linux-kernel - Re: [PATCH 0/2] sysctl: allow CLONE

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Date:	Tue, 26 Jan 2016 09:38:31 -0500
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Richard Weinberger <richard@....at>,
	Robert Święcki <robert@...ecki.net>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to be disabled

On Mon, Jan 25, 2016 at 11:57 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Kees Cook <keescook@...omium.org> writes:
>
>> On Mon, Jan 25, 2016 at 11:33 AM, Eric W. Biederman
>> <ebiederm@...ssion.com> wrote:
>>> Kees Cook <keescook@...omium.org> writes:
>>>>
>>>> Well, I don't know about less weird, but it would leave a unneeded
>>>> hole in the permission checks.
>>>
>>> To be clear the current patch has my:
>>>
>>> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>
>>> The code is buggy, and poorly thought through.  Your lack of interest in
>>> fixing the bugs in your patch is distressing.
>>
>> I'm not sure where you see me having a "lack of interest". The
>> existing cap-checking sysctls have a corner-case bug, which is
>> orthogonal to this change.
>
> That certainly doesn't sound like you have any plans to change anything
> there.
>
>>> So broken code, not willing to fix.  No. We are not merging this sysctl.
>>
>> I think you're jumping to conclusions. :)
>
> I think I am the maintainer.
>
> What you are proposing is very much something that is only of interst to
> people who are not using user namespaces.  It is fatally flawed as
> a way to avoid new attack surfaces for people who don't care as the
> sysctl leaves user namespaces enabled by default.  It is fatally flawed
> as remediation to recommend to people to change if a new user namespace
> related but is discovered.  Any running process that happens to be
> created while user namespace creation was enabled will continue to
> exist.  Effectively a reboot will be required as part of a mitigation.
> Many sysadmins will get that wrong.
>
> I can't possibly see your sysctl as proposed achieving it's goals.  A
> person has to be entirely too aware of subtlety and nuance to use it
> effectively.

What you're saying is true for the "oh crap" case of a new userns
related CVE being found.  However, there is the case where sysadmins
know for a fact that a set of machines should not allow user
namespaces to be enabled.  Currently they have 2 choices, 1) use their
distro kernel as-is, which may not meet their goal of having userns
disabled, or 2) rebuild their kernel to disable it, which may
invalidate any support contracts they have.

I tend to agree with you on the lack of value around runtime
mitigation, but allowing an admin to toggle this as a blatant on/off
switch on reboot does have value.

>> This feature is already implemented by two distros, and likely wanted
>> by others. We cannot ignore that. The sysctl default doesn't change
>> the existing behavior, so this doesn't get in your way at all. Can you
>> please respond to my earlier email where I rebutted each of your
>> arguments against it? Just saying "no" and putting words in my mouth
>> isn't very productive.
>
> Calling people who make mistakes insane is not a rebuttal.  In security
> usability matters, and your sysctl has low usability.
>
> Further you seem to have missed something crucial in your understanding.
> As was explained earlier the sysctl was added to ubuntu to allow early
> adopters to experiment not as a long term way of managing user
> namespaces.
>
>
> What sounds like a generally useful feature that would cover your use
> case and many others is a per user limit on the number of user
> namespaces users may create.

Where that number may be zero?  I don't see how that is really any
better than a sysctl.  Could you elaborate?

josh