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SubjectRe: [PATCH v4] certs: Add EFI_CERT_X509_GUID support for dbx entries
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> On Jan 27, 2021, at 7:03 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> [Cc'ing linux-integrity]
>
> On Wed, 2021-01-27 at 11:46 +0000, David Howells wrote:
>> Jarkko Sakkinen <jarkko@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.

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