watchdog: Fix potential kref imbalance when opening watchdog

[ Upstream commit 4bcd615fad6adddc68b058d498b30a9e0e0db77a ]

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: ee142889e32f5 ("watchdog: Introduce WDOG_HW_RUNNING flag")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 0826e66..e6edf37 100644 (file)
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -768,6 +768,7 @@ 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 */
@@ -787,7 +788,8 @@ 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;
        }
@@ -798,7 +800,7 @@ 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 */