linux-kernel - Re: [PATCH] drm/gud: fix NULL fb and crtc dereferences on USB disconnect

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Message-ID: <28c39f1979452b24ddde4de97e60ca721334eb49.camel@aol.com>
Date: Sat, 03 Jan 2026 17:23:33 +0000
From: Ruben Wauters <rubenru09@....com>
To: Shenghao Yang <me@...nghaoyang.info>, Maarten Lankhorst	
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] drm/gud: fix NULL fb and crtc dereferences on USB
 disconnect

Hi

On Wed, 2025-12-31 at 13:50 +0800, Shenghao Yang wrote:
> On disconnect drm_atomic_helper_disable_all() is called which
> sets both the fb and crtc for a plane to NULL before invoking a commit.
> 
> This causes a kernel oops on every display disconnect.
> 
> Add guards for those dereferences.
> 
> Cc: <stable@...r.kernel.org> # 6.18.x
> Signed-off-by: Shenghao Yang <me@...nghaoyang.info>
> ---
>  drivers/gpu/drm/gud/gud_pipe.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/gud/gud_pipe.c b/drivers/gpu/drm/gud/gud_pipe.c
> index 76d77a736d84..4b77be94348d 100644
> --- a/drivers/gpu/drm/gud/gud_pipe.c
> +++ b/drivers/gpu/drm/gud/gud_pipe.c
> @@ -457,27 +457,20 @@ int gud_plane_atomic_check(struct drm_plane *plane,
>  	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_crtc *crtc = new_plane_state->crtc;
> -	struct drm_crtc_state *crtc_state;
> +	struct drm_crtc_state *crtc_state = NULL;
>  	const struct drm_display_mode *mode;
>  	struct drm_framebuffer *old_fb = old_plane_state->fb;
>  	struct drm_connector_state *connector_state = NULL;
>  	struct drm_framebuffer *fb = new_plane_state->fb;
> -	const struct drm_format_info *format = fb->format;
> +	const struct drm_format_info *format;
>  	struct drm_connector *connector;
>  	unsigned int i, num_properties;
>  	struct gud_state_req *req;
>  	int idx, ret;
>  	size_t len;
>  
> -	if (drm_WARN_ON_ONCE(plane->dev, !fb))
> -		return -EINVAL;
> -
> -	if (drm_WARN_ON_ONCE(plane->dev, !crtc))
> -		return -EINVAL;

With the elimination of these two WARN_ON_ONCEs, it's possible that
crtc_state may not be assigned below, and therefore may be read/passed
to functions when it is NULL (e.g. line 488). Either protection for a
null crtc_state should be added to the rest of the function, or the
function shouldn't continue if crtc is NULL.

Ruben
> -	crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
> -
> -	mode = &crtc_state->mode;
> +	if (crtc)
> +		crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
>  
>  	ret = drm_atomic_helper_check_plane_state(new_plane_state, crtc_state,
>  						  DRM_PLANE_NO_SCALING,
> @@ -492,6 +485,9 @@ int gud_plane_atomic_check(struct drm_plane *plane,
>  	if (old_plane_state->rotation != new_plane_state->rotation)
>  		crtc_state->mode_changed = true;
>  
> +	mode = &crtc_state->mode;
> +	format = fb->format;
> +
>  	if (old_fb && old_fb->format != format)
>  		crtc_state->mode_changed = true;
>  
> @@ -598,7 +594,7 @@ void gud_plane_atomic_update(struct drm_plane *plane,
>  	struct drm_atomic_helper_damage_iter iter;
>  	int ret, idx;
>  
> -	if (crtc->state->mode_changed || !crtc->state->enable) {
> +	if (!crtc || crtc->state->mode_changed || !crtc->state->enable) {
>  		cancel_work_sync(&gdrm->work);
>  		mutex_lock(&gdrm->damage_lock);
>  		if (gdrm->fb) {

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