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Message-ID: <CA+5PVA5_x05a=2twmfx3FjqbaLHsQL3Yug3kW_WS=UYxs1nKqA@mail.gmail.com>
Date: Tue, 26 Jan 2016 18:47:15 -0500
From: Josh Boyer <jwboyer@...oraproject.org>
To: "Serge E. Hallyn" <serge.hallyn@...ntu.com>
Cc: Kostya Serebryany <kcc@...gle.com>, linux-doc@...r.kernel.org,
Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>, kernel-hardening@...ts.openwall.com,
Andy Lutomirski <luto@...capital.net>, "Eric W. Biederman" <ebiederm@...ssion.com>,
Alexander Potapenko <glider@...gle.com>, Kees Cook <keescook@...omium.org>,
Eric Dumazet <edumazet@...gle.com>, Robert Święcki <robert@...ecki.net>,
Miklos Szeredi <mszeredi@...e.cz>, Dmitry Vyukov <dvyukov@...gle.com>,
Richard Weinberger <richard@....at>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Sasha Levin <sasha.levin@...cle.com>
Subject: Re: Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to
be disabled
On Jan 26, 2016 3:58 PM, "Serge Hallyn" <serge.hallyn@...ntu.com> wrote:
>
> Quoting Josh Boyer (jwboyer@...oraproject.org):
> > 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
>
> Hi - can you give a specific example of this? (Where users really should
> not be able to use them - not where they might not need them) I think
> it'll help the discussion tremendously. Because so far the only good
> arguments I've seen have been about actual bugs in the user namespaces,
> which would not warrant a designed-in permanent disable switch. If
> there are good use cases where such a disable switch will always be
> needed (and compiling out can't satisfy) that'd be helpful.
I already did in the email you quoted. Limiting the surface where the
admin knows that shouldn't be used without having to rebuild a distribution
kernel. Think of it like blacklisting a module, except user namespaces
can't be modules.
josh
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