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

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Date:   Tue, 14 Nov 2017 14:34:30 -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 2:31 PM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Tue, 2017-11-14 at 14:17 -0800, Matthew Garrett wrote:
>> 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.
>
> OK, so I agree that if you have sealed something required for boot (and
> have the capability for resealing it on OS upgrade) you can use
> measurements locally.  However, I don't believe we have any systems
> today in Linux which can do this (we have theoretical ideas about how
> we might do it with LUKS root keys and one day we might actually have
> the infrastructure to make it viable for a standard laptop).

It's used for TPMTOTP, for instance.

> Absent that, secure boot provides a reasonable measure of security
> which works with today's infrastructure.
>
> Note: this doesn't mean I necessarily want signatures everywhere (like
> firmware).  We can sign elements in blobs that provide the effective
> security without needing more granular signatures.

To be clear - I'm arguing in favour of signatures here. Measured boot
is much easier to work with in their presence.