Re: [PATCH -next v4] watchdog: Fix potential dereferencing of null pointer

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Nov 09, 2020 at 01:05:12PM +0000, Wang Wensheng wrote:
> A reboot notifier, which stops the WDT by calling the stop hook without
> any check, would be registered when we set WDOG_STOP_ON_REBOOT flag.
> 
> Howerer we allow the WDT driver to omit the stop hook since commit
> "d0684c8a93549" ("watchdog: Make stop function optional") and provide
> a module parameter for user that controls the WDOG_STOP_ON_REBOOT flag
> in commit 9232c80659e94 ("watchdog: Add stop_on_reboot parameter to
> control reboot policy"). Together that commits make user potential to
> insert a watchdog driver that don't provide a stop hook but with the
> stop_on_reboot parameter set, then dereferencing of null pointer occurs
> on system reboot.
> 
> Check the stop hook before registering the reboot notifier to fix the
> issue.
> 
> Fixes: d0684c8a9354 ("watchdog: Make stop function optional")
> Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>

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

> ---
>  drivers/watchdog/watchdog_core.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
> index 423844757812..0e9a99559609 100644
> --- a/drivers/watchdog/watchdog_core.c
> +++ b/drivers/watchdog/watchdog_core.c
> @@ -267,15 +267,19 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
>  	}
>  
>  	if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
> -		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
> -
> -		ret = register_reboot_notifier(&wdd->reboot_nb);
> -		if (ret) {
> -			pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
> -			       wdd->id, ret);
> -			watchdog_dev_unregister(wdd);
> -			ida_simple_remove(&watchdog_ida, id);
> -			return ret;
> +		if (!wdd->ops->stop)
> +			pr_warn("watchdog%d: stop_on_reboot not supported\n", wdd->id);
> +		else {
> +			wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
> +
> +			ret = register_reboot_notifier(&wdd->reboot_nb);
> +			if (ret) {
> +				pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
> +					wdd->id, ret);
> +				watchdog_dev_unregister(wdd);
> +				ida_simple_remove(&watchdog_ida, id);
> +				return ret;
> +			}
>  		}
>  	}
>  
> -- 
> 2.25.0
>