linux-kernel - [PATCH AUTOSEL 6.4 21/31] drm/exynos: fix a possible null-pointer dereference due to data race in exynos_drm_crtc_atomic

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Message-Id: <20230908193201.3462957-21-sashal@kernel.org>
Date:   Fri,  8 Sep 2023 15:31:50 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Tuo Li <islituo@...il.com>, BassCheck <bass@...a.edu.cn>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Inki Dae <inki.dae@...sung.com>,
        Sasha Levin <sashal@...nel.org>, sw0312.kim@...sung.com,
        kyungmin.park@...sung.com, airlied@...il.com, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org
Subject: [PATCH AUTOSEL 6.4 21/31] drm/exynos: fix a possible null-pointer dereference due to data race in exynos_drm_crtc_atomic_disable()

From: Tuo Li <islituo@...il.com>

[ Upstream commit 2e63972a2de14482d0eae1a03a73e379f1c3f44c ]

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@...a.edu.cn>
Link: https://sites.google.com/view/basscheck/home
Signed-off-by: Tuo Li <islituo@...il.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Added relevant link.
Signed-off-by: Inki Dae <inki.dae@...sung.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 4153f302de7c4..d19e796c20613 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,
-- 
2.40.1