linux-kernel - Re: [PATCH] device_cgroup: fix unchecked cgroup parent usage

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Message-Id: <20121031163501.79a0950e.akpm@linux-foundation.org>
Date:	Wed, 31 Oct 2012 16:35:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Aristeu Rozanski <aris@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Li Zefan <lizefan@...wei.com>,
	James Morris <jmorris@...ei.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Jiri Slaby <jslaby@...e.cz>, cgroups@...r.kernel.org
Subject: Re: [PATCH] device_cgroup: fix unchecked cgroup parent usage

On Wed, 31 Oct 2012 12:04:30 -0400
Aristeu Rozanski <aris@...hat.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;
>  }

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