linux-kernel - Re: [PATCH] Move console redirect to pid namespace

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Date:	Thu, 14 Feb 2013 20:08:16 -0600
From:	Corey Minyard <tcminyard@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Bruno Prémont <bonbons@...ux-vserver.org>,
	Corey Minyard <cminyard@...sta.com>,
	containers@...ts.linux-foundation.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Move console redirect to pid namespace

On 02/13/2013 01:08 PM, Eric W. Biederman wrote:
> Bruno Prémont <bonbons@...ux-vserver.org> writes:
>
>> CCing containers list
>>
>> On Fri, 08 February 2013 minyard@....org wrote:
>>> From: Corey Minyard <cminyard@...sta.com>
>>>
>>> The console redirect - ioctl(fd, TIOCCONS) - is not in a namespace,
>>> thus a container can do a redirect and grab all the I/O on the host
>>> and all container consoles.
>>>
>>> This change puts the redirect in the pid namespace.
>>>
>>> Signed-off-by: Corey Minyard <cminyard@...sta.com>
>>> ---
>>>
>>> I'm pretty sure this patch is not correct, but I'm not quite sure the
>>> best way to fix this.  I'm not 100% sure that the pid namespace is the
>>> right place, but it seemed the most reasonable of all the choices.  The
>>> other obvious choice is the mount namespace, but it didn't seem as good
>>> a fit.
>> With recent changes, tying it to init user namespace might even be
>> better.
> With recent changes this is tied to the initial user namespace.  So the
> simple solution to this and so many other similiar security problems is
> to run your container in a user namespace.
>
> The permission check currently is capable(CAP_SYS_ADMIN) which requires
> the caller to have the CAP_SYS_ADMIN in the initial user namespace.

I'm not sure I follow.  Are these changes in k.org, or in another 
repository someplace?

>
> Is there a desire to have TIOCCONS not just fail in a container but to
> have TIOCCONS work in a container specific way?

Well, my desire is for the host console to work properly if a container 
uses TIOCCONS :-).  It seems to me that the most consistent way to 
handle this is to have TIOCCONS in a container redirect the container's 
console.

>
>>> The other problem is that I don't think you can call fput() from
>>> destroy_pid_namespace().  That can be called from interrupt context,
>>> and I don't think fput() is safe there.  I know it's not safe in 3.4
>>> with the RT patch applied.  However, the only way I've come up with to
>>> fix it is to add a workqueue, and that seems a bit heavy for this.
> Actually getting destroy_pid_namespace out of interrupt context wouldn't
> be the worst thing in the world.

I would agree, but it would still require something like a workqueue.  
Is there a better mechanism?

-corey
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