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Date: Mon, 6 Nov 2017 23:42:08 +0100
From: Christian Brauner <>
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 06, 2017 at 04:14:18PM -0600, Serge 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.

I agree that /proc/sys/kernel/unprivileged_userns_clone is the most reasonable
thing to do. This patch introduces a layer of complexity to fine-tune user
namespace creation that - in the relevant security critical scenario - should
simply be turned of entirely.

Is /proc/sys/kernel/unprivileged_userns_clone upstreamed or is this still only
carried downstream?

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