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

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Date: Sat, 4 May 2024 14:04:26 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Duoming Zhou <duoming@....edu.cn>
Cc: 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,
	lars@...bit.com
Subject: Re: [PATCH net] ax25: Fix refcount leak issues of ax25_dev

On Wed, May 01, 2024 at 02:02:18PM +0800, Duoming Zhou wrote:
> @@ -58,7 +59,6 @@ void ax25_dev_device_up(struct net_device *dev)
>  		return;
>  	}
>  
> -	refcount_set(&ax25_dev->refcount, 1);

Let's keep this here, and just delete the ax25_dev_hold().  It makes
the diff smaller and I like setting the refcount earlier anyway.

>  	dev->ax25_ptr     = ax25_dev;

Let's move this assignment under the spinlock where ax25_dev_hold() was.

>  	ax25_dev->dev     = dev;
>  	netdev_hold(dev, &ax25_dev->dev_tracker, GFP_KERNEL);
> @@ -88,7 +88,7 @@ void ax25_dev_device_up(struct net_device *dev)
>  	ax25_dev->next = ax25_dev_list;
>  	ax25_dev_list  = ax25_dev;
>  	spin_unlock_bh(&ax25_dev_lock);
> -	ax25_dev_hold(ax25_dev);
> +	refcount_set(&ax25_dev->refcount, 1);
>  
>  	ax25_register_dev_sysctl(ax25_dev);
>  }
> @@ -135,7 +135,6 @@ void ax25_dev_device_down(struct net_device *dev)
>  
>  unlock_put:
>  	spin_unlock_bh(&ax25_dev_lock);
> -	ax25_dev_put(ax25_dev);
>  	dev->ax25_ptr = NULL;
>  	netdev_put(dev, &ax25_dev->dev_tracker);
>  	ax25_dev_put(ax25_dev);

So far as I can see, the ax25_dev should be on the list.  Also, I think
the dev->ax25_ptr = NULL; assignment should be under the lock.  So this
code should just look like:

        list_for_each_entry(s, &ax25_dev_list, list) {
                if (s->forward == dev)
                        s->forward = NULL;
        }

        list_for_each_entry(s, &ax25_dev_list, list) {
                if (s == ax25_dev) {
                        list_del(&s->list);
                        break;
                }
        }
        dev->ax25_ptr = NULL;
        spin_unlock_bh(&ax25_dev_lock);
        netdev_put(dev, &ax25_dev->dev_tracker);
        ax25_dev_put(ax25_dev);
}

Also it should just be on the list once...  In fact, it's impossible for
one pointer to be on a list twice.  So it would be nice to add a break;
in ax25_addr_ax25dev().  It doesn't change the code, it just makes it
more obvious.

ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
{
        ax25_dev *ax25_dev, *res = NULL;

        spin_lock_bh(&ax25_dev_lock);
        list_for_each_entry(ax25_dev, &ax25_dev_list, list) {
                if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
                        res = ax25_dev;
			ax25_dev_hold(res);
                        break;
                }
        }
        spin_unlock_bh(&ax25_dev_lock);

        return res;
}

regards,
dan carpenter