[PATCH] drm/vc4: Fix atomicity violation in vc4_crtc_send_vblank()

[Qiu-ji Chen]

Atomicity violation occurs when the vc4_crtc_send_vblank function is
executed simultaneously with modifications to crtc->state or
crtc->state->event. Consider a scenario where both crtc->state and
crtc->state->event are non-null. They can pass the validity check, but at
the same time, crtc->state or crtc->state->event could be set to null. In
this case, the validity check in vc4_crtc_send_vblank might act on the old
crtc->state and crtc->state->event (before locking), allowing invalid
values to pass the validity check, leading to null pointer dereference.

To address this issue, it is recommended to include the validity check of
crtc->state and crtc->state->event within the locking section of the
function. This modification ensures that the values of crtc->state->event
and crtc->state do not change during the validation process, maintaining
their valid conditions.

This possible bug is found by an experimental static analysis tool
developed by our team. This tool analyzes the locking APIs
to extract function pairs that can be concurrently executed, and then
analyzes the instructions in the paired functions to identify possible
concurrency bugs including data races and atomicity violations.

Fixes: 68e4a69aec4d ("drm/vc4: crtc: Create vblank reporting function")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Qiu-ji Chen <chenqiuji666@xxxxxxxxx>
---
 drivers/gpu/drm/vc4/vc4_crtc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
index 8b5a7e5eb146..98885f519827 100644
--- a/drivers/gpu/drm/vc4/vc4_crtc.c
+++ b/drivers/gpu/drm/vc4/vc4_crtc.c
@@ -575,10 +575,12 @@ void vc4_crtc_send_vblank(struct drm_crtc *crtc)
 	struct drm_device *dev = crtc->dev;
 	unsigned long flags;
 
-	if (!crtc->state || !crtc->state->event)
+	spin_lock_irqsave(&dev->event_lock, flags);
+	if (!crtc->state || !crtc->state->event) {
+		spin_unlock_irqrestore(&dev->event_lock, flags);
 		return;
+	}
 
-	spin_lock_irqsave(&dev->event_lock, flags);
 	drm_crtc_send_vblank_event(crtc, crtc->state->event);
 	crtc->state->event = NULL;
 	spin_unlock_irqrestore(&dev->event_lock, flags);
-- 
2.34.1