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

[Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>]

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.")
---
 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