rtc: 88pm860x: prevent use-after-free on device remove

The device's remove() attempts to shut down the delayed_work scheduled
on the kernel-global workqueue by calling flush_scheduled_work().

Unfortunately, flush_scheduled_work() does not prevent the delayed_work
from re-scheduling itself. The delayed_work might run after the device
has been removed, and touch the already de-allocated info structure.
This is a potential use-after-free.

Fix by calling cancel_delayed_work_sync() during remove(): this ensures
that the delayed work is properly cancelled, is no longer running, and
is not able to re-schedule itself.

This issue was detected with the help of Coccinelle.
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-88pm860x.c

@@ -421,7 +421,7 @@ static int pm860x_rtc_remove(struct platform_device *pdev)
 	struct pm860x_rtc_info *info = platform_get_drvdata(pdev);
 
 #ifdef VRTC_CALIBRATION
-	flush_scheduled_work();
+	cancel_delayed_work_sync(&info->calib_work);
 	/* disable measurement */
 	pm860x_set_bits(info->i2c, PM8607_MEAS_EN2, MEAS2_VRTC, 0);
 #endif	/* VRTC_CALIBRATION */