linux-kernel - Re: [PATCH 9/9] MODSIGN: Allow the "db" UEFI variable to be suppressed

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Date:   Mon, 21 Nov 2016 14:05:32 -0500
From:   Peter Jones <pjones@...hat.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Josh Boyer <jwboyer@...oraproject.org>,
        David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
        Matthew Garrett <matthew.garrett@...ula.com>,
        "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>
Subject: Re: [PATCH 9/9] MODSIGN: Allow the "db" UEFI variable to be
 suppressed

On Mon, Nov 21, 2016 at 04:42:45PM +0000, Ard Biesheuvel wrote:
> On 21 November 2016 at 16:26, Josh Boyer <jwboyer@...oraproject.org> wrote:
> > On Mon, Nov 21, 2016 at 11:18 AM, Ard Biesheuvel
> > <ard.biesheuvel@...aro.org> wrote:
> >> On 16 November 2016 at 18:11, David Howells <dhowells@...hat.com> wrote:
> >>> From: Josh Boyer <jwboyer@...oraproject.org>
> >>>
> >>> If a user tells shim to not use the certs/hashes in the UEFI db variable
> >>> for verification purposes, shim will set a UEFI variable called
> >>> MokIgnoreDB.  Have the uefi import code look for this and ignore the db
> >>> variable if it is found.
> >>>
> >>
> >> Similar concern as in the previous patch: it appears to me that you
> >> can DoS a machine by setting MokIgnoreDB if, e.g., its modules are
> >> signed against a cert that resides in db, and shim/mokmanager are not
> >> being used.
> >
> > If shim/mokmanager aren't used, then you can't actually modify
> > MokIgnoreDB.  Again, it requires physical access and a reboot into
> > mokmanager to actually take effect.
> >
> 
> This does the trick as well
> 
> printf "\x07\x00\x00\x00\x01" >
> /sys/firmware/efi/efivars/MokIgnoreDB-605dab50-e046-4300-abb6-3dd810dd8b23

So that really means two things.  First, kernel should only honor any of
the Mok* variables if they're Boot Services-only variables.  Second, to
avoid the DoS, shim should create them all as Boot Services-only the
first time it boots.  That'll prevent them from being created post-boot.

-- 
        Peter