drm/exynos: fix a possible null-pointer dereference due to data race in exynos_drm_crtc_atomic_disable()

The variable crtc->state->event is often protected by the lock 
crtc->dev->event_lock when is accessed. However, it is accessed as a 
condition of an if statement in exynos_drm_crtc_atomic_disable() without
holding the lock:

  if (crtc->state->event && !crtc->state->active)

However, if crtc->state->event is changed to NULL by another thread right
after the conditions of the if statement is checked to be true, a
null-pointer dereference can occur in drm_crtc_send_vblank_event():

  e->pipe = pipe;

To fix this possible null-pointer dereference caused by data race, the 
spin lock coverage is extended to protect the if statement as well as the 
function call to drm_crtc_send_vblank_event().

Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
---
 drivers/gpu/drm/exynos/exynos_drm_crtc.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

On 30/06/2023 04:19, Tuo Li wrote:
> The variable crtc->state->event is often protected by the lock 
> crtc->dev->event_lock when is accessed. However, it is accessed as a 
> condition of an if statement in exynos_drm_crtc_atomic_disable() without
> holding the lock:
> 
>   if (crtc->state->event && !crtc->state->active)
> 
> However, if crtc->state->event is changed to NULL by another thread right
> after the conditions of the if statement is checked to be true, a
> null-pointer dereference can occur in drm_crtc_send_vblank_event():
> 
>   e->pipe = pipe;
> 
> To fix this possible null-pointer dereference caused by data race, the 
> spin lock coverage is extended to protect the if statement as well as the 
> function call to drm_crtc_send_vblank_event().
> 
> Reported-by: BassCheck <bass@buaa.edu.cn>

I cannot find this report. This is an open source work and public
collaboration. The "Reported-by" usually means that the issue was
reported to us, in some way, usually in public. Can we see the report?
Otherwise adding non-public, non-verifiable reports is useless and
clutters our report-credit-system.

Best regards,
Krzysztof

On 03/07/2023 05:00, Tuo Li wrote:
> Hello,
> 
> Thanks for your reply! The report is publicly available at
> https://sites.google.com/view/basscheck/home. And this
> patch is from the 8th report on this website.


Great, thank you!

Best regards,
Krzysztof

On 30/06/2023 04:19, Tuo Li wrote:
> The variable crtc->state->event is often protected by the lock 
> crtc->dev->event_lock when is accessed. However, it is accessed as a 
> condition of an if statement in exynos_drm_crtc_atomic_disable() without
> holding the lock:
> 
>   if (crtc->state->event && !crtc->state->active)
> 
> However, if crtc->state->event is changed to NULL by another thread right
> after the conditions of the if statement is checked to be true, a
> null-pointer dereference can occur in drm_crtc_send_vblank_event():
> 
>   e->pipe = pipe;
> 
> To fix this possible null-pointer dereference caused by data race, the 
> spin lock coverage is extended to protect the if statement as well as the 
> function call to drm_crtc_send_vblank_event().
> 
> Reported-by: BassCheck <bass@buaa.edu.cn>
> Signed-off-by: Tuo Li <islituo@gmail.com>

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Best regards,
Krzysztof

> -----Original Message-----
> From: Tuo Li <islituo@gmail.com>
> Sent: Friday, June 30, 2023 11:19 AM
> To: inki.dae@samsung.com; sw0312.kim@samsung.com;
> kyungmin.park@samsung.com; airlied@gmail.com; daniel@ffwll.ch;
> krzysztof.kozlowski@linaro.org; alim.akhtar@samsung.com
> Cc: dri-devel@lists.freedesktop.org; linux-arm-kernel@lists.infradead.org;
> linux-samsung-soc@vger.kernel.org; linux-kernel@vger.kernel.org;
> baijiaju1990@outlook.com; Tuo Li <islituo@gmail.com>; BassCheck
> <bass@buaa.edu.cn>
> Subject: [PATCH] drm/exynos: fix a possible null-pointer dereference due
> to data race in exynos_drm_crtc_atomic_disable()
> 
> The variable crtc->state->event is often protected by the lock
> crtc->dev->event_lock when is accessed. However, it is accessed as a
> condition of an if statement in exynos_drm_crtc_atomic_disable() without
> holding the lock:
> 
>   if (crtc->state->event && !crtc->state->active)
> 
> However, if crtc->state->event is changed to NULL by another thread right
> after the conditions of the if statement is checked to be true, a
> null-pointer dereference can occur in drm_crtc_send_vblank_event():
> 
>   e->pipe = pipe;
> 
> To fix this possible null-pointer dereference caused by data race, the
> spin lock coverage is extended to protect the if statement as well as the
> function call to drm_crtc_send_vblank_event().
> 
> Reported-by: BassCheck <bass@buaa.edu.cn>
> Signed-off-by: Tuo Li <islituo@gmail.com>

Applied.

Thanks,
Inki Dae

> ---
>  drivers/gpu/drm/exynos/exynos_drm_crtc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> b/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> index 4153f302de7c..d19e796c2061 100644
> --- a/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> +++ b/drivers/gpu/drm/exynos/exynos_drm_crtc.c
> @@ -39,13 +39,12 @@ static void exynos_drm_crtc_atomic_disable(struct
> drm_crtc *crtc,
>  	if (exynos_crtc->ops->atomic_disable)
>  		exynos_crtc->ops->atomic_disable(exynos_crtc);
> 
> +	spin_lock_irq(&crtc->dev->event_lock);
>  	if (crtc->state->event && !crtc->state->active) {
> -		spin_lock_irq(&crtc->dev->event_lock);
>  		drm_crtc_send_vblank_event(crtc, crtc->state->event);
> -		spin_unlock_irq(&crtc->dev->event_lock);
> -
>  		crtc->state->event = NULL;
>  	}
> +	spin_unlock_irq(&crtc->dev->event_lock);
>  }
> 
>  static int exynos_crtc_atomic_check(struct drm_crtc *crtc,
> --
> 2.34.1