linux-kernel - Re: [PATCHv3 0/2] capability controlled user-namespaces

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Date:   Mon, 8 Jan 2018 10:24:59 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     James Morris <james.l.morris@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCHv3 0/2] capability controlled user-namespaces

On Mon, Jan 8, 2018 at 10:11 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@...gle.com):
>> On Mon, Jan 8, 2018 at 7:47 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> > Quoting James Morris (james.l.morris@...cle.com):
>> >> On Mon, 8 Jan 2018, Serge E. Hallyn wrote:
>> >> I meant in terms of "marking" a user ns as "controlled" type -- it's
>> >> unnecessary jargon from an end user point of view.
>> >
>> > Ah, yes, that was my point in
>> >
>> > http://lkml.iu.edu/hypermail/linux/kernel/1711.1/01845.html
>> > and
>> > http://lkml.iu.edu/hypermail/linux/kernel/1711.1/02276.html
>> >
>> >> This may happen internally but don't make it a special case with a
>> >> different name and don't bother users with internal concepts: simply
>> >> implement capability whitelists with the default having equivalent
>
> So the challenge is to have unprivileged users be contained, while
> allowing trusted workloads in containers created by a root user to
> bypass the restriction.
>
> Now, the current proposal actually doesn't support a root user starting
> an application that it doesn't quite trust in such a way that it *is*
> subject to the whitelist.

Well, this is not hard since root process can spawn another process
and loose privileges before creating user-ns to be controlled by the
whitelist.
You need an ability to preserve the creation of user-namespaces that
exhibit 'the uncontrolled behavior' and only trusted/privileged (root)
user should have it which is maintained here.

> Which is unfortunate.  But apart from using
> ptags or a cgroup, I can't think of a good way to get us everything we
> want:
>
> 1. unprivileged users always restricted
> 2. existing unprivileged containers become restricted when whitelist
> is enabled
> 3. privileged users are able to create containers which are not restricted

all this is achieved by the patch-set without any changes to the
application with the above knob.

> 4. privileged users are able to create containers which *are* restricted
>
With this patch-set; the root user process can fork another process
with less privileges before creating a user-ns if the exec-ed process
cannot be trusted. So there is a way with little modification as
opposed to nothing available at this moment for this scenario.