linux-kernel - Re: [PATCH] drm/bridge: imx: fix potential NULL pointer dereference in imx8qxp_ldb_parse_dt

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Date:   Mon, 28 Aug 2023 21:37:48 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Zhang Shurong <zhang_shurong@...mail.com>
Cc:     victor.liu@....com, andrzej.hajda@...el.com,
        neil.armstrong@...aro.org, rfoss@...nel.org, jonas@...boo.se,
        jernej.skrabec@...il.com, airlied@...il.com, daniel@...ll.ch,
        shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
        festevam@...il.com, linux-imx@....com,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: imx: fix potential NULL pointer dereference
 in imx8qxp_ldb_parse_dt_companion()

On Tue, Aug 29, 2023 at 02:01:25AM +0800, Zhang Shurong wrote:
> 在 2023年8月29日星期二 CST 上午1:28:22，Laurent Pinchart 写道：
> > Hi Zhang,
> > 
> > Thank you for the patch.
> > 
> > On Tue, Aug 29, 2023 at 12:55:01AM +0800, Zhang Shurong wrote:
> > > of_match_device() may fail and returns a NULL pointer.
> > 
> > How can it return a NULL pointer here ?
> > 
> > > Fix this by checking the return value of of_match_device().
> > > 
> > > Fixes: 3818715f62b4 ("drm/bridge: imx: Add LDB support for i.MX8qxp")
> > > Signed-off-by: Zhang Shurong <zhang_shurong@...mail.com>
> > > ---
> > > 
> > >  drivers/gpu/drm/bridge/imx/imx8qxp-ldb.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/bridge/imx/imx8qxp-ldb.c
> > > b/drivers/gpu/drm/bridge/imx/imx8qxp-ldb.c index
> > > 7984da9c0a35..d272f35c8eac 100644
> > > --- a/drivers/gpu/drm/bridge/imx/imx8qxp-ldb.c
> > > +++ b/drivers/gpu/drm/bridge/imx/imx8qxp-ldb.c
> > > @@ -488,6 +488,8 @@ static int imx8qxp_ldb_parse_dt_companion(struct imx8qxp_ldb *imx8qxp_ldb)
> > >  	 * string.
> > >  	 */
> > >  	match = of_match_device(dev->driver->of_match_table, dev);
> > > +	if (!match)
> > > +		return -ENODEV;
> > >  	if (!of_device_is_compatible(companion, match->compatible)) {
> > >  		DRM_DEV_ERROR(dev, "companion LDB is incompatible\n");
> > >  		ret = -ENXIO;
>
> I think we can make it happen by designing the platform device in a way that 
> its name aligns with that of the driver. In such a scenario, when the driver 
> is probed, the of_match_device function will return null. You can verify this 
> functionality by reviewing the following function:
> 
> static int platform_match(struct device *dev, struct device_driver *drv)

This particular device is found in OF-based systems only, and only
instantiated through DT. You can create a platform_device manually that
may match this driver, but that would be a made-up situation, not
something that can happen in practice.

-- 
Regards,

Laurent Pinchart