linux-kernel - Re: [PATCH 0/2] Introduce security_create_user

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Date:   Tue, 28 Jun 2022 18:12:25 +0200
From:   KP Singh <kpsingh@...nel.org>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Frederick Lawler <fred@...udflare.com>,
        Paul Moore <paul@...l-moore.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Christian Brauner <brauner@...nel.org>, revest@...omium.org,
        jackmanb@...omium.org, ast@...nel.org, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, jmorris@...ei.org, serge@...lyn.com,
        bpf@...r.kernel.org, linux-security-module@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()

On Tue, Jun 28, 2022 at 6:02 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
>
> On 6/28/2022 8:14 AM, Frederick Lawler wrote:
> > On 6/27/22 6:18 PM, Casey Schaufler wrote:
> >> On 6/27/2022 3:27 PM, Paul Moore wrote:
> >>> On Mon, Jun 27, 2022 at 6:15 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >>>> On 6/27/22 11:56 PM, Paul Moore wrote:
> >>>>> On Mon, Jun 27, 2022 at 8:11 AM Christian Brauner <brauner@...nel.org> wrote:
> >>>>>> On Thu, Jun 23, 2022 at 11:21:37PM -0400, Paul Moore wrote:
> >>>>> ...
> >>>>>
> >>>>>>> This is one of the reasons why I usually like to see at least one LSM
> >>>>>>> implementation to go along with every new/modified hook.  The
> >>>>>>> implementation forces you to think about what information is necessary
> >>>>>>> to perform a basic access control decision; sometimes it isn't always
> >>>>>>> obvious until you have to write the access control :)
> >>>>>> I spoke to Frederick at length during LSS and as I've been given to
> >>>>>> understand there's a eBPF program that would immediately use this new
> >>>>>> hook. Now I don't want to get into the whole "Is the eBPF LSM hook
> >>>>>> infrastructure an LSM" but I think we can let this count as a legitimate
> >>>>>> first user of this hook/code.
> >>>>> Yes, for the most part I don't really worry about the "is a BPF LSM a
> >>>>> LSM?" question, it's generally not important for most discussions.
> >>>>> However, there is an issue unique to the BPF LSMs which I think is
> >>>>> relevant here: there is no hook implementation code living under
> >>>>> security/.  While I talked about a hook implementation being helpful
> >>>>> to verify the hook prototype, it is also helpful in providing an
> >>>>> in-tree example for other LSMs; unfortunately we don't get that same
> >>>>> example value when the initial hook implementation is a BPF LSM.
> >>>> I would argue that such a patch series must come together with a BPF
> >>>> selftest which then i) contains an in-tree usage example, ii) adds BPF
> >>>> CI test coverage. Shipping with a BPF selftest at least would be the
> >>>> usual expectation.
> >>> I'm not going to disagree with that, I generally require matching
> >>> tests for new SELinux kernel code, but I was careful to mention code
> >>> under 'security/' and not necessarily just a test implementation :)  I
> >>> don't want to get into a big discussion about it, but I think having a
> >>> working implementation somewhere under 'security/' is more
> >>> discoverable for most LSM folks.
> >>
> >> I agree. It would be unfortunate if we added a hook explicitly for eBPF
> >> only to discover that the proposed user needs something different. The
> >> LSM community should have a chance to review the code before committing
> >> to all the maintenance required in supporting it.
> >>
> >> Is there a reference on how to write an eBPF security module?
> >
> > There's a documentation page that briefly touches on a BPF LSM implementation [1].
>
> That's a brief touch, alright. I'll grant that the LSM interface isn't
> especially well documented for C developers, but we have done tutorials
> and have multiple examples. I worry that without an in-tree example for
> eBPF we might well be setting developers up for spectacular failure.
>

Casey, Daniel and I are recommending an in-tree example, it will be
in BPF selftests and we will CC you on the reviews.

Frederick, is that okay with you?

> >
> >> There should be something out there warning the eBPF programmer of the
> >> implications of providing a secid_to_secctx hook for starters.
> >>
> >
> > Links:
> > 1. https://docs.kernel.org/bpf/prog_lsm.html?highlight=bpf+lsm#
> >