[PATCH] drm/vblank: fix misuse of drm_WARN in drm_wait_one_vblank()

[Vitaliy Shevtsov <v.shevtsov@maxima.ru>]

drm_wait_one_vblank() uses drm_WARN() to check for a time-dependent
condition. Since syzkaller runs the kernel with the panic_on_warn set, this
causes the entire kernel to panic with a "vblank wait timed out on crtc %i"
message.

In this case it does not mean that there is something wrong with the kernel
but is caused by time delays in vblanks handling that the fuzzer introduces
as a side effect when fail_alloc_pages, failslab, fail_usercopy faults are
injected with maximum verbosity. With lower verbosity this issue disappears.

drm_WARN() was introduced here by e8450f51a4b3 ("drm/irq: Implement a
generic vblank_wait function") and it is intended to indicate a failure with
vblank irqs handling by the underlying driver. The issue is raised during
testing of the vkms driver, but it may be potentially reproduced with other
drivers.

Fix this by using drm_warn() instead which does not cause the kernel to
panic with panic_on_warn set, but still provides a way to tell users about
this unexpected condition.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: e8450f51a4b3 ("drm/irq: Implement a generic vblank_wait function")
Cc: stable@vger.kernel.org
Reported-by: syzbot+9a8f87865d5e2e8ef57f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9a8f87865d5e2e8ef57f
Signed-off-by: Vitaliy Shevtsov <v.shevtsov@maxima.ru>
---
 drivers/gpu/drm/drm_vblank.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 94e45ed6869d..fa09ff5b1d48 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -1304,7 +1304,8 @@ void drm_wait_one_vblank(struct drm_device *dev, unsigned int pipe)
 				 last != drm_vblank_count(dev, pipe),
 				 msecs_to_jiffies(100));
 
-	drm_WARN(dev, ret == 0, "vblank wait timed out on crtc %i\n", pipe);
+	if (!ret)
+		drm_warn(dev, "vblank wait timed out on crtc %i\n", pipe);
 
 	drm_vblank_put(dev, pipe);
 }
-- 
2.47.1