[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 you have verified the sender or the content.

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 at chromium.org<mailto:hsinyi at chromium.org>>

Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")


Reviewed-by: CK Hu <ck.hu at mediatek.com>



---

 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

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