[PATCH -next v4] watchdog: Fix potential dereferencing of null pointer

[Wang Wensheng <wangwensheng4@huawei.com>]

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.

Fixes: d0684c8a9354 ("watchdog: Make stop function optional")
Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
---
 drivers/watchdog/watchdog_core.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index 423844757812..0e9a99559609 100644
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -267,15 +267,19 @@ 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;
-
-		ret = register_reboot_notifier(&wdd->reboot_nb);
-		if (ret) {
-			pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
-			       wdd->id, ret);
-			watchdog_dev_unregister(wdd);
-			ida_simple_remove(&watchdog_ida, id);
-			return ret;
+		if (!wdd->ops->stop)
+			pr_warn("watchdog%d: stop_on_reboot not supported\n", wdd->id);
+		else {
+			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",
+					wdd->id, ret);
+				watchdog_dev_unregister(wdd);
+				ida_simple_remove(&watchdog_ida, id);
+				return ret;
+			}
 		}
 	}
 
-- 
2.25.0