linux-kernel - Re: [PATCH v2 1/9] can: ems_usb: ems_usb_disconnect(): fix NULL pointer dereference

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Message-ID: <Y5Rmp66zvlwykRLq@hovoldconsulting.com>
Date:   Sat, 10 Dec 2022 11:59:51 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org,
        Oliver Neukum <oneukum@...e.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Frank Jungclaus <frank.jungclaus@....eu>, socketcan@....eu,
        Yasushi SHOJI <yashi@...cecubics.com>,
        Stefan Mätje <stefan.maetje@....eu>,
        Hangyu Hua <hbh25y@...il.com>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Peter Fink <pfink@...ist-es.de>,
        Jeroen Hofstee <jhofstee@...tronenergy.com>,
        Christoph Möhring <cmoehring@...ist-es.de>,
        John Whittington <git@...engineering.co.uk>,
        Vasanth Sadhasivan <vasanth.sadhasivan@...sara.com>,
        Jimmy Assarsson <extja@...ser.com>,
        Anssi Hannula <anssi.hannula@...wise.fi>,
        Pavel Skripkin <paskripkin@...il.com>,
        Stephane Grosjean <s.grosjean@...k-system.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Dongliang Mu <dzm91@...t.edu.cn>,
        Sebastian Haas <haas@...-wuensche.com>,
        Maximilian Schneider <max@...neidersoft.net>,
        Daniel Berglund <db@...ser.com>,
        Olivier Sobrie <olivier@...rie.be>,
        Remigiusz Kołłątaj 
        <remigiusz.kollataj@...ica.com>,
        Jakob Unterwurzacher <jakob.unterwurzacher@...obroma-systems.com>,
        Martin Elshuber <martin.elshuber@...obroma-systems.com>,
        Bernd Krumboeck <b.krumboeck@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH v2 1/9] can: ems_usb: ems_usb_disconnect(): fix NULL
 pointer dereference

On Sat, Dec 10, 2022 at 06:01:49PM +0900, Vincent Mailhol wrote:
> ems_usb sets the driver's priv data to NULL before waiting for the
> completion of outsdanding urbs. This can results in NULL pointer
> dereference, c.f. [1] and [2].

Please stop making hand-wavy claims like this. There is no risk for a
NULL-pointer deference here, and if you think otherwise you need to
explain how that can happen in detail for each driver.

> Remove the call to usb_set_intfdata(intf, NULL). The core will take
> care of setting it to NULL after ems_usb_disconnect() at [3].
> 
> [1] c/27ef17849779 ("usb: add usb_set_intfdata() documentation")
> Link: https://git.kernel.org/gregkh/usb/c/27ef17849779

The claim in this commit is not correct either.

> [2] thread about usb_set_intfdata() on linux-usb mailing.
> Link: https://lore.kernel.org/linux-usb/Y4OD70GD4KnoRk0k@rowland.harvard.edu/
> 
> [3] function usb_unbind_interface() from drivers/usb/core/driver.c
> Link: https://elixir.bootlin.com/linux/v6.0/source/drivers/usb/core/driver.c#L497
> 
> Fixes: 702171adeed3 ("ems_usb: Added support for EMS CPC-USB/ARM7 CAN/USB interface")
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> ---
>  drivers/net/can/usb/ems_usb.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/can/usb/ems_usb.c b/drivers/net/can/usb/ems_usb.c
> index 050c0b49938a..c64cb40ac8de 100644
> --- a/drivers/net/can/usb/ems_usb.c
> +++ b/drivers/net/can/usb/ems_usb.c
> @@ -1062,8 +1062,6 @@ static void ems_usb_disconnect(struct usb_interface *intf)
>  {
>  	struct ems_usb *dev = usb_get_intfdata(intf);

The interface data pointer is only used in this function so there is no
risk for any NULL pointer dereference here. I only checked one of the
other drivers you patch, but I'm pretty sure all of your claims about
fixing NULL-pointer dereferences in this series are equally bogus.

>  
> -	usb_set_intfdata(intf, NULL);
> -
>  	if (dev) {
>  		unregister_netdev(dev->netdev);

Johan