Messages in this thread | | | Subject | Re: [RFC PATCH v3 00/12] Integrity Policy Enforcement LSM (IPE) | From | Deven Bowers <> | Date | Tue, 12 May 2020 13:46:00 -0700 |
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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?
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