netdev - Re: [PATCH net] ax25: Fix refcount leak issues of ax25

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Date: Thu, 9 May 2024 09:40:02 +0800 (GMT+08:00)
From: duoming@....edu.cn
To: "Dan Carpenter" <dan.carpenter@...aro.org>
Cc: "Lars Kellogg-Stedman" <lars@...bit.com>, linux-hams@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	pabeni@...hat.com, kuba@...nel.org, edumazet@...gle.com,
	davem@...emloft.net, jreuter@...na.de
Subject: Re: [PATCH net] ax25: Fix refcount leak issues of ax25_dev

On Tue, 7 May 2024 11:08:14 +0300 Dan Carpenter wrote:
> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> index 9169efb2f43a..4d1ab296d52c 100644
> --- a/net/ax25/af_ax25.c
> +++ b/net/ax25/af_ax25.c
> @@ -92,6 +92,7 @@ static void ax25_kill_by_device(struct net_device *dev)
>  				spin_unlock_bh(&ax25_list_lock);
>  				ax25_disconnect(s, ENETUNREACH);
>  				s->ax25_dev = NULL;
> +				ax25_dev_put(ax25_dev);
>  				ax25_cb_del(s);
>  				spin_lock_bh(&ax25_list_lock);
>  				goto again;
> @@ -101,11 +102,8 @@ static void ax25_kill_by_device(struct net_device *dev)
>  			lock_sock(sk);
>  			ax25_disconnect(s, ENETUNREACH);
>  			s->ax25_dev = NULL;
> -			if (sk->sk_socket) {
> -				netdev_put(ax25_dev->dev,
> -					   &s->dev_tracker);
> -				ax25_dev_put(ax25_dev);
> -			}
> +			netdev_put(ax25_dev->dev, &s->dev_tracker);
> +			ax25_dev_put(ax25_dev);

We should not decrease the refcount without checking "if (sk->sk_socket)", because 
there is a race condition between ax25_kill_by_device() and ax25_release(), if we
decrease the refcount in ax25_release(), we should not decrease it here, otherwise 
the refcount underflow will happen.

Best regards,
Duoming Zhou