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

[Tuo Li]

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 at buaa.edu.cn>
Signed-off-by: Tuo Li <islituo at gmail.com>
---
 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