Re: [PATCH v1] drm/simple_kms_helper: Fix NULL pointer dereference with no active CRTC

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Feb 22, 2018 at 08:12:48AM +0200, Oleksandr Andrushchenko wrote:
> On 02/22/2018 08:09 AM, Oleksandr Andrushchenko wrote:
> > From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> > 
> > It is possible that drm_simple_kms_plane_atomic_check called
> > with no CRTC set, e.g. when user-space application sets CRTC_ID/FB_ID
> > to 0 before doing any actual drawing. This leads to NULL pointer
> > dereference because in this case new CRTC state is NULL and must be
> > checked before accessing.
> > 
> > Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > ---
> > Changes since initial:
> > - re-worked checks for null CRTC as suggested by Daniel Vetter
> > ---
> >   drivers/gpu/drm/drm_simple_kms_helper.c | 10 +++-------
> >   1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_simple_kms_helper.c b/drivers/gpu/drm/drm_simple_kms_helper.c
> > index 9ca8a4a59b74..4a1dbd88b1ec 100644
> > --- a/drivers/gpu/drm/drm_simple_kms_helper.c
> > +++ b/drivers/gpu/drm/drm_simple_kms_helper.c
> > @@ -121,12 +121,6 @@ static int drm_simple_kms_plane_atomic_check(struct drm_plane *plane,
> >   	pipe = container_of(plane, struct drm_simple_display_pipe, plane);
> >   	crtc_state = drm_atomic_get_new_crtc_state(plane_state->state,
> >   						   &pipe->crtc);
> > -	if (!crtc_state->enable)
> > -		return 0; /* nothing to check when disabling or disabled */
> > -
> > -	if (crtc_state->enable)
> > -		drm_mode_get_hv_timing(&crtc_state->mode,
> > -				       &clip.x2, &clip.y2);
> >   	ret = drm_atomic_helper_check_plane_state(plane_state, crtc_state,
> >   						  &clip,
> > @@ -137,7 +131,9 @@ static int drm_simple_kms_plane_atomic_check(struct drm_plane *plane,
> >   		return ret;
> >   	if (!plane_state->visible)
> > -		return -EINVAL;
> Daniel, I have put your R-b tag, but I had removed suggested
> "WARN_ON(crtc_state && crtc_state->enable);"
> here as it fires each time when crtc_state is not NULL.
> Please let me know if this is not ok and you want me to remove
> your R-b tag.

I'm a bit confused why that fires, but oh well. Applied, thanks for your
patch.
-Daniel

> > +		return 0;
> > +
> > +	drm_mode_get_hv_timing(&crtc_state->mode, &clip.x2, &clip.y2);
> >   	if (!pipe->funcs || !pipe->funcs->check)
> >   		return 0;
> Thank you,
> Oleksandr
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch