From mboxrd@z Thu Jan  1 00:00:00 1970
From: mjg59@google.com (Matthew Garrett)
Date: Tue, 14 Nov 2017 14:17:25 -0800
Subject: Firmware signing -- Re: [PATCH 00/27] security,
	efi: Add kernel lockdown
In-Reply-To: <1510697658.7703.12.camel@HansenPartnership.com>
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	<20171113174250.GA22894@wotan.suse.de>
	<20171113210848.4dc344bd@alans-desktop>
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	<CA+55aFzrK29hbxoKs3GsvVUY4_uR7aBgZv2eeVyVq7MzfATY3w@mail.gmail.com>
	<1510662098.3711.139.camel@linux.vnet.ibm.com>
	<CA+55aFzvuvqOfsJ9arzcc1QbTGs+U-TsNmsyem9UAVVQC8YkZQ@mail.gmail.com>
	<CACdnJuvP=0AHGtfGJ5+cT+kHRy3fU4BLjwkvzP0rLO6q5ejAQQ@mail.gmail.com>
	<CA+55aFxeLwgwxh2iJTf6Dz0T_a_TZfTdhBw5TkcSsCmjt2N5pw@mail.gmail.com>
	<20171114205014.GJ729@wotan.suse.de>
	<CACdnJuvzPnMwsAF4mUJXCaJWQ=nCc9Yi5u3gj1A0+BsWf1Swgw@mail.gmail.com>
	<1510697658.7703.12.camel@HansenPartnership.com>
Message-ID: <CACdnJuuYasij2_JAvdvof-8PRgKMSAT1NOBzHG=Vr-4MN79SNg@mail.gmail.com>
To: linux-security-module@vger.kernel.org
List-Id: linux-security-module.vger.kernel.org

On Tue, Nov 14, 2017 at 2:14 PM, James Bottomley
<James.Bottomley@hansenpartnership.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.
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