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

Mimi Zohar
2022-12-15T19:59:21Z

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.

thanks,

Mimi