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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 563FFC4CECD for ; Mon, 27 Apr 2020 18:15:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3191D214AF for ; Mon, 27 Apr 2020 18:15:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=citrix.com header.i=@citrix.com header.b="RICYFDiW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726510AbgD0SPN (ORCPT ); Mon, 27 Apr 2020 14:15:13 -0400 Received: from esa6.hc3370-68.iphmx.com ([216.71.155.175]:21981 "EHLO esa6.hc3370-68.iphmx.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726250AbgD0SPM (ORCPT ); Mon, 27 Apr 2020 14:15:12 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=citrix.com; s=securemail; t=1588011312; h=subject:to:cc:references:from:message-id:date: mime-version:in-reply-to:content-transfer-encoding; bh=KBBurnKwQk0mJMwfeqD0uyO9fh4p+KVZdnBjFEeHlv4=; b=RICYFDiWA2SWatqKnhjWWBhAgHgVnhVrTNm4ZXSrt1WNaD/RlDuCMcRF SlsMzz/VXZLs+Y4bKg+FusohTZY7yJMrphwWTA1qBCVjc0xfrcwN5ekHB RPs87Retey/GxdO3rIB8Ym0NSDhHm0DuIsrXW6H11sZOpiD12NTPNezV0 Y=; Authentication-Results: esa6.hc3370-68.iphmx.com; dkim=none (message not signed) header.i=none; spf=None smtp.pra=andrew.cooper3@citrix.com; spf=Pass smtp.mailfrom=Andrew.Cooper3@citrix.com; spf=None smtp.helo=postmaster@mail.citrix.com Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender authenticity information available from domain of andrew.cooper3@citrix.com) identity=pra; client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com; envelope-from="Andrew.Cooper3@citrix.com"; x-sender="andrew.cooper3@citrix.com"; x-conformance=sidf_compatible Received-SPF: Pass (esa6.hc3370-68.iphmx.com: domain of Andrew.Cooper3@citrix.com designates 162.221.158.21 as permitted sender) identity=mailfrom; client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com; envelope-from="Andrew.Cooper3@citrix.com"; x-sender="Andrew.Cooper3@citrix.com"; x-conformance=sidf_compatible; x-record-type="v=spf1"; x-record-text="v=spf1 ip4:209.167.231.154 ip4:178.63.86.133 ip4:195.66.111.40/30 ip4:85.115.9.32/28 ip4:199.102.83.4 ip4:192.28.146.160 ip4:192.28.146.107 ip4:216.52.6.88 ip4:216.52.6.188 ip4:162.221.158.21 ip4:162.221.156.83 ip4:168.245.78.127 ~all" Received-SPF: None (esa6.hc3370-68.iphmx.com: no sender authenticity information available from domain of postmaster@mail.citrix.com) identity=helo; client-ip=162.221.158.21; receiver=esa6.hc3370-68.iphmx.com; envelope-from="Andrew.Cooper3@citrix.com"; x-sender="postmaster@mail.citrix.com"; x-conformance=sidf_compatible IronPort-SDR: ELrzJwrXWLafnbRhhOpo9sUmi2ptJ2dbSjQcB604T4YpDGeUhU1Zj28fONXyGGCdtv0J2MzLme XdKmX8FYTey36XrM/OsB2JaDUUC99LUtGFxpHQvrJIgJJR9qtSTItmghnWelKsCmEv4Es0/loA 7nD6hg1DAWb6O67JNQn9pDkGbYOW6P1mCIEA7IhM8rR37q2YS/ikdhotFDI33VW5CLeFwRmvlX VgHNdN4Igthunl6nAHb98NDXTO7YJJbRh5jj5abE4kLhAEiCXMk9j5b/g9dQiXF8CDbK52pYgh Kw8= X-SBRS: 2.7 X-MesageID: 16722682 X-Ironport-Server: esa6.hc3370-68.iphmx.com X-Remote-IP: 162.221.158.21 X-Policy: $RELAYED X-IronPort-AV: E=Sophos;i="5.73,325,1583211600"; d="scan'208";a="16722682" Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace) To: Andy Lutomirski CC: Joerg Roedel , Joerg Roedel , "Dave Hansen" , Tom Lendacky , "Mike Stunes" , Dan Williams , "Dave Hansen" , "H. Peter Anvin" , Juergen Gross , Jiri Slaby , Kees Cook , kvm list , LKML , Peter Zijlstra , "Thomas Hellstrom" , Linux Virtualization , X86 ML , "Sean Christopherson" References: <20200425191032.GK21900@8bytes.org> <910AE5B4-4522-4133-99F7-64850181FBF9@amacapital.net> <20200425202316.GL21900@8bytes.org> From: Andrew Cooper Message-ID: <57aa7412-b9e1-3331-ba30-bb6daaa28ff3@citrix.com> Date: