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

[Jarkko Sakkinen]

On Wed, Jan 20, 2021 at 03:13:11PM -0700, Eric Snowberg wrote:
> 
> > On Jan 20, 2021, at 4:26 AM, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> > 
> > On Fri, Jan 15, 2021 at 09:49:02AM -0700, Eric Snowberg wrote:
> >> 
> >>> On Jan 15, 2021, at 2:15 AM, Jarkko Sakkinen <jarkko@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx> wrote:
> >>>>> 
> >>>>> On Tue, Jan 12, 2021 at 02:57:39PM +0000, David Howells wrote:
> >>>>>> Eric Snowberg <eric.snowberg@xxxxxxxxxx> wrote:
> >>>>>> 
> >>>>>>>> On Dec 10, 2020, at 2:49 AM, David Howells <dhowells@xxxxxxxxxx> wrote:
> >>>>>>>> 
> >>>>>>>> Eric Snowberg <eric.snowberg@xxxxxxxxxx> 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?
> 
> This change simply adds support for a missing type.  The kernel 
> already supports cert and hash entries (EFI_CERT_X509_SHA256_GUID,
> EFI_CERT_SHA256_GUID) that originate from the dbx and are loaded 
> into the blacklist keyring during boot.  I’m not sure why a cert 
> defined with EFI_CERT_X509_GUID should be handled in a different 
> manner.
> 
> 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.

/Jarkko