netdev - Re: [PATCH v4 0/4] Introduce security_create_user

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Date:   Tue, 09 Aug 2022 16:52:06 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        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, serge@...lyn.com,
        stephen.smalley.work@...il.com, eparis@...isplace.org,
        shuah@...nel.org, brauner@...nel.org, 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
Subject: Re: [PATCH v4 0/4] Introduce security_create_user_ns()

Casey Schaufler <casey@...aufler-ca.com> writes:

> Smack has no interest in using the proposed hook because user namespaces
> are neither subjects nor objects. They are collections of DAC and/or
> privilege configuration alternatives. Or something like that. From the
> viewpoint of a security module that only implements old fashioned MAC
> there is no value in constraining user namespaces.

The goal is to simply enable things that become safe when you don't have
to worry about confusing setuid executables.

> SELinux, on the other hand, seeks to be comprehensive well beyond
> controlling accesses between subjects and objects. Asking the question
> "should there be a control on this operation?" seems sufficient to justify
> adding the control to SELinux policy. This is characteristic of
> "Fine Grain" control.

I see that from a theoretical standpoint.  On the flip side I prefer
arguments grounded in something more than what an abstract framework
could appreciate.  We are so far from any theoretical purity in the
linux kernel that I can't see theoretical purity being enough to justify
a decision like this.

> So I'm of two minds on this. I don't need the hook, but I also understand
> why SELinux and BPF want it. I don't necessarily agree with their logic,
> but it is consistent with existing behavior. Any system that uses either
> of those security modules needs to be ready for unexpected denials based
> on any potential security concern. Keeping namespaces completely orthogonal
> to LSM seems doomed to failure eventually.

I can cross that bridge when there is a healthy conversation about such
a change.

Too often I get "ouch! Creating a user namespace was used as the easiest
way to exploit a security bug. Let's solve the issue by denying user
namespaces."  Which leads to half thought out policies made out of fear.

Which is where I think this conversation started.  I haven't seen it
make it's way to any healthy reasons to deny user namespaces yet.

Eric