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

[syzbot]

> 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
>

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