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Date: Mon, 25 Jan 2016 10:51:49 -0800
From: Kees Cook <>
To: Andy Lutomirski <>
Cc: "Eric W. Biederman" <>, Andrew Morton <>, 
	Al Viro <>, Richard Weinberger <>, 
	Robert Święcki <>, 
	Dmitry Vyukov <>, David Howells <>, 
	Miklos Szeredi <>, Kostya Serebryany <>, 
	Alexander Potapenko <>, Eric Dumazet <>, 
	Sasha Levin <>, 
	"" <>, 
	"" <>, 
	"" <>
Subject: Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to be disabled

On Sun, Jan 24, 2016 at 2:22 PM, Andy Lutomirski <> wrote:
> On Fri, Jan 22, 2016 at 7:02 PM, Eric W. Biederman
> <> wrote:
>> Kees Cook <> writes:
>>> There continues to be unexpected side-effects and security exposures
>>> via CLONE_NEWUSER. For many end-users running distro kernels with
>>> CONFIG_USER_NS enabled, there is no way to disable this feature when
>>> desired. As such, this creates a sysctl to restrict CLONE_NEWUSER so
>>> admins not running containers or Chrome can avoid the risks of this
>>> feature.
>> I don't actually think there do continue to be unexpected side-effects
>> and security exposures with CLONE_NEWUSER.  It takes a while for all of
>> the fixes to trickle out to distros.  At most what I have seen recently
>> are problems with other kernel interfaces being amplified with user
>> namespaces.  AKA the current mess with devpts, and the unexpected
>> issues with bind mounts in mount namespaces.
>> So to keep this productive.  Please tell me about the threat model
>> you envision, and how you envision knobs in the kernel being used to
>> counter those threats.
> I consider the ability to use CLONE_NEWUSER to acquire CAP_NET_ADMIN
> over /any/ network namespace and to thus access the network
> configuration API to be a huge risk.  For example, unprivileged users
> can program iptables.  I'll eat my hat if there are no privilege
> escalations in there.  (They can't request module loading, but still.)

Should I consider this an Ack for the patch? :)


Kees Cook
Chrome OS & Brillo Security

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