linux-kernel - Re: Re: [PATCH 2/2] ax25: add refcount in ax25

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Message-ID: <387eb6ac.8e2c7.17eb5c703d1.Coremail.22021233@zju.edu.cn>
Date:   Tue, 1 Feb 2022 22:53:45 +0800 (GMT+08:00)
From:   周多明 <22021233@....edu.cn>
To:     "Dan Carpenter" <dan.carpenter@...cle.com>
Cc:     linux-hams@...r.kernel.org, jreuter@...na.de, ralf@...ux-mips.org,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH 2/2] ax25: add refcount in ax25_dev to avoid UAF
 bugs

Thank you very much for your time and pointing out problems in my patch.
Another two questions you asked is shown below:

> @@ -112,20 +115,22 @@ void ax25_dev_device_down(struct net_device *dev)
>  
>  	if ((s = ax25_dev_list) == ax25_dev) {
>  		ax25_dev_list = s->next;
> +		ax25_dev_put(ax25_dev);

Do we not have to call ax25_dev_hold(s->next)?

>  		spin_unlock_bh(&ax25_dev_lock);
>  		dev->ax25_ptr = NULL;
>  		dev_put_track(dev, &ax25_dev->dev_tracker);
> -		kfree(ax25_dev);
> +		ax25_dev_put(ax25_dev);
>  		return;
>  	}
>  
>  	while (s != NULL && s->next != NULL) {
>  		if (s->next == ax25_dev) {
>  			s->next = ax25_dev->next;
> +			ax25_dev_put(ax25_dev);

ax25_dev_hold(ax25_dev->next)?

Answer:
We don't have to call ax25_dev_hold(s->next) or ax25_dev_hold(ax25_dev->next)
in ax25_dev_device_down() because we have already increased the refcount when 
we insert ax25_dev into the linked list in ax25_dev_device_up().

> @@ -83,6 +85,7 @@ void ax25_dev_device_up(struct net_device *dev)
>  	spin_lock_bh(&ax25_dev_lock);
>  	ax25_dev->next = ax25_dev_list;
>  	ax25_dev_list  = ax25_dev;
> +	ax25_dev_hold(ax25_dev);
>  	spin_unlock_bh(&ax25_dev_lock);

Best wishes,
Duoming Zhou