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

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Date:   Mon, 21 Nov 2016 16:42:45 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Josh Boyer <jwboyer@...oraproject.org>
Cc:     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 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