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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 7EC32C4707A for ; Fri, 21 May 2021 18:30:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5CC46613F8 for ; Fri, 21 May 2021 18:30:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234822AbhEUSby (ORCPT ); Fri, 21 May 2021 14:31:54 -0400 Received: from mga05.intel.com ([192.55.52.43]:39036 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229762AbhEUSbx (ORCPT ); Fri, 21 May 2021 14:31:53 -0400 IronPort-SDR: FDPv9F+qYq4BT20lLRpM1dXDoccaLABOvihrWYJurvzrKFeOxdQuU8wfTK6DF7E7m6GvYaEe7S gft5qvoMIsnw== X-IronPort-AV: E=McAfee;i="6200,9189,9991"; a="287087255" X-IronPort-AV: E=Sophos;i="5.82,319,1613462400"; d="scan'208";a="287087255" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 May 2021 11:30:28 -0700 IronPort-SDR: 2/ibAWiIBOcZjx8ReYRPPf0JUqh04zOHN9CUMSnVNuzNXkKiN1uPl7pS1Sxyv3CUBlmP+Nm8E5 RFY1LzbMhBLA== X-IronPort-AV: E=Sophos;i="5.82,319,1613462400"; d="scan'208";a="613342975" Received: from pburton-mobl.amr.corp.intel.com (HELO [10.209.36.169]) ([10.209.36.169]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 May 2021 11:30:27 -0700 Subject: Re: [RFC v2-fix-v2 1/1] x86/boot: Avoid #VE during boot for TDX platforms To: Sean Christopherson Cc: Kuppuswamy Sathyanarayanan , Peter Zijlstra , Andy Lutomirski , Tony Luck , Andi Kleen , Kirill Shutemov , Kuppuswamy Sathyanarayanan , Dan Williams , Raj Ashok , linux-kernel@vger.kernel.org, Sean Christopherson References: <20210521143524.2527690-1-sathyanarayanan.kuppuswamy@linux.intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/21/21 11:18 AM, Sean Christopherson wrote: > On Fri, May 21, 2021, Dave Hansen wrote: >>> + /* >>> + * Preserve current value of EFER for comparison and to skip >>> + * EFER writes if no change was made (for TDX guest) >>> + */ >>> + movl %eax, %edx >>> btsl $_EFER_SCE, %eax /* Enable System Call */ >>> btl $20,%edi /* No Execute supported? */ >>> jnc 1f >>> btsl $_EFER_NX, %eax >>> btsq $_PAGE_BIT_NX,early_pmd_flags(%rip) >>> -1: wrmsr /* Make changes effective */ >>> >>> + /* Avoid writing EFER if no change was made (for TDX guest) */ >>> +1: cmpl %edx, %eax >>> + je 1f >>> + xor %edx, %edx >>> + wrmsr /* Make changes effective */ >>> +1: >> >> Just curious, but what if this goes wrong? Say the TDX firmware didn't >> set up EFER correctly and this code does the WRMSR. > > By firmware, do you mean TDX-module, or guest firmware? EFER is read-only in a > TDX guest, i.e. the guest firmware can't change it either. I guess I was assuming that the trusted BIOS was going to do the setup of EFER before it hands control over to the kernel. So, I *meant* the BIOS. But, I see from below that it's probably the TDX-module that's responsible for this behavior. >> What ends up happening? Do we get anything out on the console, or is it >> essentially undebuggable? > > Assuming "firmware" means TDX-module, if TDX-Module botches EFER (and only EFER) > then odds are very, very good that the guest will never get to the kernel as it > will have died long before in guest BIOS. > > If the bug is such that EFER is correct in hardware, but RDMSR returns the wrong > value (due to MSR interception), IIRC this will triple fault and so nothing will > get logged. But, the odds of that type of bug being hit in production are > practically zero because the EFER setup is very static, i.e. any such bug should > be hit during qualification of the VMM+TDX-Module. > > In any case, even if a bug escapes, the shutdown is relatively easy to debug even > without logs because the failure will cleary point at the WRMSR (that info can be > had by running a debug TD or a debug TDX-Module). By TDX standards, debugging > shutdowns on a specific instruction is downright trivial :-). That sounds sane to me. It would be nice to get this into the changelog. Perhaps: This theoretically makes guest boot more fragile. If, for instance, EER was set up incorrectly and a WRMSR was performed, the resulting (unhandled) #VE would triple fault. However, this is likely to trip up the guest BIOS long before control reaches the kernel. In any case, these kinds of problems are unlikely to occur in production environments, and developers have good debug tools to fix them quickly. That would put my mind at ease a bit.