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

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Hi

Am 29.07.22 um 18:40 schrieb Chrisanthus, Anitha:
> Agree with Thomas, drm_atomic_commit() will not call kmb_atomic_update() with a NULL plane. This is not an actual bug.

Indeed, it's the atomic_update function. I didn't notice at first.

> 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)

To add to my previous reply, none of these variables can legally be NULL 
here. If plane if NULL, DRM helpers would run atomic_disable instead.

If you want to fix anything, just remove the tests entirely and save a 
few CPU cycles.

Best regards
Thomas

> > > >            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)
> > Geschäftsführer: Ivo Totev

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

Attachment: OpenPGP_signature
Description: OpenPGP digital signature