Re: [PATCH v3 00/10] Add CA enforcement keyring restrictions

Mimi Zohar
2022-12-15T21:03:59Z

On Thu, 2022-12-15 at 20:28 +0000, Eric Snowberg wrote:
>
> > On Dec 15, 2022, at 12:58 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >
> > On Thu, 2022-12-15 at 16:26 +0000, Eric Snowberg wrote:
> >>
> >>> On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >>>
> >>> On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
> >>>> Prior to the introduction of the machine keyring, most distros simply
> >>>> allowed all keys contained within the platform keyring to be used
> >>>> for both kernel and module verification. This was done by an out of
> >>>> tree patch. Some distros took it even further and loaded all these keys
> >>>> into the secondary trusted keyring. This also allowed the system owner
> >>>> to add their own key for IMA usage.
> >>>>
> >>>> Each distro contains similar documentation on how to sign kernel modules
> >>>> and enroll the key into the MOK. The process is fairly straightforward.
> >>>> With the introduction of the machine keyring, the process remains
> >>>> basically the same, without the need for any out of tree patches.
> >>>>
> >>>> The machine keyring allowed distros to eliminate the out of tree patches
> >>>> for kernel module signing. However, it falls short in allowing the end
> >>>> user to add their own keys for IMA. Currently the machine keyring can not
> >>>> be used as another trust anchor for adding keys to the ima keyring, since
> >>>> CA enforcement does not currently exist. This would expand the current
> >>>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> >>>> Kconfig states that keys may be added to the ima keyrings if the key is
> >>>> validly signed by a CA cert in the system built-in or secondary trusted
> >>>> keyring. Currently there is not code that enforces the contents of a
> >>>> CA cert. Any key in the builtin or secondary keyring can be used.
> >>>>
> >>>> To allow IMA to be enabled with the machine keyring, this series introduces
> >>>> enforcement of key usage in the certificate. This series also applies
> >>>> this enforcement across all kernel keyrings.
> >>>>
> >>>> The machine keyring shares similarities with both the builtin and
> >>>> secondary keyrings. Similar to the builtin, no keys may be added to the
> >>>> machine keyring following boot. The secondary keyring allows user
> >>>> provided keys to be added following boot; however, a previously enrolled
> >>>> kernel key must vouch for the key before it may be included. The system
> >>>> owner may include their own keys into the machine keyring prior to boot.
> >>>> If the end-user is not the system owner, they may not add their own keys
> >>>> to the machine keyring.
> >>>>
> >>>> The machine keyring is only populated when Secure Boot is enabled. A
> >>>> system owner has the ability to control the entire Secure Boot keychain
> >>>> (PK, KEK, DB, and DBX). The system owner can also turn Secure Boot off.
> >>>> With this control, they may use insert-sys-cert to include their own key
> >>>> and re-sign their kernel and have it boot. The system owner also has
> >>>> control to include or exclude MOK keys. This series does not try to
> >>>> interpret how a system owner has configured their machine. If the system
> >>>> owner has taken the steps to add their own MOK keys, they will be
> >>>> included in the machine keyring and used for verification, exactly
> >>>> the same way as keys contained in the builtin and secondary keyrings.
> >>>> Since the system owner has the ability to add keys before booting to
> >>>> either the machine or builtin keyrings, it is viewed as inconsequential
> >>>> if the key originated from one or the other.
> >>>>
> >>>> This series introduces two different ways to configure the machine keyring.
> >>>> By default, nothing changes and all MOK keys are loaded into it. Whenever
> >>>> a CA cert is found within the machine, builtin, or secondary, a flag
> >>>> indicating this is stored in the public key struct. The other option is
> >>>> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs
> >>>> will be loaded into the machine keyring. All remaining MOK keys will be
> >>>> loaded into the platform keyring.
> >>>>
> >>>> A CA cert shall be defined as any X509 certificate that contains the
> >>>> keyCertSign key usage and has the CA bit set to true.
> >>>
> >>> Hi Eric,
> >>>
> >>> Allowing CA certificates with the digitalSignature key usage flag
> >>> enabled defeats the purpose of the new Kconfig. Please update the
> >>> above definition to exclude the digitalSignature key usage flag and
> >>> modify the code accordingly.
> >>
> >> Within v2, the request was made to allow Intermediate CA certificates to be
> >> loaded directly. The Intermediate CA referenced was the one used by kernel.org.
> >> This Intermediate CA contains both digitalSignature and keyCertSign. If the code
> >> is changed to exclude this certificate, now the root CA has to be loaded again. Is that
> >> the intent?
> >
> > That definitely was not the intent. Nor would it address the issue of
> > a particular intermediate CA certificate having both keyCertSign and
> > digitalSignature.
>
> Sorry, I’m not following. Why is it an issue that an intermediate CA certificate contains
> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> CA cert like the one used on kernel.org?

I must be missing something. Isn't the purpose of "keyUsage" to
minimize how a certificate may be used? Why would we want the same
certificate to be used for both certificate signing and code signing?

thanks,

Mimi