Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC PATCH v3 00/12] Integrity Policy Enforcement LSM (IPE)
To:     Deven Bowers <deven.desai@linux.microsoft.com>, agk@redhat.com,
        axboe@kernel.dk, snitzer@redhat.com, jmorris@namei.org,
        serge@hallyn.com, zohar@linux.ibm.com,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, dm-devel@redhat.com,
        linux-block@vger.kernel.org, jannh@google.com
Cc:     tyhicks@linux.microsoft.com, pasha.tatashin@soleen.com,
        sashal@kernel.org, jaskarankhurana@linux.microsoft.com,
        nramas@linux.microsoft.com, mdsakib@linux.microsoft.com,
        linux-kernel@vger.kernel.org, corbet@lwn.net
References: <20200415162550.2324-1-deven.desai@linux.microsoft.com>
 <b07ac7e1-7cf5-92c9-81d0-64174c3d5024@digikod.net>
 <0001755a-6b2a-b13b-960c-eb0b065c8e3c@linux.microsoft.com>
 <8ba7b15f-de91-40f7-fc95-115228345fce@linux.microsoft.com>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>
Message-ID: <44fb36ae-959d-4ff7-ed1f-ccfc2e292232@digikod.net>
Date:   Thu, 14 May 2020 21:28:15 +0200
User-Agent: 
MIME-Version: 1.0
In-Reply-To: <8ba7b15f-de91-40f7-fc95-115228345fce@linux.microsoft.com>
Content-Type: text/plain; charset=iso-8859-15
Content-Language: en-US
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 12/05/2020 22:46, Deven Bowers wrote:
> 
> 
> On 5/11/2020 11:03 AM, Deven Bowers wrote:
>>
>>
>> On 5/10/2020 2:28 AM, Micka?l Sala?n wrote:
>>
>> [...snip]
>>
>>>>
>>>> Additionally, rules are evaluated top-to-bottom. As a result, any
>>>> revocation rules, or denies should be placed early in the file to
>>>> ensure
>>>> that these rules are evaluated before a rule with "action=ALLOW" is
>>>> hit.
>>>>
>>>> IPE policy is designed to be forward compatible and backwards
>>>> compatible,
>>>> thus any failure to parse a rule will result in the line being ignored,
>>>> and a warning being emitted. If backwards compatibility is not
>>>> required,
>>>> the kernel commandline parameter and sysctl, ipe.strict_parse can be
>>>> enabled, which will cause these warnings to be fatal.
>>>
>>> Ignoring unknown command may lead to inconsistent beaviors. To achieve
>>> forward compatibility, I think it would be better to never ignore
>>> unknown rule but to give a way to userspace to known what is the current
>>> kernel ABI. This could be done with a securityfs file listing the
>>> current policy grammar.
>>>
>>
>> That's a fair point. From a manual perspective, I think this is fine.
>> A human-user can interpret a grammar successfully on their own when new
>> syntax is introduced.
>>
>> ?From a producing API perspective, I'd have to think about it a bit
>> more. Ideally, the grammar would be structured in such a way that the
>> userland
>> interpreter of this grammar would not have to be updated once new syntax
>> is introduced, avoiding the need to update the userland binary. To do so
>> generically ("op=%s") is easy, but doesn't necessarily convey sufficient
>> information (what happens when a new "op" token is introduced?). I think
>> this may come down to regular expression representations of valid values
>> for these tokens, which worries me as regular expressions are incredibly
>> error-prone[1].
>>
>> I'll see what I can come up with regarding this.
> 
> I have not found a way that I like to expose some kind of grammar
> through securityfs that can be understood by usermode to parse the
> policy. Here's what I propose as a compromise:
> 
> ????1. I remove the unknown command behavior. This address your
> first point about inconsistent behaviors, and effectively removes the
> strict_parse sysctl (as it is always enabled).
> 
> ????2. I introduce a versioning system for the properties
> themselves. The valid set of properties and their versions
> can be found in securityfs, under say, ipe/config in a key=value
> format where `key` indicates the understood token, and `value`
> indicates their current version. For example:
> 
> ????$ cat $SECURITYFS/ipe/config
> ????op=1
> ????action=1
> ????policy_name=1
> ????policy_version=1
> ????dmverity_signature=1
> ????dmverity_roothash=1
> ????boot_verified=1

The name ipe/config sounds like a file to configure IPE. Maybe something
like ipe/config_abi or ipe/config_grammar?

> 
> if new syntax is introduced, the version number is increased.
> 
> ????3. The format of those versions are documented as part of
> the admin-guide around IPE. If user-mode at that point wants to rip
> the documentation formats and correlate with the versioning, then
> it fulfills the same functionality as above, with out the complexity
> around exposing a parsing grammar and interpreting it on-the-fly.
> Many of these are unlikely to move past version 1, however.
> 
> Thoughts?
> 

That seems reasonable.