[SRU][Jammy][Focal][PATCH 1/1] Input: cyttsp4_core - change del_timer_sync() to timer_shutdown_sync()

[Bethany Jamison]

From: Duoming Zhou <duoming at zju.edu.cn>

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.

Fixes: 17fb1563d69b ("Input: cyttsp4 - add core driver for Cypress TMA4XX touchscreen devices")
Signed-off-by: Duoming Zhou <duoming at zju.edu.cn>
Link: https://lore.kernel.org/r/20230421082919.8471-1-duoming@zju.edu.cn
Signed-off-by: Dmitry Torokhov <dmitry.torokhov at gmail.com>
(cherry picked from commit dbe836576f12743a7d2d170ad4ad4fd324c4d47a)
CVE-2023-4134
Signed-off-by: Bethany Jamison <bethany.jamison at canonical.com>
---
 drivers/input/touchscreen/cyttsp4_core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/input/touchscreen/cyttsp4_core.c b/drivers/input/touchscreen/cyttsp4_core.c
index dccbcb942fe5..f999265896f4 100644
--- a/drivers/input/touchscreen/cyttsp4_core.c
+++ b/drivers/input/touchscreen/cyttsp4_core.c
@@ -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)
-- 
2.34.1