[PATCH AUTOSEL for 4.14 23/67] watchdog: Fix potential kref imbalance when opening watchdog

Sasha Levin
2018-03-08T06:48:30Z

From: Guenter Roeck <linux@roeck-us.net>

[ 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>
---
drivers/watchdog/watchdog_dev.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 0826e663bd5a..e6edf3737ea7 100644
--- 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 */
--
2.14.1