linux-kernel - Re: [PATCH 6/7] efi: Handle secure boot from UEFI-2.6 [ver #7]

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Message-ID: <10454.1485889162@warthog.procyon.org.uk>
Date:   Tue, 31 Jan 2017 18:59:22 +0000
From:   David Howells <dhowells@...hat.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     dhowells@...hat.com, Matt Fleming <matt@...eblueprint.co.uk>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        keyrings@...r.kernel.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 6/7] efi: Handle secure boot from UEFI-2.6 [ver #7]

Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> > UEFI-2.6 adds a new variable, DeployedMode.  If it exists, this must be 1
> > if we're to engage lockdown mode.
> >
> > Reported-by: James Bottomley <James.Bottomley@...senPartnership.com>
> > Signed-off-by: David Howells <dhowells@...hat.com>
> 
> Interestingly, the string 'DeployedMode' appears zero times in the
> EDK2 codebase, so I wonder if it makes any sense to merge this now.
> The string 'AuditMode' does appear once, but in a comment

It's in the standard, so shouldn't we check for it?

> In any case, the logic is not entirely correct either: apologies if it
> was me who caused any confusion here, but it seems DeployedMode could
> legally be 0 or 1 while secure boot is in fact enabled. It is actually
> AuditMode that should be taken into account here, i.e., if AuditMode
> == 1, the firmware ignores invalid or missing signatures. If
> SecureBoot == 0x1, SetupMode == 0x0 and AuditMode == 0x0 (or
> non-existent), signature verification is performed regardless of the
> value (or existence) of DeployedMode.
> 
> So I propose to respin this patch to treat AuditMode == 0x1 as 'secure
> boot disabled', and ignore if it is missing.

Ummm...  This might conflict what said:

| Since you seem to be using this to mean "is the platform locked down?",
| this looks to be no longer complete in the UEFI 2.6 world.  If
| DeployedMode == 0, even if SecureBoot == 1 and SetupMode == 0, you can
| remove the platform key by writing 1 to AuditMode and gain control of
| the secure variables.  The lock down state becomes DeployedMode == 1,
| SecureBoot == 1 and SetupMode == 0
| 
| See the diagram on page 1817
| 
| http://www.uefi.org/sites/default/files/resources/UEFI%20Spec%202_6.pdf

Looking again at that diagram, should I be checking all four variables
(SecureBoot, SetupMode, DeployedMode and AuditMode)?  And/or should I treat
audit mode differently to deployed mode?

Further, there doesn't seem to be a state in which SecureBoot is shown as
being 1.

David