Re: [PATCH] drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip

[CK Hu (胡俊光)]

Hi, Hsin-yi:

On Fri, 2024-02-23 at 13:23 -0800, Hsin-Yi Wang wrote:
>  	 
> External email : Please do not click links or open attachments until
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>  It's possible that mtk_crtc->event is NULL in
> mtk_drm_crtc_finish_page_flip().
> 
> pending_needs_vblank value is set by mtk_crtc->event, but in
> mtk_drm_crtc_atomic_flush(), it's is not guarded by the same
> lock in mtk_drm_finish_page_flip(), thus a race condition happens.
> 
> Consider the following case:
> 
> CPU1                              CPU2
> step 1:
> mtk_drm_crtc_atomic_begin()
> mtk_crtc->event is not null,
>                                   step 1:
>                                   mtk_drm_crtc_atomic_flush:
>                                   mtk_drm_crtc_update_config(
>                                       !!mtk_crtc->event)
> step 2:
> mtk_crtc_ddp_irq ->
> mtk_drm_finish_page_flip:
> lock
> mtk_crtc->event set to null,
> pending_needs_vblank set to false
> unlock
>                                   pending_needs_vblank set to true,
> 
>                                   step 2:
>                                   mtk_crtc_ddp_irq ->
>                                   mtk_drm_finish_page_flip called
> again,
>                                   pending_needs_vblank is still true
>                                   //null pointer
> 
> Instead of guarding the entire mtk_drm_crtc_atomic_flush(), it's more
> efficient to just check if mtk_crtc->event is null before use.
> 
> Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
> Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC
> MT8173.")

Reviewed-by: CK Hu <ck.hu@xxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/mediatek/mtk_drm_crtc.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> index db43f9dff912..d645b85f9721 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
> @@ -95,11 +95,13 @@ static void mtk_drm_crtc_finish_page_flip(struct
> mtk_drm_crtc *mtk_crtc)
>  	struct drm_crtc *crtc = &mtk_crtc->base;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&crtc->dev->event_lock, flags);
> -	drm_crtc_send_vblank_event(crtc, mtk_crtc->event);
> -	drm_crtc_vblank_put(crtc);
> -	mtk_crtc->event = NULL;
> -	spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
> +	if (mtk_crtc->event) {
> +		spin_lock_irqsave(&crtc->dev->event_lock, flags);
> +		drm_crtc_send_vblank_event(crtc, mtk_crtc->event);
> +		drm_crtc_vblank_put(crtc);
> +		mtk_crtc->event = NULL;
> +		spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
> +	}
>  }
>  
>  static void mtk_drm_finish_page_flip(struct mtk_drm_crtc *mtk_crtc)
> -- 
> 2.44.0.rc0.258.g7320e95886-goog