Re: [RFC PATCH v3 00/13] Clavis LSM
From: Paul Moore
Date: Tue Mar 04 2025 - 19:20:21 EST
On Tue, Mar 4, 2025 at 7:54 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> On Mon, 2025-03-03 at 17:38 -0500, Paul Moore wrote:
> > On Fri, Feb 28, 2025 at 12:19 PM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> > > On Fri, 2025-02-28 at 11:14 -0500, Paul Moore wrote:
> > > > On Fri, Feb 28, 2025 at 9:09 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> > > > > On Thu, 2025-02-27 at 17:22 -0500, Paul Moore wrote:
> >
> > ...
> >
> > > Ok, let's go through different scenarios to see if it would scale.
> > >
> > > Scenario 1: Mostly distro signed userspace applications, minimum number of
> > > developer, customer, 3rd party applications.
> > >
> > > Scenario 2: Multiple developer, customer, 3rd party applications, signed by the
> > > same party.
> > >
> > > Scenario 3: extreme case - every application signed by different party.
> > >
> > > With the minimum case, there would probably be a default key or sets of
> > > permissible keys. In the extreme case, the number of keyrings would be
> > > equivalent to the number of application/software packages.
> >
> > Perhaps we're not understanding each other, but my understanding of
> > the above three scenarios is that they are all examples of signed
> > applications where something (likely something in the kernel like IMA)
> > verifies the signature on the application. While there are going to
> > be differing numbers of keys in each of the three scenarios, I believe
> > they would all be on/linked-to the same usage oriented keyring as they
> > all share the same usage: application signatures.
>
> Yes they're all verifying file signatures, but the software packages are from
> different sources (e.g. distro, chrome), signed by different keys.
Yep.
> Only a
> particular key should be used to verify the file signatures for a particular
> application.
That's definitely one access control policy, but I can also envision a
scenario where I have just one keyring for application signatures with
multiple keys from multiple vendors.
> Clavis limits key usage based on LSM hooks (e.g. kernel modules, kernel image,
> firmware, etc). It's a good start, but even this probably is not fine enough
> granularity.
Which is fine, but like I said earlier, it makes far more sense to me
to move towards usage oriented keyrings and then apply whatever
additional access control granularity is required to meet a given
scenario.
It's also worth (re)mentioning that what makes Clavis not-a-LSM in my
mind is how it is implemented, not necessarily its security goals. If
Clavis were to be implemented in such a way that it only relied on
security/LSM blobs and not keys/keyrings it might be more suitable.
> > > > My takeaway from Clavis was that it was more about establishing a set
> > > > of access controls around keys already present in the keyrings and my
> > > > comments about usage/spplication oriented keyrings have been in that
> > > > context. While the access control policy, regardless of how it is
> > > > implemented, should no doubt incorporate the trust placed in the
> > > > individual keys, how that trust is established is a separate issue
> > > > from access control as far as I'm concerned.
> > >
> > > Clavis defined both a mechanism for establishing trust and access control rules.
> > >
> > > Clavis defined a single Clavis key to establish trust. The Clavis policy rules
> > > were signed by the Clavis key. The Clavis policy rules defined the access
> > > control.
> >
> > Unfortunately I think we're getting a little ambiguous with how we are
> > using the word "trust". Just as "security" can mean different things
> > depending on context, so can "trust" as the qualities we are trusting
> > will vary depending on context. I'll leave it at that for now as I
> > believe we are talking about different things in the paragraphs above.
> >
> > Regardless, I'll also say this regarding Clavis and key/keyring access
> > controls - as implemented, Clavis doesn't look like a LSM to me for
> > the reasons already given. If all of the various keys subsystem
> > maintainers believe it is the Right Thing To Do inside the keys
> > subsystem then it isn't my place to have a say in that. I personally
> > believe that doing the work to support usage oriented keyrings before,
> > or while, implementing a Clavis-like mechanism is the better option,
> > but that is a decision for you and the other key maintainers.
>
> "Usage oriented keyrings" similarly implies any key on a particular keyring is
> acceptable.
Yep.
> Without understanding what you mean by "usage oriented keyrings", I
> would assume it would work initially, but eventually it too will not be fine
> enough granularity.
It all depends on what your goals are, but like I said above, it
really seems to me like this is a good first step which can be
followed up with additional granularity.
--
paul-moore.com