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@vger.kernel.org Signed-off-by: Qiu-ji Chen chenqiuji666@gmail.com --- 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);