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Date:   Tue, 28 Jun 2022 10:14:29 -0500
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Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()
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To:     Casey Schaufler <casey@schaufler-ca.com>,
        Paul Moore <paul@paul-moore.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Cc:     Christian Brauner <brauner@kernel.org>, kpsingh@kernel.org,
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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@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's a documentation page that briefly touches on a BPF LSM 
implementation [1].

> 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#