drm/exynos: fix a possible null-pointer dereference due to data race in exynos_drm_cr...

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>
Link: https://sites.google.com/view/basscheck/home
Signed-off-by: Tuo Li <islituo@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Added relevant link.
Signed-off-by: Inki Dae <inki.dae@samsung.com>

diff --git a/drivers/gpu/drm/exynos/exynos_drm_crtc.c b/drivers/gpu/drm/exynos/exynos_drm_crtc.c
index 4153f30..d19e796 100644 (file)
--- 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,