[PATCH -next v2 2/2] watchdog: Fix potential dereferencing of null pointer

[Wang Wensheng]

A reboot notifier, which stops the WDT by calling the stop hook without
any check, would be registered when we set WDOG_STOP_ON_REBOOT flag.

Howerer we allow the WDT driver to omit the stop hook since commit
"d0684c8a93549" ("watchdog: Make stop function optional") and provide
a module parameter for user that controls the WDOG_STOP_ON_REBOOT flag
in commit 9232c80659e94 ("watchdog: Add stop_on_reboot parameter to
control reboot policy"). Together that commits make user potential to
insert a watchdog driver that don't provide a stop hook but with the
stop_on_reboot parameter set, then dereferencing of null pointer occurs
on system reboot.

Check the stop hook before registering the reboot notifier to fix the
issue.

Signed-off-by: Wang Wensheng <wangwensheng4@xxxxxxxxxx>
---
 drivers/watchdog/watchdog_core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index c73871f41142..5269761ba072 100644
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -265,8 +265,12 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
 	}
 
 	if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
-		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
+		if (!wdd->ops->stop) {
+			ret = -EINVAL;
+			goto dev_unregister;
+		}
 
+		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
 		ret = register_reboot_notifier(&wdd->reboot_nb);
 		if (ret) {
 			pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
-- 
2.25.0