linux-kernel - Re: [PATCH net 2/2] ax25: fix potential reference counting leak in ax25_addr

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Date: Thu, 2 May 2024 22:56:25 +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, jreuter@...na.de, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	Lars Kellogg-Stedman <lars@...bit.com>
Subject: Re: [PATCH net 2/2] ax25: fix potential reference counting leak in
 ax25_addr_ax25dev

On Thu, May 02, 2024 at 10:43:38PM +0800, Duoming Zhou wrote:
> The reference counting of ax25_dev potentially increase more
> than once in ax25_addr_ax25dev(), which will cause memory leak.
> 
> In order to fix the above issue, only increase the reference
> counting of ax25_dev once, when the res is not null.
> 
> Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
> Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> ---
>  net/ax25/ax25_dev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
> index 07723095c60..945af92a7b6 100644
> --- a/net/ax25/ax25_dev.c
> +++ b/net/ax25/ax25_dev.c
> @@ -37,8 +37,9 @@ ax25_dev *ax25_addr_ax25dev(ax25_address *addr)
>  	for (ax25_dev = ax25_dev_list; ax25_dev != NULL; ax25_dev = ax25_dev->next)
>  		if (ax25cmp(addr, (const ax25_address *)ax25_dev->dev->dev_addr) == 0) {
>  			res = ax25_dev;
> -			ax25_dev_hold(ax25_dev);
>  		}
> +	if (res)
> +		ax25_dev_hold(ax25_dev);
                              ^^^^^^^^
It should be ax25_dev_hold(res);

This is the NULL dereference that Lars saw.  Thanks for testing, by the
way Lars.

regards,
dan carpenter