watchdog: Fix potential kref imbalance when opening watchdog

Web:        https://git.kernel.org/torvalds/c/4bcd615fad6adddc68b058d498b30a9e0e0db77a
Commit:     4bcd615fad6adddc68b058d498b30a9e0e0db77a
Parent:     f3b5ad89de16f5d42e8ad36fbdf85f705c1ae051
Refname:    refs/heads/master
Author:     Guenter Roeck <[email protected]>
AuthorDate: Mon Sep 25 09:17:01 2017 -0700
Committer:  Wim Van Sebroeck <[email protected]>
CommitDate: Sun Dec 17 11:01:47 2017 +0100

    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: ee142889e32f5 ("watchdog: Introduce WDOG_HW_RUNNING flag")
    Signed-off-by: Guenter Roeck <[email protected]>
    Signed-off-by: Wim Van Sebroeck <[email protected]>
---
 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 1e971a50d7fb..12fe47d8237b 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/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 */
--
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