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Date:   Mon, 27 Jun 2022 16:18:32 -0700
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Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()
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To:     Paul Moore <paul@paul-moore.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Cc:     Christian Brauner <brauner@kernel.org>,
        Frederick Lawler <fred@cloudflare.com>, kpsingh@kernel.org,
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        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@cloudflare.com,
        Casey Schaufler <casey@schaufler-ca.com>
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From:   Casey Schaufler <casey@schaufler-ca.com>
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On 6/27/2022 3:27 PM, Paul Moore wrote:
> On Mon, Jun 27, 2022 at 6:15 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>> On 6/27/22 11:56 PM, Paul Moore wrote:
>>> On Mon, Jun 27, 2022 at 8:11 AM Christian Brauner <brauner@kernel.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 should be something out there warning the eBPF programmer of the
implications of providing a secid_to_secctx hook for starters.