linux-kernel - Re: [PATCH 2/3] drm/display/dp_mst: Fix drm_atomic_get_mst_topology

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Date:   Thu, 02 Jun 2022 16:43:53 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     dri-devel@...ts.freedesktop.org, stable@...r.kernel.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jani Nikula <jani.nikula@...el.com>,
        Wayne Lin <Wayne.Lin@....com>,
        Bhawanpreet Lakha <Bhawanpreet.Lakha@....com>,
        Imran Khan <imran.f.khan@...cle.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Fangzhi Zuo <Jerry.Zuo@....com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] drm/display/dp_mst: Fix
 drm_atomic_get_mst_topology_state()

On Thu, 2022-06-02 at 23:42 +0300, Ville Syrjälä wrote:
> On Thu, Jun 02, 2022 at 04:17:56PM -0400, Lyude Paul wrote:
> > I noticed a rather surprising issue here while working on removing all of
> > the non-atomic MST code: drm_atomic_get_mst_topology_state() doesn't check
> > the return value of drm_atomic_get_private_obj_state() and instead just
> > passes it directly to to_dp_mst_topology_state(). This means that if we
> > hit a deadlock or something else which would return an error code pointer,
> > we'll likely segfault the kernel.
> > 
> > This is definitely another one of those fixes where I'm astonished we
> > somehow managed never to discover this issue until now…
> 
> It has been discussed before.
> 
> struct drm_dp_mst_topology_state {
>         struct drm_private_state base;
>         ...
> }
> 
> so offsetof(base)==0.

fhjdffds yes you're right, I guess we can just drop this patch then

> 
> > 
> > Signed-off-by: Lyude Paul <lyude@...hat.com>
> > Fixes: a4370c777406 ("drm/atomic: Make private objs proper objects")
> > Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > Cc: <stable@...r.kernel.org> # v4.14+
> > ---
> >  drivers/gpu/drm/display/drm_dp_mst_topology.c | 2 +-
> >  include/drm/display/drm_dp_mst_helper.h       | 2 ++
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > index d84673b3294b..d6e595b95f07 100644
> > --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> > @@ -5468,7 +5468,7 @@ EXPORT_SYMBOL(drm_dp_mst_topology_state_funcs);
> >  struct drm_dp_mst_topology_state
> > *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
> >                                                                     struct
> > drm_dp_mst_topology_mgr *mgr)
> >  {
> > -       return
> > to_dp_mst_topology_state(drm_atomic_get_private_obj_state(state, &mgr-
> > >base));
> > +       return
> > to_dp_mst_topology_state_safe(drm_atomic_get_private_obj_state(state,
> > &mgr->base));
> >  }
> >  EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);
> >  
> > diff --git a/include/drm/display/drm_dp_mst_helper.h
> > b/include/drm/display/drm_dp_mst_helper.h
> > index 10adec068b7f..fe7577e7f305 100644
> > --- a/include/drm/display/drm_dp_mst_helper.h
> > +++ b/include/drm/display/drm_dp_mst_helper.h
> > @@ -541,6 +541,8 @@ struct drm_dp_payload {
> >  };
> >  
> >  #define to_dp_mst_topology_state(x) container_of(x, struct
> > drm_dp_mst_topology_state, base)
> > +#define to_dp_mst_topology_state_safe(x) \
> > +       container_of_safe(x, struct drm_dp_mst_topology_state, base)
> 
> Wasn't aware of container_of_safe(). I suppose no real harm 
> in using it. Not sure why we'd even keep the non-safe version
> around?
> 
> Though the use of container_of_safe() everywhere won't help
> when "casting" the other way (&foo->base, when foo==NULL/errptr).
> In order to make that work for non-zero offsets we'd have to
> introduce a casting macro for that direction as well.
> 
> >  
> >  struct drm_dp_vcpi_allocation {
> >         struct drm_dp_mst_port *port;
> > -- 
> > 2.35.3
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat