

Convert to .exit_rtnl, introduced in commit 7a60d91c690b ("net:
Add ->exit_rtnl() hook to struct pernet_operations."), where the
framework already holds RTNL and batches all callbacks under a
single rtnl_lock()/rtnl_unlock() pair, eliminating the contention
window.

The rcu_assign_pointer(wg->creating_net, NULL) is safe to move
from .pre_exit to .exit_rtnl (which runs after synchronize_rcu())
because all RCU readers of creating_net either use maybe_get_net()
- which returns NULL for a dying namespace with zero refcount - or
access net->user_ns which remains valid throughout the entire
ops_undo_list sequence.

Reported-by: syzbot+pFBD3bslSSshiJCd3rxy@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?id=cb64c22a492202ca929e18262fdb8cb89e635c70
Signed-off-by: Shardul Bankar <shardul.b@xxxxxxxxxxxxxxxxxx>
---
 drivers/net/wireguard/device.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/net/wireguard/device.c b/drivers/net/wireguard/device.c
index 46a71ec36af8..eb854c5294a3 100644
--- a/drivers/net/wireguard/device.c
+++ b/drivers/net/wireguard/device.c
@@ -411,12 +411,11 @@ static struct rtnl_link_ops link_ops __read_mostly = {
 	.newlink		= wg_newlink,
 };
 
-static void wg_netns_pre_exit(struct net *net)
+static void wg_netns_exit_rtnl(struct net *net, struct list_head *dev_kill_list)
 {
 	struct wg_device *wg;
 	struct wg_peer *peer;
 
-	rtnl_lock();
 	list_for_each_entry(wg, &device_list, device_list) {
 		if (rcu_access_pointer(wg->creating_net) == net) {
 			pr_debug("%s: Creating namespace exiting\n", wg->dev->name);
@@ -429,11 +428,10 @@ static void wg_netns_pre_exit(struct net *net)
 			mutex_unlock(&wg->device_update_lock);
 		}
 	}
-	rtnl_unlock();
 }
 
 static struct pernet_operations pernet_ops = {
-	.pre_exit = wg_netns_pre_exit
+	.exit_rtnl = wg_netns_exit_rtnl
 };
 
 int __init wg_device_init(void)
-- 
2.34.1