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

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Date:   Sun, 11 Dec 2022 20:24:29 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Johan Hovold <johan@...nel.org>
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 Tue. 10 déc. 2022 à 20:02, Johan Hovold <johan@...nel.org> wrote:
> 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.

Understood.

*My* mistake comes from this message from Alan [1]:

| But if a driver does make the call, it should be careful to
| ensure that the call happens _after_ the driver is finished
| using the interface-data pointer.  For example, after all
| outstanding URBs have completed, if the completion handlers
| will need to call usb_get_intfdata().

I did not pay enough attention to the "if the completion handlers will
need to call usb_get_intfdata()" part and jumped into the incorrect
conclusion that any use of usb_set_intfdata(intf, NULL) before URB
completion was erroneous.

My deep apologies for all the noise. Please forget this series and one
more time, thank you for your patience.

[1] https://lore.kernel.org/linux-usb/Y4OD70GD4KnoRk0k@rowland.harvard.edu/