Re: [PATCH v4] certs: Add EFI_CERT_X509_GUID support for dbx entries

[Jarkko Sakkinen]

On Wed, Jan 27, 2021 at 08:41:29AM -0700, Eric Snowberg wrote:
> 
> > On Jan 27, 2021, at 7:03 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> > 
> > [Cc'ing linux-integrity]
> > 
> > On Wed, 2021-01-27 at 11:46 +0000, David Howells wrote:
> >> Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> >> 
> >>>> I suppose a user space tool could be created. But wouldn’t what is
> >>>> currently done in the kernel in this area need to be removed?
> >>> 
> >>> Right. I don't think this was a great idea in the first place to
> >>> do to the kernel but since it exists, I guess the patch does make
> >>> sense.
> >> 
> >> This information needs to be loaded from the UEFI tables before the system
> >> starts loading any kernel modules or running any programs (if we do
> >> verification of such, which I think IMA can do).
> > 
> > There needs to a clear distinction between the pre-boot and post-boot
> > keys.  UEFI has its own trust model, which should be limited to UEFI. 
> > The .platform keyring was upstreamed and limited to verifying the kexec
> > kernel image.   Any other usage of the .platform keyring keys is
> > abusing its intended purpose.
> > 
> > The cover letter says,   "Anytime the .platform keyring is used, the
> > keys in the .blacklist keyring are referenced, if a matching key is
> > found, the key will be rejected."   I don't have a problem with loading
> > the UEFI X509 dbx entries as long as its usage is limited to verifying
> > the kexec kernel image.
> 
> Correct, with my patch, when EFI_CERT_X509_GUID entries are found in the
> dbx, they will only be used during kexec.  I believe the latest dbx file on 
> uefi.org contains three of these entires.
> 
> Based on my understanding of why the platform keyring was introduced, 
> I intentionally only used these for kexec.  I do question the current 
> upstream mainline code though.  Currently, when EFI_CERT_X509_SHA256_GUID
> or EFI_CERT_SHA256_GUID entries are found in the dbx, they are applied 
> everywhere.  It seems like there should be a dbx revocation keyring 
> equivalent to the current platform keyring that is only used for pre-boot. 
> 
> If that is a direction you would like to see this go in the future, let
> me know, I’d be happy to work on it.

I would tend to agree with this.

/Jarkko