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

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Feb 13, 2018 at 10:44:16AM +0200, 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>
> ---
>  drivers/gpu/drm/drm_simple_kms_helper.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_simple_kms_helper.c b/drivers/gpu/drm/drm_simple_kms_helper.c
> index 9ca8a4a59b74..a05eca9cec8b 100644
> --- a/drivers/gpu/drm/drm_simple_kms_helper.c
> +++ b/drivers/gpu/drm/drm_simple_kms_helper.c
> @@ -121,8 +121,10 @@ 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 || !crtc_state->enable)
> +		/* nothing to check when disabling or disabled or no CRTC set */
> +		return 0;
>  
>  	if (crtc_state->enable)
>  		drm_mode_get_hv_timing(&crtc_state->mode,

Hm, this is a bit annoying, since the can_position = false parameter to
drm_atomic_helper_check_plane_state is supposed to catch all this. Would
moving all the checks after the call to that helper, and gating them on
plane_state->visible also work?

We'd need to add a guarantee to drm_atomic_helper_check_plane_state that
it can cope with crtc_state == NULL, but I think that's a good idea
anyway. Atm it shouldn't end up looking at the crtc_state pointer in that
case.

Otherwise we'll just go with your fix, but it feels all a bit too fragile,
hence why I want to explore more robust options a bit.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch