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

[Eric Snowberg]

> On Jan 15, 2021, at 2:15 AM, Jarkko Sakkinen <jarkko at kernel.org> wrote:
> 
> On Wed, Jan 13, 2021 at 05:11:10PM -0700, Eric Snowberg wrote:
>> 
>>> On Jan 13, 2021, at 1:41 PM, Jarkko Sakkinen <jarkko.sakkinen at linux.intel.com> wrote:
>>> 
>>> On Tue, Jan 12, 2021 at 02:57:39PM +0000, David Howells wrote:
>>>> Eric Snowberg <eric.snowberg at oracle.com> wrote:
>>>> 
>>>>>> On Dec 10, 2020, at 2:49 AM, David Howells <dhowells at redhat.com> wrote:
>>>>>> 
>>>>>> Eric Snowberg <eric.snowberg at oracle.com> wrote:
>>>>>> 
>>>>>>> Add support for EFI_CERT_X509_GUID dbx entries. When a EFI_CERT_X509_GUID
>>>>>>> is found, it is added as an asymmetrical key to the .blacklist keyring.
>>>>>>> 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.
>>>>>> 
>>>>>> Ummm...  Why this way and not as a blacklist key which takes up less space?
>>>>>> I'm guessing that you're using the key chain matching logic.  We really only
>>>>>> need to blacklist the key IDs.
>>>>> 
>>>>> I implemented it this way so that certs in the dbx would only impact 
>>>>> the .platform keyring. I was under the impression we didn’t want to have 
>>>>> Secure Boot UEFI db/dbx certs dictate keyring functionality within the kernel
>>>>> itself. Meaning if we have a matching dbx cert in any other keyring (builtin,
>>>>> secondary, ima, etc.), it would be allowed. If that is not how you’d like to 
>>>>> see it done, let me know and I’ll make the change.
>>>> 
>>>> I wonder if that is that the right thing to do.  I guess this is a policy
>>>> decision and may depend on the particular user.
>>> 
>>> Why would you want to allow dbx entry in any keyring?
>> 
>> Today, DB and MOK certs go into the platform keyring.  These certs are only
>> referenced during kexec.  They can’t be used for other things like validating
>> kernel module signatures.  If we follow the same pattern, the DBX and MOKX entries
>> in the blacklist keyring should only impact kexec. 
>> 
>> Currently, Mickaël Salaün has another outstanding series to allow root to update 
>> the blacklist keyring.  I assume the use case for this is around certificates used 
>> within the kernel, for example revoking kernel module signatures.  The question I have
>> is, should another keyring be introduced?  One that carries DBX and MOKX, which just
>> correspond to certs/hashes in the platform keyring; this keyring would only be
>> referenced for kexec, just like the platform keyring is today. Then, the current
>> blacklist keyring would be used for everything internal to the kernel.
> 
> Right, I'm following actively that series.
> 
> Why couldn't user space drive this process and use that feature to do it?

I could see where the user would want to use both. With Mickaël Salaün’s
series, the blacklist keyring is updated immediately.  However it does
not survive a reboot.  With my patch, the blacklist keyring is updated
during boot, based on what is in the dbx. Neither approach needs a new 
kernel build.