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Date:   Sun, 14 Aug 2022 10:55:08 -0500
From:   "Serge E. Hallyn" <serge@hallyn.com>
To:     Paul Moore <paul@paul-moore.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Frederick Lawler <fred@cloudflare.com>, kpsingh@kernel.org,
        revest@chromium.org, jackmanb@chromium.org, ast@kernel.org,
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        bpf@vger.kernel.org, linux-security-module@vger.kernel.org,
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        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        kernel-team@cloudflare.com, cgzones@googlemail.com,
        karl@bigbadwolfsecurity.com
Subject: Re: [PATCH v4 0/4] Introduce security_create_user_ns()
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On Mon, Aug 08, 2022 at 03:16:16PM -0400, Paul Moore wrote:
> On Mon, Aug 8, 2022 at 2:56 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > Paul Moore <paul@paul-moore.com> writes:
> > > On Mon, Aug 1, 2022 at 10:56 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > >> Frederick Lawler <fred@cloudflare.com> writes:
> > >>
> > >> > While creating a LSM BPF MAC policy to block user namespace creation, we
> > >> > used the LSM cred_prepare hook because that is the closest hook to prevent
> > >> > a call to create_user_ns().
> > >>
> > >> Re-nack for all of the same reasons.
> > >> AKA This can only break the users of the user namespace.
> > >>
> > >> Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com>
> > >>
> > >> You aren't fixing what your problem you are papering over it by denying
> > >> access to the user namespace.
> > >>
> > >> Nack Nack Nack.
> > >>
> > >> Stop.
> > >>
> > >> Go back to the drawing board.
> > >>
> > >> Do not pass go.
> > >>
> > >> Do not collect $200.
> > >
> > > If you want us to take your comments seriously Eric, you need to
> > > provide the list with some constructive feedback that would allow
> > > Frederick to move forward with a solution to the use case that has
> > > been proposed.  You response above may be many things, but it is
> > > certainly not that.
> >
> > I did provide constructive feedback.  My feedback to his problem
> > was to address the real problem of bugs in the kernel.
> 
> We've heard from several people who have use cases which require
> adding LSM-level access controls and observability to user namespace
> creation.  This is the problem we are trying to solve here; if you do
> not like the approach proposed in this patchset please suggest another
> implementation that allows LSMs visibility into user namespace
> creation.

Regarding the observability - can someone concisely lay out why just
auditing userns creation would not suffice?  Userspace could decide
what to report based on whether the creating user_ns == /proc/1/ns/user...

Regarding limiting the tweaking of otherwise-privileged code by
unprivileged users, i wonder whether we could instead add smarts to
ns_capable().  Point being, uid mapping would still work, but we'd
break the "privileged against resources you own" part of user
namespaces.  I would want it to default to allow, but then when a
0-day is found which requires reaching ns_capable() code, admins 
could easily prevent exploitation until reboot from a fixed kernel.