Mon, 27 Apr 2020 19:15:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Content-Language: en-GB X-ClientProxiedBy: AMSPEX02CAS01.citrite.net (10.69.22.112) To AMSPEX02CL02.citrite.net (10.69.22.126) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/04/2020 18:37, Andy Lutomirski wrote: > On Sat, Apr 25, 2020 at 3:10 PM Andy Lutomirski wrote: >> On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel wrote: >>> On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote: >>>> I assume the race you mean is: >>>> >>>> #VC >>>> Immediate NMI before IST gets shifted >>>> #VC >>>> >>>> Kaboom. >>>> >>>> How are you dealing with this? Ultimately, I think that NMI will need >>>> to turn off IST before engaging in any funny business. Let me ponder >>>> this a bit. >>> Right, I dealt with that by unconditionally shifting/unshifting the #VC IST entry >>> in do_nmi() (thanks to Davin Kaplan for the idea). It might cause >>> one of the IST stacks to be unused during nesting, but that is fine. The >>> stack memory for #VC is only allocated when SEV-ES is active (in an >>> SEV-ES VM). >> Blech. It probably works, but still, yuck. It's a bit sad that we >> seem to be growing more and more poorly designed happens-anywhere >> exception types at an alarming rate. We seem to have #NM, #MC, #VC, >> #HV, and #DB. This doesn't really scale. > I have a somewhat serious question: should we use IST for #VC at all? > As I understand it, Rome and Naples make it mandatory for hypervisors > to intercept #DB, which means that, due to the MOV SS mess, it's sort > of mandatory to use IST for #VC. But Milan fixes the #DB issue, so, > if we're running under a sufficiently sensible hypervisor, we don't > need IST for #VC. > > So I think we have two choices: > > 1. Use IST for #VC and deal with all the mess that entails. > > 2. Say that we SEV-ES client support on Rome and Naples is for > development only and do a quick boot-time check for whether #DB is > intercepted. (Just set TF and see what vector we get.) If #DB is > intercepted, print a very loud warning and refuse to boot unless some > special sev_es.insecure_development_mode or similar option is set. > > #2 results in simpler and more robust entry code. #1 is more secure. > > So my question is: will anyone actually use SEV-ES in production on > Rome or Naples? As I understand it, it's not really ready for prime > time on those chips. And do we care if the combination of a malicious > hypervisor and malicious guest userspace on Milan can compromise the > guest kernel? I don't think SEV-ES is really mean to resist a > concerted effort by the hypervisor to compromise the guest. More specifically, it is mandatory for hypervisors to intercept #DB to defend against CVE-2015-8104, unless they're willing to trust the guest not to tickle that corner case. This is believed fixed with SEV-SNP to allow the encrypted guest to use debugging functionality without posing a DoS risk to the host.  In this case, the hypervisor is expected not to intercept #DB. If #DB is intercepted, and #VC doesn't use IST, malicious userspace can cause problems with a movss-delayed breakpoint over SYSCALL. The question basically whether it is worth going to the effort of making #VC IST and all the problems that entails, to cover one corner case in earlier hardware. Ultimately, this depends on whether anyone plans to put SEV-ES into production on pre SEV-SNP hardware, and if developers using pre-SEV-SNP hardware are happy with "don't run malicious userspace" or "don't run malicious kernels and skip the #DB intercept" as a fair tradeoff to avoid the #VC IST fun. ~Andrew