> 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.
