linux-kernel - Re: [RFC PATCH v3 00/13] Clavis LSM

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Message-ID: <c580811716f550ed5d6777db5e143afe4ad06edc.camel@linux.ibm.com>
Date: Fri, 28 Feb 2025 09:08:33 -0500
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Paul Moore <paul@...l-moore.com>
Cc: Eric Snowberg <eric.snowberg@...cle.com>,
        David Howells
 <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "open
 list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>,
        David
 Woodhouse <dwmw2@...radead.org>,
        "herbert@...dor.apana.org.au"
 <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Ard Biesheuvel <ardb@...nel.org>, James Morris <jmorris@...ei.org>,
        "Serge
 E. Hallyn" <serge@...lyn.com>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Mickaël Salaün <mic@...ikod.net>,
        "casey@...aufler-ca.com" <casey@...aufler-ca.com>,
        Stefan Berger
 <stefanb@...ux.ibm.com>,
        "ebiggers@...nel.org" <ebiggers@...nel.org>,
        Randy
 Dunlap <rdunlap@...radead.org>,
        open list <linux-kernel@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>
Subject: Re: [RFC PATCH v3 00/13] Clavis LSM

On Thu, 2025-02-27 at 17:22 -0500, Paul Moore wrote:
> On Thu, Feb 27, 2025 at 3:41 PM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> > On Mon, 2025-01-06 at 17:15 +0000, Eric Snowberg wrote:
> > > > On Jan 5, 2025, at 8:40 PM, Paul Moore <paul@...l-moore.com> wrote:
> > > > On Fri, Jan 3, 2025 at 11:48 PM Paul Moore <paul@...l-moore.com> wrote:
> > > > > 
> > > > > Regardless, back to Clavis ... reading quickly through the cover
> > > > > letter again, I do somewhat wonder if this isn't better integrated
> > > > > into the keyring proper; have you talked to both David and Jarkko
> > > > > about this?
> > > > 
> > > > I realize I should probably expand on my thinking a bit, especially
> > > > since my comment a while regarding LSMs dedicated to enforcing access
> > > > control on keys is what was given as a reason for making Clavis a LSM.
> > > > 
> > > > I still stand by my comment from over a year ago that I see no reason
> > > > why we couldn't support a LSM that enforces access controls on
> > > > keyrings/keys.  What gives me pause with the Clavis LSM is that so
> > > > much of Clavis is resident in the keyrings themselves, e.g. Clavis
> > > > policy ACLs and authorization keys, that it really feels like it
> > > > should be part of the keys subsystem and not a LSM.  Yes, existing
> > > > LSMs do have LSM specific data that resides outside of the LSM and in
> > > > an object's subsystem, but that is usually limited to security
> > > > identifiers and similar things, not the LSM's security policy.
> > 
> > Hi Jarkko, David,
> > 
> > Both Paul's and my main concerns with this patch set is storing policy in the
> > keyring.  We would appreciate your chiming in here about storing key policy in
> > the keyring itself.
> 
> I'd still also like to see some discussion about moving towards the
> addition of keyrings oriented towards usage instead of limiting
> ourselves to keyrings that are oriented on the source of the keys.
> Perhaps I'm missing some important detail which makes this
> impractical, but it seems like an obvious improvement to me and would
> go a long way towards solving some of the problems that we typically
> see with kernel keys.

The proliferation of keyrings won't solve the key usage problem for IMA-
appraisal.  IMA-appraisal can be used to verify the kexec image, kernel modules,
firwmare, etc, but it also verifies file signatures contained in userspace
packages.  To support the latter case, keyrings would need to be application
specific.  (This version of Clavis doesn't solve the latter key usage for IMA-
appraisal either.)

The keys baked into the kernel are trusted because the kernel itself was signed
and verified (secure boot).  Anyone building a kernel can build a key into the
kernel image, which establishes a "root of trust".  That key can then be used to
verify and load other keys onto the IMA keyring.

The problem is how to safely establish a root of trust without baking the key
into the kernel image and then limiting that trust to specific usages or
applications.

Mimi