From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AB040C00140 for ; Wed, 10 Aug 2022 17:46:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232038AbiHJRqt (ORCPT ); Wed, 10 Aug 2022 13:46:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51188 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230282AbiHJRqr (ORCPT ); Wed, 10 Aug 2022 13:46:47 -0400 Received: from cavan.codon.org.uk (irc.codon.org.uk [IPv6:2a00:1098:84:22e::2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3445C6CF46; Wed, 10 Aug 2022 10:46:41 -0700 (PDT) Received: by cavan.codon.org.uk (Postfix, from userid 1000) id 815D840A8A; Wed, 10 Aug 2022 18:46:38 +0100 (BST) Date: Wed, 10 Aug 2022 18:46:38 +0100 From: Matthew Garrett To: Brendan Trotter Cc: The development of GNU GRUB , Ard Biesheuvel , Daniel Kiper , Alec Brown , Kanth Ghatraju , Ross Philipson , "piotr.krol@3mdeb.com" , "krystian.hebel@3mdeb.com" , "persaur@gmail.com" , "Yoder, Stuart" , Andrew Cooper , "michal.zygowski@3mdeb.com" , James Bottomley , "lukasz@hawrylko.pl" , linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, James Morris Subject: Re: Linux DRTM on UEFI platforms Message-ID: <20220810174638.GA7906@srcf.ucam.org> References: <7aab2990-9c57-2456-b08d-299ae96ac919@apertussolutions.com> <203110bb-b70b-b4f1-9453-46136659f84c@apertussolutions.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 10, 2022 at 06:37:18PM +0930, Brendan Trotter wrote: > [1] doesn't provide any useful information. How does a kernel know > that the callback provided by boot loader actually measures what it's > supposed to measure, or even does anything at all? The kernel has no way to know this - *any* code you've run before performing a measurement could tamper with the kernel such that it believes it's fine. This is just as true in DRTM as it is in SRTM. But you know what the expected measurements should be, so you're able to either seal secrets to those PCR values or rely on remote attestation. > [1] doesn't provide any useful information. Senter and skinit don't > provide a method for kernel to detect that (e.g.) a MiTM boot loader > has always measured a forgery and has changed unmeasured code in a > different way every time you boot. Measurements are not opaque objects. If you're not able to reconstruct the expected measurement then you're doing it wrong.