watchdog: Fix possible use-after-free in wdt_startup()

mainline inclusion
from mainline-v5.14-rc1
commit c08a6b31
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9ROH3
CVE: CVE-2021-47324

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c08a6b31e4917034f0ed0cb457c3bb209576f542



--------------------------------

This module's remove path calls del_timer(). However, that function
does not wait until the timer handler finishes. This means that the
timer handler may still be running after the driver's remove function
has finished, which would result in a use-after-free.

Fix by calling del_timer_sync(), which makes sure the timer handler
has finished, and unable to re-schedule itself.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1620716495-108352-1-git-send-email-zou_wei@huawei.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>
Signed-off-by: Zeng Heng <zengheng4@huawei.com>