linux-kernel - Re: [PATCH] drm/vc4: hdmi: Fix pointer dereference before check

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Message-ID: <Y2jy1y3hTXsXmgjS@intel.com>
Date:   Mon, 7 Nov 2022 13:58:15 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     José Expósito <jose.exposito89@...il.com>,
        dri-devel@...ts.freedesktop.org, emma@...olt.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vc4: hdmi: Fix pointer dereference before check

On Mon, Nov 07, 2022 at 09:26:30AM +0100, Maxime Ripard wrote:
> On Wed, Nov 02, 2022 at 12:10:03PM +0100, José Expósito wrote:
> > Hi Maxime,
> > 
> > Thanks a lot for looking into the patch.
> > 
> > On Wed, Nov 02, 2022 at 10:01:53AM +0100, Maxime Ripard wrote:
> > > Hi,
> > > 
> > > On Sat, Oct 29, 2022 at 11:34:13AM +0200, José Expósito wrote:
> > > > Commit 6bed2ea3cb38 ("drm/vc4: hdmi: Reset link on hotplug") introduced
> > > > the vc4_hdmi_reset_link() function. This function dereferences the
> > > > "connector" pointer before checking whether it is NULL or not.
> > > > 
> > > > Rework variable assignment to avoid this issue.
> > > > 
> > > > Fixes: 6bed2ea3cb38 ("drm/vc4: hdmi: Reset link on hotplug")
> > > > Signed-off-by: José Expósito <jose.exposito89@...il.com>
> > > > ---
> > > >  drivers/gpu/drm/vc4/vc4_hdmi.c | 10 +++++++---
> > > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > > index 4a73fafca51b..07d058b6afb7 100644
> > > > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > > > @@ -319,9 +319,9 @@ static int reset_pipe(struct drm_crtc *crtc,
> > > >  static int vc4_hdmi_reset_link(struct drm_connector *connector,
> > > >  			       struct drm_modeset_acquire_ctx *ctx)
> > > >  {
> > > > -	struct drm_device *drm = connector->dev;
> > > > -	struct vc4_hdmi *vc4_hdmi = connector_to_vc4_hdmi(connector);
> > > > -	struct drm_encoder *encoder = &vc4_hdmi->encoder.base;
> > > > +	struct drm_device *drm;
> > > > +	struct vc4_hdmi *vc4_hdmi;
> > > > +	struct drm_encoder *encoder;
> > > >  	struct drm_connector_state *conn_state;
> > > >  	struct drm_crtc_state *crtc_state;
> > > >  	struct drm_crtc *crtc;
> > > > @@ -332,6 +332,10 @@ static int vc4_hdmi_reset_link(struct drm_connector *connector,
> > > >  	if (!connector)
> > > >  		return 0;
> > > >  
> > > > +	drm = connector->dev;
> > > > +	vc4_hdmi = connector_to_vc4_hdmi(connector);
> > > > +	encoder = &vc4_hdmi->encoder.base;
> > > > +
> > > 
> > > I don't think that's right. Connector shouldn't be NULL to begin with,
> > > how did you notice this?
> > > 
> > > Maxime
> > 
> > This issue was reported by Coverity. At the moment this function is not
> > invoked with a NULL connector by any code path. However, since the NULL
> > check is present, in my opinion, it makes sense to either remove it or
> > make it usefull just in case the preconditions change in the future.
> 
> Yeah, it makes sense
> 
> I'd ask for a small cosmetic change then, could you add the assignments
> where they are actually needed instead of at the top of the function?
> 
> Something like
> 
>  if (!connector)
>  	return 0;

Dunno why you want to keep around dead code like that.
I'd just nuke the bogus null check.

> 
> +drm = connector->dev;
>  ret = drm_modeset_lock(&drm->mode_config.connection_mutex, ctx);
> 
> ...
> 
> +vc4_hdmi = connector_to_vc4_hdmi(connector);
>  if (!vc4_hdmi_supports_scrambling(vc4_hdmi))
> 
> ...
> 
> Changing the prototype of vc4_hdmi_supports_scrambling to take a struct
> vc4_hdmi pointer would also help, it's much more convenient.
> 
> Maxime

-- 
Ville Syrjälä
Intel