linux-kernel - Re: [PATCH v4] certs: Add EFI_CERT_X509

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Date:   Tue, 12 Jan 2021 14:57:39 +0000
From:   David Howells <dhowells@...hat.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     dhowells@...hat.com, dwmw2@...radead.org,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        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
Subject: Re: [PATCH v4] certs: Add EFI_CERT_X509_GUID support for dbx entries

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.

> > Also, what should happen if a revocation cert rejected by the blacklist?
> 
> I’m not sure I understand the question. How would it be rejected?

The SHA256 of a revocation cert being loaded could match an
already-blacklisted SHA256 sum, either compiled in or already loaded from
UEFI.

David