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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D10E3C433E0 for ; Sun, 28 Feb 2021 08:00:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F4B864E58 for ; Sun, 28 Feb 2021 08:00:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230299AbhB1H7r (ORCPT ); Sun, 28 Feb 2021 02:59:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44544 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229984AbhB1H7q (ORCPT ); Sun, 28 Feb 2021 02:59:46 -0500 Received: from cavan.codon.org.uk (cavan.codon.org.uk [IPv6:2a00:1098:84:22e::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 18070C06174A; Sat, 27 Feb 2021 23:59:05 -0800 (PST) Received: by cavan.codon.org.uk (Postfix, from userid 1000) id 1B1A540A0A; Sun, 28 Feb 2021 07:59:02 +0000 (UTC) Date: Sun, 28 Feb 2021 07:59:02 +0000 From: Matthew Garrett To: James Bottomley Cc: Matthew Garrett , linux-kernel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-pm@vger.kernel.org, keyrings@vger.kernel.org, zohar@linux.ibm.com, jarkko@kernel.org, corbet@lwn.net, rjw@rjwysocki.net, Matthew Garrett Subject: Re: [PATCH 2/9] tpm: Allow PCR 23 to be restricted to kernel-only use Message-ID: <20210228075902.GA9183@codon.org.uk> References: <20210220013255.1083202-1-matthewgarrett@google.com> <20210220013255.1083202-3-matthewgarrett@google.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: keyrings@vger.kernel.org On Wed, Feb 24, 2021 at 10:00:53AM -0800, James Bottomley wrote: > On Sat, 2021-02-20 at 01:32 +0000, Matthew Garrett wrote: > > Under certain circumstances it might be desirable to enable the > > creation of TPM-backed secrets that are only accessible to the > > kernel. In an ideal world this could be achieved by using TPM > > localities, but these don't appear to be available on consumer > > systems. > > I don't understand this ... the localities seem to work fine on all the > systems I have ... is this some embedded thing? I haven't made it work on an HP Z440 or a Lenovo P520. So now I'm wondering whether having chipsets with TXT support (even if it's turned off) confuse this point. Sigh. I'd really prefer to use localities than a PCR, so if it works on client platforms I'd be inclined to say we'll do a self-test and go for that, and workstation vendors can just recommend their customers use UPSes or something. > > An alternative is to simply block userland from modifying one of the > > resettable PCRs, leaving it available to the kernel. If the kernel > > ensures that no userland can access the TPM while it is carrying out > > work, it can reset PCR 23, extend it to an arbitrary value, create or > > load a secret, and then reset the PCR again. Even if userland somehow > > obtains the sealed material, it will be unable to unseal it since PCR > > 23 will never be in the appropriate state. > > This seems a bit arbitrary: You're removing this PCR from user space > accessibility, but PCR 23 is defined as "Application Support" how can > we be sure no application will actually want to use it (and then fail)? Absolutely no way of guaranteeing that, and enabling this option is certainly an ABI break. > Since PCRs are very scarce, why not use a NV index instead. They're > still a bounded resource, but most TPMs have far more of them than they > do PCRs, and the address space is much bigger so picking a nice > arbitrary 24 bit value reduces the chance of collisions. How many write cycles do we expect the NV to survive? But I'll find a client system with a TPM and play with locality support there - maybe we can just avoid this problem anyway.