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

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Date:	Wed, 13 Feb 2013 11:08:23 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Bruno Prémont <bonbons@...ux-vserver.org>
Cc:	Corey Minyard <minyard@....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

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.

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

>> 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.

Eric
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