[PATCH 6.18.y] drm/vkms: Fix ABBA deadlock in vblank disable and timer callback

[w15303746062]

From: Mingyu Wang <25181214217@xxxxxxxxxxxxxxxxx>

[Note: This patch addresses a legacy VKMS implementation deadlock specific
to older stable trees (e.g., 6.18.y). Mainline has removed this code during
the generic DRM_CRTC_VBLANK_TIMER_FUNCS refactoring.]

During local fuzzing with Syzkaller, an RCU preempt stall (soft lockup)
was observed. This is caused by an ABBA deadlock between the
drm_vblank_disable_and_save() function and the vkms_vblank_simulate()
hrtimer callback.

The race condition occurs as follows:

Thread A (CPU 3 - DRM_IOCTL_MODE_SETCRTC):
  - drm_vblank_disable_and_save() acquires `&dev->vblank_time_lock`.
  - Calls __disable_vblank() -> vkms_disable_vblank().
  - Calls hrtimer_cancel() to synchronously stop the vblank timer.
  - BLOCK: hrtimer_cancel() spins indefinitely waiting for the timer
    callback to finish executing on CPU 0.

Thread B (CPU 0 - hrtimer interrupt):
  - Executes the hrtimer callback vkms_vblank_simulate().
  - Calls drm_crtc_handle_vblank() -> drm_handle_vblank().
  - BLOCK: drm_handle_vblank() tries to acquire `&dev->vblank_time_lock`
    and spins forever because Thread A is holding it.

This patch fixes the deadlock by replacing hrtimer_cancel() with
hrtimer_try_to_cancel(). If the timer callback is running, try_to_cancel()
will safely return -1 and allow Thread A to proceed and release the lock.

Additionally, vkms_vblank_simulate() is modified to conditionally return
HRTIMER_NORESTART if drm_crtc_handle_vblank() fails (which it will,
because Thread A sets `vblank->enabled = false` immediately after
try_to_cancel). This acts as a self-destruct mechanism, preventing the
timer from blindly re-arming itself and causing an infinite loop of
DRM_ERROR messages.

Signed-off-by: Mingyu Wang <25181214217@xxxxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/vkms/vkms_crtc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
index e60573e0f3e9..a62153b73548 100644
--- a/drivers/gpu/drm/vkms/vkms_crtc.c
+++ b/drivers/gpu/drm/vkms/vkms_crtc.c
@@ -57,7 +57,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
 
 	dma_fence_end_signalling(fence_cookie);
 
-	return HRTIMER_RESTART;
+	return ret ? HRTIMER_RESTART : HRTIMER_NORESTART;
 }
 
 static int vkms_enable_vblank(struct drm_crtc *crtc)
@@ -77,7 +77,7 @@ static void vkms_disable_vblank(struct drm_crtc *crtc)
 {
 	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
 
-	hrtimer_cancel(&out->vblank_hrtimer);
+	hrtimer_try_to_cancel(&out->vblank_hrtimer);
 }
 
 static bool vkms_get_vblank_timestamp(struct drm_crtc *crtc,
-- 
2.34.1