linux-kernel - Re: [PATCH v5 0/4] Introduce security_create_user

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Message-ID: <20220826152319.GA12466@mail.hallyn.com>
Date:   Fri, 26 Aug 2022 10:23:19 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederick Lawler <fred@...udflare.com>, kpsingh@...nel.org,
        revest@...omium.org, jackmanb@...omium.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        jmorris@...ei.org, stephen.smalley.work@...il.com,
        eparis@...isplace.org, shuah@...nel.org, brauner@...nel.org,
        casey@...aufler-ca.com, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel-team@...udflare.com,
        cgzones@...glemail.com, karl@...badwolfsecurity.com,
        tixxdz@...il.com
Subject: Re: [PATCH v5 0/4] Introduce security_create_user_ns()

On Thu, Aug 25, 2022 at 01:15:46PM -0500, Eric W. Biederman wrote:
> Paul Moore <paul@...l-moore.com> writes:
> 
> > On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@...lyn.com> wrote:
> >>  I am hoping we can come up with
> >> "something better" to address people's needs, make everyone happy, and
> >> bring forth world peace.  Which would stack just fine with what's here
> >> for defense in depth.
> >>
> >> You may well not be interested in further work, and that's fine.  I need
> >> to set aside a few days to think on this.
> >
> > I'm happy to continue the discussion as long as it's constructive; I
> > think we all are.  My gut feeling is that Frederick's approach falls
> > closest to the sweet spot of "workable without being overly offensive"
> > (*cough*), but if you've got an additional approach in mind, or an
> > alternative approach that solves the same use case problems, I think
> > we'd all love to hear about it.
> 
> I would love to actually hear the problems people are trying to solve so
> that we can have a sensible conversation about the trade offs.
> 
> As best I can tell without more information people want to use
> the creation of a user namespace as a signal that the code is
> attempting an exploit.

I don't think that's it at all.  I think the problem is that it seems
you can pretty reliably get a root shell at some point in the future
by creating a user namespace, leaving it open for a bit, and waiting
for a new announcement of the latest netfilter or whatever exploit
that requires root in a user namespace.  Then go back to your userns
shell and run the exploit.

So i was hoping we could do something more targeted.  Be it splitting
off the ability to run code under capable_ns code from uid mapping (to
an extent), or maybe some limited-livepatch type of thing where
certain parts of code become inaccessible to code in a non-init userns
after some sysctl has been toggled, or something cooloer that I've
failed to think of.

-serge