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

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Message-ID: <1cbe8280-4429-204f-c0a8-4f666707052e@roeck-us.net>
Date:   Mon, 2 Nov 2020 00:15:52 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Wang Wensheng <wangwensheng4@...wei.com>, wim@...ux-watchdog.org,
        linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     rui.xiang@...wei.com, guohanjun@...wei.com
Subject: Re: [PATCH -next] watchdog: Fix potential dereferencing of null
 pointer

On 11/1/20 11:44 PM, 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.
> 

This does a bit more: It also cleans up error handling. Which makes sense,
but should be done in a second patch.

Guenter

> Fixes: d0684c8a9354 ("watchdog: Make stop function optional")
> Signed-off-by: Wang Wensheng <wangwensheng4@...wei.com>
> ---
>  drivers/watchdog/watchdog_core.c | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
> index 423844757812..5269761ba072 100644
> --- a/drivers/watchdog/watchdog_core.c
> +++ b/drivers/watchdog/watchdog_core.c
> @@ -252,10 +252,8 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
>  		wdd->id = id;
>  
>  		ret = watchdog_dev_register(wdd);
> -		if (ret) {
> -			ida_simple_remove(&watchdog_ida, id);
> -			return ret;
> -		}
> +		if (ret)
> +			goto id_remove;
>  	}
>  
>  	/* Module parameter to force watchdog policy on reboot. */
> @@ -267,15 +265,17 @@ 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;
> +		if (!wdd->ops->stop) {
> +			ret = -EINVAL;
> +			goto dev_unregister;
> +		}
>  
> +		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;
> +			goto dev_unregister;
>  		}
>  	}
>  
> @@ -289,6 +289,13 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
>  	}
>  
>  	return 0;
> +
> +dev_unregister:
> +	watchdog_dev_unregister(wdd);
> +id_remove:
> +	ida_simple_remove(&watchdog_ida, id);
> +
> +	return ret;
>  }
>  
>  /**
>