Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <d70d3b2d-6c3f-b1fc-f40c-f5ec01a627c0@schaufler-ca.com>
Date:   Tue, 28 Jun 2022 09:02:13 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.10.0
Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()
Content-Language: en-US
To:     Frederick Lawler <fred@cloudflare.com>,
        Paul Moore <paul@paul-moore.com>,
        Daniel Borkmann <daniel@iogearbox.net>
Cc:     Christian Brauner <brauner@kernel.org>, kpsingh@kernel.org,
        revest@chromium.org, jackmanb@chromium.org, ast@kernel.org,
        andrii@kernel.org, kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
        john.fastabend@gmail.com, jmorris@namei.org, serge@hallyn.com,
        bpf@vger.kernel.org, linux-security-module@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@cloudflare.com,
        Casey Schaufler <casey@schaufler-ca.com>
References: <20220621233939.993579-1-fred@cloudflare.com>
 <ce1653b1-feb0-1a99-0e97-8dfb289eeb79@schaufler-ca.com>
 <b72c889a-4a50-3330-baae-3bbf065e7187@cloudflare.com>
 <CAHC9VhSTkEMT90Tk+=iTyp3npWEm+3imrkFVX2qb=XsOPp9F=A@mail.gmail.com>
 <20220627121137.cnmctlxxtcgzwrws@wittgenstein>
 <CAHC9VhSQH9tE-NgU6Q-GLqSy7R6FVjSbp4Tc4gVTbjZCqAWy5Q@mail.gmail.com>
 <6a8fba0a-c9c9-61ba-793a-c2e0c2924f88@iogearbox.net>
 <CAHC9VhQQJH95jTWMOGDB4deS=whSfnaF_e73zoabOOeHJMv+0Q@mail.gmail.com>
 <685096bb-af0a-08c0-491a-e176ac009e85@schaufler-ca.com>
 <9ae473c4-cd42-bb45-bce2-8aa2e4784a43@cloudflare.com>
From:   Casey Schaufler <casey@schaufler-ca.com>
In-Reply-To: <9ae473c4-cd42-bb45-bce2-8aa2e4784a43@cloudflare.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@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].

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.

>
>> 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#
>