linux-kernel - Re: [PATCH v4] driver core: Fix use-after-free and double free on glue directory

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Date:   Tue, 18 Jun 2019 21:40:13 +0800
From:   Muchun Song <smuchun@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Prateek Sood <prsood@...eaurora.org>,
        Mukesh Ojha <mojha@...eaurora.org>, gkohli@...eaurora.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        zhaowuyun@...gtech.com
Subject: Re: [PATCH v4] driver core: Fix use-after-free and double free on
 glue directory

Ping guys ? I think this is worth fixing.

Muchun Song <smuchun@...il.com> 于2019年5月25日周六 下午8:15写道：

>
> Hi greg k-h,
>
> Greg KH <gregkh@...uxfoundation.org> 于2019年5月25日周六 上午3:04写道：
> >
> > On Thu, May 16, 2019 at 10:23:42PM +0800, Muchun Song wrote:
> > > There is a race condition between removing glue directory and adding a new
> > > device under the glue directory. It can be reproduced in following test:
> >
> > <snip>
> >
> > Is this related to:
> >         Subject: [PATCH v3] drivers: core: Remove glue dirs early only when refcount is 1
> >
> > ?
> >
> > If so, why is the solution so different?
>
> In the v1 patch, the solution is that remove glue dirs early only when
> refcount is 1. So
> the v1 patch like below:
>
> @@ -1825,7 +1825,7 @@ static void cleanup_glue_dir(struct device *dev,
> struct kobject *glue_dir)
>                 return;
>
>         mutex_lock(&gdp_mutex);
> -       if (!kobject_has_children(glue_dir))
> +       if (!kobject_has_children(glue_dir) && kref_read(&glue_dir->kref) == 1)
>                 kobject_del(glue_dir);
>         kobject_put(glue_dir);
>         mutex_unlock(&gdp_mutex);
> -----------------------------------------------------------------------
>
> But from Ben's suggestion as below:
>
> I find relying on the object count for such decisions rather fragile as
> it could be taken temporarily for other reasons, couldn't it ? In which
> case we would just fail...
>
> Ideally, the looking up of the glue dir and creation of its child
> should be protected by the same lock instance (the gdp_mutex in that
> case).
> -----------------------------------------------------------------------
>
> So another solution is used from Ben's suggestion in the v2 patch. But
> I forgot to update the commit message until the v4 patch. Thanks.
>
> Yours,
> Muchun