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Date:   Wed, 20 Jan 2021 13:26:35 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        David Howells <dhowells@...hat.com>, dwmw2@...radead.org,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        jmorris@...ei.org, serge@...lyn.com, nayna@...ux.ibm.com,
        Mimi Zohar <zohar@...ux.ibm.com>, erichte@...ux.ibm.com,
        mpe@...erman.id.au, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        James.Bottomley@...senpartnership.com
Subject: Re: [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@...nel.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@...ux.intel.com> wrote:
> >>> 
> >>> On Tue, Jan 12, 2021 at 02:57:39PM +0000, David Howells wrote:
> >>>> Eric Snowberg <eric.snowberg@...cle.com> wrote:
> >>>> 
> >>>>>> On Dec 10, 2020, at 2:49 AM, David Howells <dhowells@...hat.com> wrote:
> >>>>>> 
> >>>>>> Eric Snowberg <eric.snowberg@...cle.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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