Re: [RFC PATCH v3 00/12] Integrity Policy Enforcement LSM (IPE)

Jaskaran Singh Khurana
2020-05-26T20:44:50Z

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Hello Mickael,

Could you please share your thoughts for the below proposal.

Regards,
JK

On Sat, 16 May 2020, Jaskaran Singh Khurana wrote:

>
> Hello Mickael,
>
> On Thu, 14 May 2020, Mickaël Salaün wrote:
>
>>
>> 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, Mickaël Salaün 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.
>>
>
> There is a use case for not having strict parsing in the cloud world where
> there are multiple versions of OS deployed across a large number of systems
> say 100,000 nodes. An OS update can take weeks to complete across all the
> nodes, and we end up having a heterogeneous mix of OS versions.
>
> Without non-strict parsing, to fix an issue in a policy we will need to
> update the various versions of the policy (one each for all OS versions
> which have different IPE policy schema). We will lose the agility we need to
> fix and deploy something urgently in the policy, the nodes might be failing
> some critical workloads meanwhile. All the various versions of the policy
> will need to be changed and production signed then deployed etc. Further some
> versions might introduce newer issues and we will need to see what all
> versions of the policy have that bug.
>
> I propose keeping the non-strict option as well to cater to this use case.
> Let me know your thoughts on this.
>
> Regards,
> JK
>