> On Dec 23, 2022, at 9:34 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: > > 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? I will add the three options in the next round. > There were a couple of other comments having to > do with variable names. Will you address them as well? And take care of the variable name changes. I won’t get back to this until January.
