linux-kernel - Re: [PATCH v0] hamradio: delete unnecessary free

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Date:   Thu, 11 Nov 2021 06:04:39 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lin Ma <linma@....edu.cn>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jirislaby@...nel.org,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v0] hamradio: delete unnecessary free_netdev()

On Thu, 11 Nov 2021 22:00:07 +0800 Lin Ma wrote:
> The former patch "defer 6pack kfree after unregister_netdev" adds
> free_netdev() function in sixpack_close(), which is a bad copy from the
> similar code in mkiss_close(). However, this free is unnecessary as the
> flag needs_free_netdev is set to true in sp_setup(), hence the
> unregister_netdev() will free the netdev automatically.
> 
> Signed-off-by: Lin Ma <linma@....edu.cn>
> ---
>  drivers/net/hamradio/6pack.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
> index bfdf89e54752..180c8f08169b 100644
> --- a/drivers/net/hamradio/6pack.c
> +++ b/drivers/net/hamradio/6pack.c
> @@ -677,8 +677,6 @@ static void sixpack_close(struct tty_struct *tty)
>  	/* Free all 6pack frame buffers after unreg. */
>  	kfree(sp->rbuff);
>  	kfree(sp->xbuff);
> -
> -	free_netdev(sp->dev);

sp is netdev_priv() tho, so this is now a UAF. I'd go for removing the
needs_free_netdev = true instead.

>  }
>  
>  /* Perform I/O control on an active 6pack channel. */