[PATCH] auxdisplay: ht16k33: fix potential user-after-free on module unload

From: Miguel Ojeda
Date: Fri Feb 08 2019 - 19:15:34 EST

On module unload/remove, we need to ensure that work does not run
after we have freed resources. Concretely, cancel_delayed_work()
may return while the callback function is still running.

>From kernel/workqueue.c:

The work callback function may still be running on return,
unless it returns true and the work doesn't re-arm itself.
Explicitly flush or use cancel_delayed_work_sync() to wait on it.

Link: https://lore.kernel.org/lkml/20190204220952.30761-1-TheSven73@xxxxxxxxxxxxxx/
Reported-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
drivers/auxdisplay/ht16k33.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/auxdisplay/ht16k33.c b/drivers/auxdisplay/ht16k33.c
index a43276c76fc6..21393ec3b9a4 100644
--- a/drivers/auxdisplay/ht16k33.c
+++ b/drivers/auxdisplay/ht16k33.c
@@ -509,7 +509,7 @@ static int ht16k33_remove(struct i2c_client *client)
struct ht16k33_priv *priv = i2c_get_clientdata(client);
struct ht16k33_fbdev *fbdev = &priv->fbdev;

- cancel_delayed_work(&fbdev->work);
+ cancel_delayed_work_sync(&fbdev->work);
free_page((unsigned long) fbdev->buffer);