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Subject: Re: RFC: New LSM to control usage of x509 certificates
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Paul Moore <paul@paul-moore.com>
Cc:     =?ISO-8859-1?Q?Micka=EBl_Sala=FCn?= <mic@digikod.net>,
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Date:   Tue, 17 Oct 2023 13:58:27 -0400
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On Tue, 2023-10-17 at 13:29 -0400, Paul Moore wrote:
> On Tue, Oct 17, 2023 at 1:09 PM Mimi Zohar <zohar@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@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.  For example, different
applications could be signed with different keys and should only be
verified with the specific key.

-- 
thanks,

Mimi