linux-kernel - Re: [PATCH v2 2/2] drm/vc4: hdmi: Fix pointer dereference before check

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Message-ID: <Y3ZwEc4x/wIsroTI@dev-arch.thelio-3990X>
Date:   Thu, 17 Nov 2022 10:32:01 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     José Expósito <jose.exposito89@...il.com>,
        mripard@...nel.org, emma@...olt.net, airlied@...il.com,
        daniel@...ll.ch, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2 2/2] drm/vc4: hdmi: Fix pointer dereference before
 check

On Thu, Nov 17, 2022 at 09:28:49AM -0800, Nick Desaulniers wrote:
> On Thu, Nov 10, 2022 at 02:47:52PM +0100, 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 | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > index a49f88e5d2b9..6b223a5fcf6f 100644
> > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > @@ -318,8 +318,8 @@ 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_device *drm;
> > +	struct vc4_hdmi *vc4_hdmi;
> 
> Hi, I think this change, or another in this area recently, is causing
> the following warning. PTAL
> 
> drivers/gpu/drm/vc4/vc4_hdmi.c:351:14: warning: variable 'vc4_hdmi' is uninitialized when used here [-Wuninitialized]
>         mutex_lock(&vc4_hdmi->mutex);
>                     ^~~~~~~~
> drivers/gpu/drm/vc4/vc4_hdmi.c:322:27: note: initialize the variable 'vc4_hdmi' to silence this warning
>         struct vc4_hdmi *vc4_hdmi;
>                                  ^
>                                   = NULL

Guess we just crossed paths but this is just a problem with -next due to
a bad conflict resolution on Stephen's part:

https://lore.kernel.org/Y3ZvffZiR+SgtY6h@dev-arch.thelio-3990X/

> >  	struct drm_connector_state *conn_state;
> >  	struct drm_crtc_state *crtc_state;
> >  	struct drm_crtc *crtc;
> > @@ -330,6 +330,7 @@ static int vc4_hdmi_reset_link(struct drm_connector *connector,
> >  	if (!connector)
> >  		return 0;
> >  
> > +	drm = connector->dev;
> >  	ret = drm_modeset_lock(&drm->mode_config.connection_mutex, ctx);
> >  	if (ret)
> >  		return ret;
> > @@ -347,6 +348,7 @@ static int vc4_hdmi_reset_link(struct drm_connector *connector,
> >  	if (!crtc_state->active)
> >  		return 0;
> >  
> > +	vc4_hdmi = connector_to_vc4_hdmi(connector);

This version of the patch is fine, as there is no mutex_lock() call
here.

> >  	if (!vc4_hdmi_supports_scrambling(vc4_hdmi))
> >  		return 0;
> >  
> > -- 
> > 2.25.1
> > 
> > 
> 

Cheers,
Nathan