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=-1.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, HK_RANDOM_FROM,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 C41EFFA372B for ; Wed, 16 Oct 2019 13:13:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A3F5120663 for ; Wed, 16 Oct 2019 13:13:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393515AbfJPNNK (ORCPT ); Wed, 16 Oct 2019 09:13:10 -0400 Received: from mga11.intel.com ([192.55.52.93]:54456 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390087AbfJPNNK (ORCPT ); Wed, 16 Oct 2019 09:13:10 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 16 Oct 2019 06:13:10 -0700 X-IronPort-AV: E=Sophos;i="5.67,303,1566889200"; d="scan'208";a="186145335" Received: from xiaoyaol-mobl.ccr.corp.intel.com (HELO [10.239.13.123]) ([10.239.13.123]) by orsmga007-auth.jf.intel.com with ESMTP/TLS/AES256-SHA; 16 Oct 2019 06:13:06 -0700 Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock To: Paolo Bonzini , Thomas Gleixner Cc: Sean Christopherson , Fenghua Yu , Ingo Molnar , Borislav Petkov , H Peter Anvin , Peter Zijlstra , Andrew Morton , Dave Hansen , Radim Krcmar , Ashok Raj , Tony Luck , Dan Williams , Sai Praneeth Prakhya , Ravi V Shankar , linux-kernel , x86 , kvm@vger.kernel.org References: <1560897679-228028-1-git-send-email-fenghua.yu@intel.com> <1560897679-228028-10-git-send-email-fenghua.yu@intel.com> <20190626203637.GC245468@romley-ivt3.sc.intel.com> <20190925180931.GG31852@linux.intel.com> <3ec328dc-2763-9da5-28d6-e28970262c58@redhat.com> <57f40083-9063-5d41-f06d-fa1ae4c78ec6@redhat.com> From: Xiaoyao Li Message-ID: <8808c9ac-0906-5eec-a31f-27cbec778f9c@intel.com> Date: Wed, 16 Oct 2019 21:13:04 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 10/16/2019 7:26 PM, Paolo Bonzini wrote: > On 16/10/19 13:23, Xiaoyao Li wrote: >> KVM always traps #AC, and only advertises split-lock detection to guest >> when the global variable split_lock_detection_enabled in host is true. >> >> - If guest enables #AC (CPL3 alignment check or split-lock detection >> enabled), injecting #AC back into guest since it's supposed capable of >> handling it. >> - If guest doesn't enable #AC, KVM reports #AC to userspace (like other >> unexpected exceptions), and we can print a hint in kernel, or let >> userspace (e.g., QEMU) tell the user guest is killed because there is a >> split-lock in guest. >> >> In this way, malicious guests always get killed by userspace and old >> sane guests cannot survive as well if it causes split-lock. If we do >> want old sane guests work we have to disable the split-lock detection >> (through booting parameter or debugfs) in the host just the same as we >> want to run an old and split-lock generating userspace binary. > > Old guests are prevalent enough that enabling split-lock detection by > default would be a big usability issue. And even ignoring that, you > would get the issue you describe below: Right, whether enabling split-lock detection is made by the administrator. The administrator is supposed to know the consequence of enabling it. Enabling it means don't want any split-lock happens in userspace, of course VMM softwares are under control. >> But there is an issue that we advertise split-lock detection to guest >> based on the value of split_lock_detection_enabled to be true in host, >> which can be turned into false dynamically when split-lock happens in >> host kernel. > > ... which means that supposedly safe guests become unsafe, and that is bad. > >> This causes guest's capability changes at run time and I >> don't if there is a better way to inform guest? Maybe we need a pv >> interface? > > Even a PV interface would not change the basic fact that a supposedly > safe configuration becomes unsafe. I don't catch you about the unsafe? If host disables split-lock detection dynamically, then the MST_TEST_CTL.split_lock is clear in the hardware and we can use the PV interface to notify the guest so that guest knows it loses the capability of split-lock detection. In this case, I think safety is meaningless for both host and guest. > Paolo >