linux-kernel - Re: [PATCH] mtd: nand: mxc_nand: fix a possible NULL dereference

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Message-ID: <20151112090154.GG24008@pengutronix.de>
Date:	Thu, 12 Nov 2015 10:01:54 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Frans Klaver <fransklaver@...il.com>
Cc:	LABBE Corentin <clabbe.montjoie@...il.com>, baruch@...s.co.il,
	Brian Norris <computersforpeace@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Krzysztof Kozłowski <k.kozlowski.k@...il.com>,
	Luis de Bethencourt <luis@...ethencourt.com>,
	kernel@...gutronix.de,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devicetree@...r.kernel.org
Subject: Re: [PATCH] mtd: nand: mxc_nand: fix a possible NULL dereference

On Thu, Nov 12, 2015 at 09:57:07AM +0100, Frans Klaver wrote:
> On Thu, Nov 12, 2015 at 9:53 AM, Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > CC += devicetree@...r.kernel.org, gregkh
> 
> You added linux@...gutronix instead of devicetree.

Well I substituted Sascha by kernel@...gutronix.de on purpose, but
considered that too unimportant for the outer world :-) But I really
forgot devicetree@...r.kernel.org. Added now.

> > On Thu, Nov 12, 2015 at 09:36:55AM +0100, Frans Klaver wrote:
> >> On Thu, Nov 12, 2015 at 9:26 AM, Uwe Kleine-König
> >> <u.kleine-koenig@...gutronix.de> wrote:
> >> > On Thu, Nov 12, 2015 at 09:03:11AM +0100, Frans Klaver wrote:
> >> >> Hi,
> >> >>
> >> >> On Thu, Nov 12, 2015 at 8:46 AM, LABBE Corentin
> >> >> <clabbe.montjoie@...il.com> wrote:
> >> >> > of_match_device could return NULL, and so cause a NULL pointer
> >> >> > dereference later.
> >> >>
> >> >> Did you actually run into this? It seems to me that this driver is
> >> >> only probed if and only if we have a match and that therefore
> >> >> of_match_device will always return a valid pointer (it is using the
> >> >> same match table). Am I missing something?
> >> >
> >> > Yes, you're missing something. The driver would probe for a dt snippet
> >> > like:
> >> >
> >> >         mxc_nand {
> >> >                 compatible = "foobar";
> >> >         }
> >> >
> >> > In this case dev->of_node is non-NULL but of_match_device(mxcnd_dt_ids,
> >> > dev) is.
> >> >
> >> > (I didn't actually test this, so there is a chance I'm wrong here. And
> >> > if not I wonder if it is sensible at all to match the device name on
> >> > driver name for of-created platform devices.)
> >>
> >> Yea, looks like you're right. platform devices check a number of
> >> things to determine a match, among which is driver name if all else
> >> fails (platform.c, platform_match()).
> >
> > Maybe something like this would help to reduce surprises:
> >
> > diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> > index f80aaaf9f610..a9fc22c86552 100644
> > --- a/drivers/base/platform.c
> > +++ b/drivers/base/platform.c
> > @@ -840,8 +840,8 @@ static int platform_match(struct device *dev, struct device_driver *drv)
> >                 return !strcmp(pdev->driver_override, drv->name);
> >
> >         /* Attempt an OF style match first */
> > -       if (of_driver_match_device(dev, drv))
> > -               return 1;
> > +       if (pdev->dev.of_node)
> > +               return of_driver_match_device(dev, drv);
> >
> >         /* Then try ACPI style match */
> >         if (acpi_driver_match_device(dev, drv))
> 
> That looks sensible, yea. There is a chance that misbehaving DT nodes
> will fail after this change, of course.

Which is ok if this behaviour is considered a misbehave :-)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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