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    Subject[PATCH 4.15 26/84] watchdog: Fix potential kref imbalance when opening watchdog
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/watchdog/watchdog_dev.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    --- a/drivers/watchdog/watchdog_dev.c
    +++ b/drivers/watchdog/watchdog_dev.c
    @@ -769,6 +769,7 @@ static int watchdog_open(struct inode *i
    {
    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 *i
    * 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 *i

    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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