Input: cyttsp4_core - change del_timer_sync() to timer_shutdown_sync()

The watchdog_timer can schedule tx_timeout_task and watchdog_work can also arm watchdog_timer. The process is shown below:

----------- timer schedules work ------------
cyttsp4_watchdog_timer() //timer handler
schedule_work(&cd->watchdog_work)

----------- work arms timer ------------ cyttsp4_watchdog_work() //workqueue callback function
cyttsp4_start_wd_timer()

    mod_timer(&cd->watchdog_timer, ...)

Although del_timer_sync() and cancel_work_sync() are called in cyttsp4_remove(), the timer and workqueue could still be rearmed. As a result, the possible use after free bugs could happen. The process is shown below:

  (cleanup routine)           |  (timer and workqueue routine)
cyttsp4_remove()              | cyttsp4_watchdog_timer() //timer
  cyttsp4_stop_wd_timer()     |   schedule_work()
    del_timer_sync()          |
                              | cyttsp4_watchdog_work() //worker
                              |   cyttsp4_start_wd_timer()
                              |     mod_timer()
    cancel_work_sync()        |
                              | cyttsp4_watchdog_timer() //timer
                              |   schedule_work()
    del_timer_sync()          |
  kfree(cd) //FREE            |
                              | cyttsp4_watchdog_work() // reschedule!
                              |   cd-> //USE

This patch changes del_timer_sync() to timer_shutdown_sync(), which could prevent rearming of the timer from the workqueue.

---
 drivers/input/touchscreen/cyttsp4_core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 0cd6f626ade..7cb26929dc7 100644
@@ -1263,9 +1263,8 @@ static void cyttsp4_stop_wd_timer(struct cyttsp4 *cd)
 	 * Ensure we wait until the watchdog timer
 	 * running on a different CPU finishes
 	 */
-	del_timer_sync(&cd->watchdog_timer);
+	timer_shutdown_sync(&cd->watchdog_timer);
 	cancel_work_sync(&cd->watchdog_work);
-	del_timer_sync(&cd->watchdog_timer);
 }
 
 static void cyttsp4_watchdog_timer(struct timer_list *t)

2.17.1

Applied, thank you.

Dmitry