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

Mickaël Salaün
2020-05-14T19:28:23Z

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.