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Date: Wed, 31 Oct 2012 16:35:01 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Aristeu Rozanski <aris@redhat.com>
Cc: linux-kernel@vger.kernel.org, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>, James Morris <jmorris@namei.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Serge Hallyn <serge.hallyn@canonical.com>, Jiri Slaby <jslaby@suse.cz>,
        cgroups@vger.kernel.org
Subject: Re: [PATCH] device_cgroup: fix unchecked cgroup parent usage
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On Wed, 31 Oct 2012 12:04:30 -0400
Aristeu Rozanski <aris@redhat.com> wrote:

> In 4cef7299b4786879a3e113e84084a72b24590c5b the cgroup parent usage is
> unchecked. root will not have a parent and trying to use
> device.{allow,deny} will cause problems.

>>From my reading of the code "problems" means "kernel null pointer
dereference".

> For some reason my stressing
> scripts didn't test the root directory so I didn't catch it on my
> regular tests.
> 
> --- github.orig/security/device_cgroup.c	2012-10-26 17:18:01.739366780 -0400
> +++ github/security/device_cgroup.c	2012-10-29 10:03:33.221918003 -0400
> @@ -352,6 +352,8 @@
>   */
>  static inline int may_allow_all(struct dev_cgroup *parent)

offtopic: this function could quite neatly have a bool return type.

>  {
> +	if (!parent)
> +		return 1;

hm.  Does it need a comment explaining what and why?  I guess not...  just.

>  	return parent->behavior == DEVCG_DEFAULT_ALLOW;
>  }