2024-02-23 21:24:34

by Hsin-Yi Wang

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

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 <[email protected]>
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

2024-02-27 08:14:27

by CK Hu (胡俊光)

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

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 <[email protected]>
> Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC
> MT8173.")

Reviewed-by: CK Hu <[email protected]>

> ---
> 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