netdev - Re: [PATCH 2/4] USB: wdm: close race between wdm_open and wdm_wwan_port

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Message-ID: <1fc14a6e-c77e-4030-bb62-6f6d5bb18d63@rowland.harvard.edu>
Date: Mon, 31 Mar 2025 10:37:57 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Oliver Neukum <oneukum@...e.com>
Cc: gregkh@...uxfoundation.org, bjorn@...k.no, loic.poulain@...aro.org,
	linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/4] USB: wdm: close race between wdm_open and
 wdm_wwan_port_stop

On Mon, Mar 31, 2025 at 03:25:02PM +0200, Oliver Neukum wrote:
> Clearing WDM_WWAN_IN_USE must be the last action or
> we can open a chardev whose URBs are still poisoned
> 
> Fixes: cac6fb015f71 ("usb: class: cdc-wdm: WWAN framework integration")
> Signed-off-by: Oliver Neukum <oneukum@...e.com>
> ---
>  drivers/usb/class/cdc-wdm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
> index 12038aa43942..e67844618da6 100644
> --- a/drivers/usb/class/cdc-wdm.c
> +++ b/drivers/usb/class/cdc-wdm.c
> @@ -870,8 +870,9 @@ static void wdm_wwan_port_stop(struct wwan_port *port)
>  	poison_urbs(desc);
>  	desc->manage_power(desc->intf, 0);
>  	clear_bit(WDM_READ, &desc->flags);
> -	clear_bit(WDM_WWAN_IN_USE, &desc->flags);
>  	unpoison_urbs(desc);
> +	/* this must be last lest we open a poisoned device */
> +	clear_bit(WDM_WWAN_IN_USE, &desc->flags);
>  }

This is a good example of a place where a memory barrier is needed.  
Neither unpoison_urbs() nor clear_bit() includes an explicit memory 
barrier.  So even though patch ensures that unpoison_urb() occurs before 
clear_bit() in the source code, there is still no guarantee that a CPU 
will execute them in that order.  Or even if they are executed in order, 
there is no guarantee that a different CPU will see their effects 
occurring in that order.

In this case you almost certainly need to have an smp_wmb() between the 
two statements, with a corresponding smp_rmb() (or equivalent) in the 
code that checks whether the WDM_WWAIN_IN_USE flag is set.

Alan Stern