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

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Message-ID: <21aff512.26d4.1741e3e3bef.Coremail.dinghao.liu@zju.edu.cn>
Date:   Mon, 24 Aug 2020 10:13:49 +0800 (GMT+08:00)
From:   dinghao.liu@....edu.cn
To:     "Guenter Roeck" <linux@...ck-us.net>
Cc:     kjlu@....edu, "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: 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")
> 

Thank you for your advice. I will send a new patch to fix this soon.

> >  		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().
> 

You are right. The callback will free wd_data->dev after all references
have gone away. So we should use put_device() here. I will fix this soon.

Regards,
Dinghao