RE: [PATCH] drm/kmb: fix dereference before NULL check in kmb_plane_atomic_update()

[Chrisanthus, Anitha]

Agree with Thomas, drm_atomic_commit() will not call kmb_atomic_update() with a NULL plane. This is not an actual bug.

Thanks,
Anitha 

> -----Original Message-----
> From: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Sent: Friday, July 29, 2022 7:26 AM
> To: Zeng Jingxiang <zengjx95@xxxxxxxxx>; Chrisanthus, Anitha
> <anitha.chrisanthus@xxxxxxxxx>; edmund.j.dea@xxxxxxxxx; airlied@xxxxxxxx;
> daniel@xxxxxxxx; laurent.pinchart@xxxxxxxxxxxxxxxx; maxime@xxxxxxxxxx;
> ville.syrjala@xxxxxxxxxxxxxxx
> Cc: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH] drm/kmb: fix dereference before NULL check in
> kmb_plane_atomic_update()
> 
> Hi
> 
> Am 29.07.22 um 16:23 schrieb Thomas Zimmermann:
> > Hi
> >
> > Am 29.07.22 um 05:07 schrieb Zeng Jingxiang:
> >> From: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>
> >>
> >> The "plane" pointer was access before checking if it was NULL.
> >>
> >> The drm_atomic_get_old_plane_state() function will dereference
> >> the pointer "plane".
> >> 345    struct drm_plane_state *old_plane_state =
> >>         drm_atomic_get_old_plane_state(state, plane);
> >> 346    struct drm_plane_state *new_plane_state =
> >>         drm_atomic_get_new_plane_state(state, plane);
> >>
> >> A NULL check for "plane" indicates that it may be NULL
> >> 363    if (!plane || !new_plane_state || !old_plane_state)
> >
> > Is this an actual bug that happens?
> >
> > All planes should always have a state. Therefore the tests for
> > !new_plane_state and !old_plane_state can be removed, I'd say.
> 
> Just to clarify: moving the test for !plane before using one of the
> get_plane_state functions is a correct bugfix.
> 
> Best regards
> Thomas
> 
> >
> > Best regards
> > Thomas
> >
> >>
> >> Fixes: 977697e20b3d ("drm/atomic: Pass the full state to planes atomic
> >> disable and update")
> >> Fixes: 37418bf14c13 ("drm: Use state helper instead of the plane state
> >> pointer")
> >> Signed-off-by: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>
> >> ---
> >>   drivers/gpu/drm/kmb/kmb_plane.c | 13 ++++++++-----
> >>   1 file changed, 8 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/kmb/kmb_plane.c
> >> b/drivers/gpu/drm/kmb/kmb_plane.c
> >> index 2735b8eb3537..d2bc998b65ce 100644
> >> --- a/drivers/gpu/drm/kmb/kmb_plane.c
> >> +++ b/drivers/gpu/drm/kmb/kmb_plane.c
> >> @@ -342,10 +342,7 @@ static void kmb_plane_set_alpha(struct
> >> kmb_drm_private *kmb,
> >>   static void kmb_plane_atomic_update(struct drm_plane *plane,
> >>                       struct drm_atomic_state *state)
> >>   {
> >> -    struct drm_plane_state *old_plane_state =
> >> drm_atomic_get_old_plane_state(state,
> >> -                                         plane);
> >> -    struct drm_plane_state *new_plane_state =
> >> drm_atomic_get_new_plane_state(state,
> >> -                                         plane);
> >> +    struct drm_plane_state *old_plane_state, *new_plane_state;
> >>       struct drm_framebuffer *fb;
> >>       struct kmb_drm_private *kmb;
> >>       unsigned int width;
> >> @@ -360,7 +357,13 @@ static void kmb_plane_atomic_update(struct
> >> drm_plane *plane,
> >>       static dma_addr_t addr[MAX_SUB_PLANES];
> >>       struct disp_cfg *init_disp_cfg;
> >> -    if (!plane || !new_plane_state || !old_plane_state)
> >> +    if (!plane)
> >> +        return;
> >> +
> >> +    old_plane_state = drm_atomic_get_old_plane_state(state, plane);
> >> +    new_plane_state = drm_atomic_get_new_plane_state(state, plane);
> >> +
> >> +    if (!new_plane_state || !old_plane_state)
> >>           return;
> >>       fb = new_plane_state->fb;
> >
> 
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
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