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Date: Thu, 28 Jan 2016 12:17:22 -0800
From: Kees Cook <>
To: Robert Święcki <>
Cc: "Eric W. Biederman" <>, Andrew Morton <>, 
	Al Viro <>, "Serge E. Hallyn" <>, 
	Andy Lutomirski <>, "Austin S. Hemmelgarn" <>, 
	Richard Weinberger <>, Dmitry Vyukov <>, David Howells <>, 
	Kostya Serebryany <>, Alexander Potapenko <>, Eric Dumazet <>, 
	Sasha Levin <>, 
	"" <>, LKML <>, 
	"" <>
Subject: Re: [PATCH v2] sysctl: allow CLONE_NEWUSER to be disabled

On Thu, Jan 28, 2016 at 11:11 AM, Robert Święcki <> wrote:
> 2016-01-28 18:48 GMT+01:00 Eric W. Biederman <>:
>> Kees Cook <> writes:
>>> +     if (sysctl_userns_restrict && !(capable(CAP_SYS_ADMIN) &&
>>> +                                     capable(CAP_SETUID) &&
>>> +                                     capable(CAP_SETGID)))
>>> +             return -EPERM;
>>> +
>> I will also note that the way I have seen containers used this check
>> adds no security and is not mentioned or justified in any way in your
>> patch description.
>> Furthermore this looks like blame shifting.  And quite frankly shifting
>> the responsibility to users if they get hacked is not an acceptable
>> attitude.
> I think I might start understanding your point. Which, if I'm not
> mistaken, is that it's not user namespaces which are buggy, but rather
> some pieces of the kernel which would otherwise not be reachable from
> the typical low-priv level of regular users (e.g. bugs in SOCK_RAW
> sockets or iptables or mounts)?
> If so, I can agree with such wording, but the proposed sysctl might
> still be needed in such case. I guess those bits of the kernel which
> were not reachable previously from non-priv users historically got
> much less attention in terms of time spent on security reviews and
> security fuzzing. And as much as users of the kernel would like to see
> those pieces of the kernel to be tested to a level that the attack
> surface reachable from unprivileged users level were tested, it will
> not happen tomorrow. And our best option now might be to have some
> switchable setting to disable this attack surface for those users who
> feel they need it. In the meantime, we can concentrate on sec
> reviewing those newly reachable kernel APIs, so some day we could
> remove this sysctl.

Yes, exactly. I want to find a way to offer admins a way to reduce
attack surface. I have no interest in blame; this is a matter of
practicality. It exposes a less well tested set of APIs to
non-privileged users, so let's offer a way to remove that when


Kees Cook
Chrome OS & Brillo Security

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