linux-kernel - Re: [PATCH v2] KEYS: Make use of platform keyring for module signature verify

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Date:   Thu, 25 Apr 2019 12:46:45 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Robert Holmes <robeholmes@...il.com>, jeyu@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] KEYS: Make use of platform keyring for module
 signature verify

On Thu, 2019-04-25 at 07:55 -0400, Mimi Zohar wrote:
> On Wed, 2019-04-24 at 14:33 +0000, Robert Holmes wrote:
> > This patch completes commit 278311e417be ("kexec, KEYS: Make use of
> > platform keyring for signature verify") which, while adding the
> > platform keyring for bzImage verification, neglected to also add
> > this keyring for module verification.
> > 
> > As such, kernel modules signed with keys from the MokList variable
> > were not successfully verified.
> 
> Using the platform keyring keys for verifying kernel modules was not
> neglected, but rather intentional.  This patch description should
> clearly explain the reason for needing to verify kernel module
> signatures based on the pre-boot keys.  (Hint: verifying kernel
> modules based on the pre-boot keys was previously rejected.)

To clarify here: most Linux systems use shim/mok to pivot the root of
trust away from the Secure Boot db variable to the new MokList/shim
built in keys.  This makes the actual secure boot db outside the
expected Linux Kernel trust boundary *unless* the user has taken
ownership of the system and is actually using db for their own trusted
keys.  This makes the policy for what pre-boot keys to trust within the
Linux boundary very complex, which is why we default to not using the
pre-boot keys at all.

James