[PATCH -next] 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.

Fixes: d0684c8a9354 ("watchdog: Make stop function optional")
Signed-off-by: Wang Wensheng <wangwensheng4@xxxxxxxxxx>
---
 drivers/watchdog/watchdog_core.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index 423844757812..5269761ba072 100644
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -252,10 +252,8 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
 		wdd->id = id;
 
 		ret = watchdog_dev_register(wdd);
-		if (ret) {
-			ida_simple_remove(&watchdog_ida, id);
-			return ret;
-		}
+		if (ret)
+			goto id_remove;
 	}
 
 	/* Module parameter to force watchdog policy on reboot. */
@@ -267,15 +265,17 @@ 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",
 			       wdd->id, ret);
-			watchdog_dev_unregister(wdd);
-			ida_simple_remove(&watchdog_ida, id);
-			return ret;
+			goto dev_unregister;
 		}
 	}
 
@@ -289,6 +289,13 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
 	}
 
 	return 0;
+
+dev_unregister:
+	watchdog_dev_unregister(wdd);
+id_remove:
+	ida_simple_remove(&watchdog_ida, id);
+
+	return ret;
 }
 
 /**
-- 
2.25.0