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SubjectRe: [PATCH v4] certs: Add EFI_CERT_X509_GUID support for dbx entries
On Fri, Jan 15, 2021 at 09:49:02AM -0700, Eric Snowberg wrote:
>
> > On Jan 15, 2021, at 2:15 AM, Jarkko Sakkinen <jarkko@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@linux.intel.com> wrote:
> >>>
> >>> On Tue, Jan 12, 2021 at 02:57:39PM +0000, David Howells wrote:
> >>>> Eric Snowberg <eric.snowberg@oracle.com> wrote:
> >>>>
> >>>>>> On Dec 10, 2020, at 2:49 AM, David Howells <dhowells@redhat.com> wrote:
> >>>>>>
> >>>>>> Eric Snowberg <eric.snowberg@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.

I don't want to purposely challenge this, but why does it matter
that it doesn't survive the boot? I'm referring here to the golden
principle of kernel defining a mechanism, not policy. User space
can do the population however it wants to for every boot.

E.g. systemd service could do this.

What am I missing here?

/Jarkko

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