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

[Jaskaran Singh Khurana <jaskarankhurana@linux.microsoft.com>]

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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
>