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Date: Mon, 8 Jan 2018 10:24:59 -0800
From: Mahesh Bandewar (महेश बंडेवार) <>
To: "Serge E. Hallyn" <>
Cc: James Morris <>, LKML <>, 
	Netdev <>, 
	Kernel-hardening <>, Linux API <>, 
	Kees Cook <>, "Eric W . Biederman" <>, 
	Eric Dumazet <>, David Miller <>, 
	Mahesh Bandewar <>
Subject: Re: [PATCHv3 0/2] capability controlled user-namespaces

On Mon, Jan 8, 2018 at 10:11 AM, Serge E. Hallyn <> wrote:
> Quoting Mahesh Bandewar (महेश बंडेवार) (
>> On Mon, Jan 8, 2018 at 7:47 AM, Serge E. Hallyn <> wrote:
>> > Quoting James Morris (
>> >> 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
>> >
>> >
>> > and
>> >
>> >
>> >> 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
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.

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