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    SubjectRe: [PATCH v3 00/10] Add CA enforcement keyring restrictions
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    > On Dec 15, 2022, at 2:03 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
    >
    > 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?

    Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
    Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
    set. Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
    challenging and will severely limit usage.

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