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

Eric Snowberg
2022-12-23T16:14:45Z

> On Dec 22, 2022, at 8:41 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
>>
>>> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>
>>> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
>>>>
>>>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>>
>>>>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
>>>>>> Hi Eric and Mimi,
>>>>>>
>>>>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>>>>>>>
>>>>>>>
>>>>>>>>>>>>> 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.
>>>>>>
>>>>>> How about allowing both keyCertSign and digitalSignature asserted but
>>>>>> issuing a warning for this case?
>>>>>>
>>>>>> Here's my rationale for this proposal.
>>>>>>
>>>>>> I assume we should conform to some X.509 specifications. So I checked
>>>>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
>>>>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
>>>>>> [2].
>>>>>>
>>>>>> [1] states in 4.2.1.3. Key Usage,
>>>>>> "If the keyUsage extension is present, then the subject public key
>>>>>> MUST NOT be used to verify signatures on certificates or CRLs unless
>>>>>> the corresponding keyCertSign or cRLSign bit is set. If the subject
>>>>>> public key is only to be used for verifying signatures on
>>>>>> certificates and/or CRLs, then the digitalSignature and
>>>>>> nonRepudiation bits SHOULD NOT be set. However, the digitalSignature
>>>>>> and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>>>>>> and/or cRLSign bits if the subject public key is to be used to verify
>>>>>> signatures on certificates and/or CRLs as well as other objects."
>>>>>>
>>>>>> and [2] states in 8.2.2.3 Key usage extension that,
>>>>>> "More than one bit may be set in an instance of the keyUsage extension.
>>>>>> The setting of multiple bits shall not change the meaning of each
>>>>>> individual bit but shall indicate that the certificate may be used for
>>>>>> all of the purposes indicated by the set bits. There may be risks
>>>>>> incurred when setting multiple bits. A review of those risks is
>>>>>> documented in Annex I."
>>>>>>
>>>>>> I interpret the above texts as we should allow both keyCertSign and
>>>>>> digitalSignature. However [2] warns about the risks of setting multiple
>>>>>> bits. Quoting Annex I,
>>>>>>
>>>>>> "Combining the contentCommitment bit in the keyUsage certificate
>>>>>> extension with other keyUsage bits may have security implications
>>>>>> depending on the security environment in which the certificate is to be
>>>>>> used. If the subject's environment can be fully controlled and trusted,
>>>>>> then there are no specific security implications. For example, in cases
>>>>>> where the subject is fully confident about exactly which data is signed
>>>>>> or cases where the subject is fully confident about the security
>>>>>> characteristics of the authentication protocol being used. If the
>>>>>> subject's environment is not fully controlled or not fully trusted, then
>>>>>> unintentional signing of commitments is possible. Examples include the
>>>>>> use of badly formed authentication exchanges and the use of a rogue
>>>>>> software component. If untrusted environments are used by a subject,
>>>>>> these security implications can be limited through use of the following
>>>>>> measures:
>>>>>> – to not combine the contentCommitment key usage setting in
>>>>>> certificates with any other key usage setting and to use the
>>>>>> corresponding private key only with this certificate;
>>>>>>
>>>>>> – to limit the use of private keys associated with certificates that
>>>>>> have the contentCommitment key usage bit set, to environments which
>>>>>> are considered adequately controlled and trustworthy"
>>>>>>
>>>>>> So maybe it's useful to add a warning if both keyCertSign and
>>>>>> digitalSignature are asserted.
>>>>>
>>>>> Coiby, thank you for adding these details. I was hoping others would
>>>>> chime in as well. I agree at minimum there should be a warning.
>>>>
>>>> A warning could be added.
>>>>
>>>>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
>>>>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
>>>>> more restrictive certificate use case requirements: all certificates,
>>>>> CA certificate signing and digital signature, only CA certificate
>>>>> signing.
>>>>
>>>> As could support for additional restrictions.
>>>>
>>>> Would these additions be required within this series? What is missing from this
>>>> discussion is why would these additions be necessary? Why should the kernel
>>>> enforce a restriction that is beyond the scope of the X.509 spec? If a warning was
>>>> to be added, what would be the justification for adding this additional code? From
>>>> my research every single 3rd party code signing intermediate CA would be flagged
>>>> with the warning. Isn’t this just going to cause confusion? Or is there a benefit that
>>>> I am missing that needs to be stated?
>>>
>>> You're focusing on third party kernel modules and forgetting about the
>>> simple use case of allowing an end user (or business) to sign their own
>>> code. True they could use the less restrictive CA certificates, but it
>>> is unnecessary.
>>
>> My focus is on signing user-space applications, as outlined in the cover letter. This
>> series has nothing to do with kernel modules. Most end-users and businesses rely on
>> a third party to deal with code signing. All third party code signing services I have
>> found use an intermediate CA containing more than just the keyCertSign usage set.
>> It seems to be an industry accepted practice that does not violate the spec. Before writing
>> new code to either warn or exclude a third party intermediate CA, I would like to understand
>> the motivation behind this request.
>
> In older discussions there are comments like, "Any CA certificate, no
> matter if it's a root or an intermediate, must have the keyCertSign
> extension. If you want to sign a revocation list (CRL) with the CA
> certificate as well (you usually do want that), than you have to add
> cRLSign as well. Any other keyUsages can and should be avoided for CA
> certificates."
>
> The question as to "why" this changed to include "digitalSignature" was
> posed here [2] with the response being for "OCSP". It also includes a
> link to Entrusts root and intermediate CAs with just keyCertSign and
> cRLSign keyUsages.
>
> The matchine keyring is a means of establishing a new root of trust.
> The motivation for further restricting CA certificates to just
> keyCertSign and cRLSign keyUsages is to limit how the CA certificates
> may be used. They should not be used for code signing.

Fair enough. If this will be viewed as justification for adding the additional
code, I can work on adding it. Above you mentioned a warning would be needed
at a minimum and a restriction could be placed behind a Kconfig. How about for
the default case I add the warning and when compiling with
INTEGRITY_CA_MACHINE_KEYRING the restriction will be enforced.