Patch "drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip" has been added to the 4.19-stable tree

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This is a note to let you know that I've just added the patch titled

    drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     drm-mediatek-fix-a-null-pointer-crash-in-mtk_drm_crt.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit a3133bb16f8dd5d00c80266169b753f7af37e297
Author: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Date:   Fri Feb 23 13:23:29 2024 -0800

    drm/mediatek: Fix a null pointer crash in mtk_drm_crtc_finish_page_flip
    
    [ Upstream commit c958e86e9cc1b48cac004a6e245154dfba8e163b ]
    
    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.
    
    Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
    Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
    Reviewed-by: CK Hu <ck.hu@xxxxxxxxxxxx>
    Link: https://patchwork.kernel.org/project/dri-devel/patch/20240223212404.3709690-1-hsinyi@xxxxxxxxxxxx/
    Signed-off-by: Chun-Kuang Hu <chunkuang.hu@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
index eac9caf322f90..fb7262ed9b699 100644
--- a/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
+++ b/drivers/gpu/drm/mediatek/mtk_drm_crtc.c
@@ -79,11 +79,13 @@ 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)




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