linux-kernel - Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel lockdown

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Date:   Tue, 14 Nov 2017 14:17:25 -0800
From:   Matthew Garrett <mjg59@...gle.com>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Berg <johannes@...solutions.net>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jan Blunck <jblunck@...radead.org>,
        Julia Lawall <julia.lawall@...6.fr>,
        Marcus Meissner <meissner@...e.de>, Gary Lin <GLin@...e.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel lockdown

On Tue, Nov 14, 2017 at 2:14 PM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Tue, 2017-11-14 at 15:55 -0500, Matthew Garrett wrote:
>> TPM-backed Trusted Boot means you don't /need/ to sign anything,
>> since the measurements of what you loaded will end up in the TPM. But
>> signatures make it a lot easier, since you can just assert that only
>> signed material will be loaded and so you only need to measure the
>> kernel and the trusted keys.
>
> Actually, I'd disagree with that quite a lot: measured boot only works
> if you're attesting to something outside of your system that has the
> capability for doing something about a wrong measurement.  Absent that,
> measured boot has no safety whatsoever.  Secure boot, on the other
> hand, can enforce not booting with elements that fail the signature
> check.

Measured boot has a great deal of value in the sealing of private
material, even in the absence of attestation. The way Microsoft make
use of PCR7 is a good example of how signatures make this easier -
achieving the same goal with a full measurement of the boot chain
instead of relying on signature validation results in significantly
more fragility.