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Date: Mon, 6 Nov 2017 18:17:21 -0500
From: Boris Lukashev <>
To: "Serge E. Hallyn" <>
Cc: Daniel Micay <>, 
	Mahesh Bandewar (महेश बंडेवार) <>, 
	Mahesh Bandewar <>, LKML <>, 
	Netdev <>, 
	Kernel-hardening <>, Linux API <>, 
	Kees Cook <>, "Eric W . Biederman" <>, 
	Eric Dumazet <>, David Miller <>
Subject: Re: Re: [PATCH resend 2/2] userns: control
 capabilities of some user namespaces

On Mon, Nov 6, 2017 at 5:14 PM, Serge E. Hallyn <> wrote:
> Quoting Daniel Micay (
>> Substantial added attack surface will never go away as a problem. There
>> aren't a finite number of vulnerabilities to be found.
> There's varying levels of usefulness and quality.  There is code which I
> want to be able to use in a container, and code which I can't ever see a
> reason for using there.  The latter, especially if it's also in a
> staging driver, would be nice to have a toggle to disable.
> You're not advocating dropping the added attack surface, only adding a
> way of dealing with an 0day after the fact.  Privilege raising 0days can
> exist anywhere, not just in code which only root in a user namespace can
> exercise.  So from that point of view, ksplice seems a more complete
> solution.  Why not just actually fix the bad code block when we know
> about it?
> Finally, it has been well argued that you can gain many new caps from
> having only a few others.  Given that, how could you ever be sure that,
> if an 0day is found which allows root in a user ns to abuse
> CAP_NET_ADMIN against the host, just keeping CAP_NET_ADMIN from them
> would suffice?  It seems to me that the existing control in
> /proc/sys/kernel/unprivileged_userns_clone might be the better duct tape
> in that case.
> -serge

This seems to be heading toward "we need full zones in Linux" with
their own procfs and sysfs namespace and a stricter isolation model
for resources and capabilities. So long as things can happen in a
namespace which have a privileged relationship with host resources,
this is going to be cat-and-mouse to one degree or another.

Containers and namespaces dont have a one-to-one relationship, so i'm
not sure that's the best term to use in the kernel security context
since there's a bunch of userspace and implementation delta across the
different systems (with their own security models and so forth).
Without accounting for what a specific implementation may or may not
do, and only looking at "how do we reduce privileged impact on parent
context from unprivileged namespaces," this patch does seem to provide
a logical way of reducing the privileges available in such a namespace
and often needed to mount escapes/impact parent context.


Boris Lukashev
Systems Architect
Semper Victus

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