RFC: New LSM to control usage of x509 certificates

[Paul Moore]

On Tue, Oct 17, 2023 at 1:59 PM Mimi Zohar <zohar at linux.ibm.com> wrote:
> On Tue, 2023-10-17 at 13:29 -0400, Paul Moore wrote:
> > On Tue, Oct 17, 2023 at 1:09 PM Mimi Zohar <zohar at linux.ibm.com> wrote:
> > > On Tue, 2023-10-17 at 11:45 -0400, Paul Moore wrote:
> > > > On Tue, Oct 17, 2023 at 9:48 AM Mimi Zohar <zohar at linux.ibm.com> wrote:
> > > > > On Thu, 2023-10-05 at 12:32 +0200, Mickaël Salaün wrote:
> > > > > > > > > A complementary approach would be to create an
> > > > > > > > > LSM (or a dedicated interface) to tie certificate properties to a set of
> > > > > > > > > kernel usages, while still letting users configure these constraints.
> > > > > > > >
> > > > > > > > That is an interesting idea.  Would the other security maintainers be in
> > > > > > > > support of such an approach?  Would a LSM be the correct interface?
> > > > > > > > Some of the recent work I have done with introducing key usage and CA
> > > > > > > > enforcement is difficult for a distro to pick up, since these changes can be
> > > > > > > > viewed as a regression.  Each end-user has different signing procedures
> > > > > > > > and policies, so making something work for everyone is difficult.  Letting the
> > > > > > > > user configure these constraints would solve this problem.
> > > > >
> > > > > Something definitely needs to be done about controlling the usage of
> > > > > x509 certificates.  My concern is the level of granularity.  Would this
> > > > > be at the LSM hook level or even finer granaularity?
> > > >
> > > > You lost me, what do you mean by finer granularity than a LSM-based
> > > > access control?  Can you give an existing example in the Linux kernel
> > > > of access control granularity that is finer grained than what is
> > > > provided by the LSMs?
> > >
> > > The current x509 certificate access control granularity is at the
> > > keyring level.  Any key on the keyring may be used to verify a
> > > signature.  Finer granularity could associate a set of certificates on
> > > a particular keyring with an LSM hook - kernel modules, BPRM, kexec,
> > > firmware, etc.  Even finer granularity could somehow limit a key's
> > > signature verification to files in particular software package(s) for
> > > example.
> > >
> > > Perhaps Mickaël and Eric were thinking about a new LSM to control usage
> > > of x509 certificates from a totally different perspective.  I'd like to
> > > hear what they're thinking.
> > >
> > > I hope this addressed your questions.
> >
> > Okay, so you were talking about finer granularity when compared to the
> > *current* LSM keyring hooks.  Gotcha.
> >
> > If we need additional, or modified, hooks that shouldn't be a problem.
> > Although I'm guessing the answer is going to be moving towards
> > purpose/operation specific keyrings which might fit in well with the
> > current keyring level controls.
>
> I don't believe defining per purpose/operation specific keyrings will
> resolve the underlying problem of granularity.

Perhaps not completely, but for in-kernel operations I believe it is
an attractive idea.

-- 
paul-moore.com