watchdog: Fix potential kref imbalance when opening watchdog

If a watchdog driver's open function sets WDOG_HW_RUNNING with the
expectation that the watchdog can not be stopped, but then stops the
watchdog anyway in its stop function, kref_get() wil not be called in
watchdog_open(). If the watchdog then stops on close, WDOG_HW_RUNNING
will be cleared and kref_put() will be called, causing a kref imbalance.
As result the character device data structure will be released, which in
turn will cause the system to crash on the next call to watchdog_open().

Fixes: ee142889 ("watchdog: Introduce WDOG_HW_RUNNING flag")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/watchdog_dev.c

@@ -769,6 +769,7 @@ static int watchdog_open(struct inode *inode, struct file *file)
 {
 	struct watchdog_core_data *wd_data;
 	struct watchdog_device *wdd;
+	bool hw_running;
 	int err;
 
 	/* Get the corresponding watchdog device */
@@ -788,7 +789,8 @@ static int watchdog_open(struct inode *inode, struct file *file)
 	 * If the /dev/watchdog device is open, we don't want the module
 	 * to be unloaded.
 	 */
-	if (!watchdog_hw_running(wdd) && !try_module_get(wdd->ops->owner)) {
+	hw_running = watchdog_hw_running(wdd);
+	if (!hw_running && !try_module_get(wdd->ops->owner)) {
 		err = -EBUSY;
 		goto out_clear;
 	}
@@ -799,7 +801,7 @@ static int watchdog_open(struct inode *inode, struct file *file)
 
 	file->private_data = wd_data;
 
-	if (!watchdog_hw_running(wdd))
+	if (!hw_running)
 		kref_get(&wd_data->kref);
 
 	/* dev/watchdog is a virtual (and thus non-seekable) filesystem */