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

[Eric Snowberg]

> On Jan 27, 2021, at 7:03 AM, Mimi Zohar <zohar at linux.ibm.com> wrote:
> 
> [Cc'ing linux-integrity]
> 
> On Wed, 2021-01-27 at 11:46 +0000, David Howells wrote:
>> Jarkko Sakkinen <jarkko at kernel.org> 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.