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

[Deven Bowers <deven.desai@linux.microsoft.com>]

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

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?