Subject: Re: [PATCH] net: usb: fix possible use-after-free in smsc75xx_bind

From: Pavel Skripkin <[email-masked]>
Date: Mon, 14 Jun 2021 19:00:45 +0300

On Mon, 14 Jun 2021 23:37:12 +0800
Dongliang Mu <[email-masked]> wrote:

> The commit 46a8b29c6306 ("net: usb: fix memory leak in smsc75xx_bind")
> fails to clean up the work scheduled in smsc75xx_reset->
> smsc75xx_set_multicast, which leads to use-after-free if the work is
> scheduled to start after the deallocation. In addition, this patch
> also removes one dangling pointer - dev->data[0].
> 
> This patch calls cancel_work_sync to cancel the schedule work and set
> the dangling pointer to NULL.
> 
> Fixes: 46a8b29c6306 ("net: usb: fix memory leak in smsc75xx_bind")
> Signed-off-by: Dongliang Mu <[email-masked]>
> ---
>  drivers/net/usb/smsc75xx.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/usb/smsc75xx.c b/drivers/net/usb/smsc75xx.c
> index b286993da67c..f81740fcc8d5 100644
> --- a/drivers/net/usb/smsc75xx.c
> +++ b/drivers/net/usb/smsc75xx.c
> @@ -1504,7 +1504,10 @@ static int smsc75xx_bind(struct usbnet *dev,
> struct usb_interface *intf) return 0;
>  
>  err:
> +	cancel_work_sync(&pdata->set_multicast);
>  	kfree(pdata);
> +	pdata = NULL;
> +	dev->data[0] = 0;
>  	return ret;
>  }
>  

Hi, Dongliang!

Just my thougth about this patch:

INIT_WORK(&pdata->set_multicast, smsc75xx_deferred_multicast_write);
does not queue anything, it just initalizes list structure and assigns
callback function. The actual work sheduling happens in
smsc75xx_set_multicast() which is smsc75xx_netdev_ops member.

In case of any error in smsc75xx_bind() the device registration fails
and smsc75xx_netdev_ops won't be registered, so, i guess, there is no
chance of UAF. 

Am I missing something? :)

With regards,
Pavel Skripkin