linux-kernel - Re: [PATCH] watchdog: Fix double-free in watchdog_cdev

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Message-ID: <03f9c262-3174-21e4-2211-df9fb2097457@roeck-us.net>
Date:   Sun, 23 Aug 2020 12:06:27 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Dinghao Liu <dinghao.liu@....edu.cn>, kjlu@....edu
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Alexander Sverdlin <alexander.sverdlin@...ia.com>,
        Krzysztof Sobota <krzysztof.sobota@...ia.com>,
        linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: Fix double-free in watchdog_cdev_register

On 8/23/20 12:13 AM, Dinghao Liu wrote:
> When misc_register() fails, wd_data will be released by the
> release callback function watchdog_core_data_release(), so
> we don't need to free it again. But when watchdog_kworker is
> NULL, we should free wd_data to prevent memleak.
> 
> Fixes: cb36e29bb0e4b ("watchdog: initialize device before misc_register")
> Signed-off-by: Dinghao Liu <dinghao.liu@....edu.cn>
> ---
>  drivers/watchdog/watchdog_dev.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index 6798addabd5a..8ee78e26feb1 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -994,8 +994,10 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
>  	wd_data->wdd = wdd;
>  	wdd->wd_data = wd_data;
>  
> -	if (IS_ERR_OR_NULL(watchdog_kworker))
> +	if (IS_ERR_OR_NULL(watchdog_kworker)) {
> +		kfree(wd_data);

This should be a separate patch, with

Fixes: 664a39236e718 ("watchdog: Introduce hardware maximum heartbeat in watchdog core")

>  		return -ENODEV;
> +	}
>  
>  	device_initialize(&wd_data->dev);
>  	wd_data->dev.devt = MKDEV(MAJOR(watchdog_devt), wdd->id);
> @@ -1021,7 +1023,6 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
>  				pr_err("%s: a legacy watchdog module is probably present.\n",
>  					wdd->info->identity);
>  			old_wd_data = NULL;
> -			kfree(wd_data);

Are you sure about this ? put_device() isn't called on &wd_data->dev
(unlike the code further below). How do you trigger the double free,
or, in other words, how is watchdog_core_data_release() ever called
in this code path ?

device_initialize() says:
	NOTE: Use put_device() to give up your reference instead of freeing
	@dev directly once you have called this function.
so it looks like put_device() should be called instead of kfree().

Thanks,
Guenter

>  			return err;
>  		}
>  	}
>