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

Eric Snowberg
2022-12-15T21:46:15Z

> 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.