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

[Mimi Zohar]

On Fri, 2022-12-23 at 16:13 +0000, Eric Snowberg wrote:
> 
> > On Dec 22, 2022, at 8:41 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> > 
> > On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
> >> 
> >>> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> >>> 
> >>> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
> >>>> 
> >>>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> 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.

Sounds good to me.  To avoid misunderstandings, will there be a Kconfig
menu with 3 options?   There were a couple of other comments having to
do with variable names.  Will you address them as well?

-- 
thanks,

Mimi