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 C720BC10DCE for ; Thu, 12 Mar 2020 11:42:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A657F206E7 for ; Thu, 12 Mar 2020 11:42:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727063AbgCLLm6 (ORCPT ); Thu, 12 Mar 2020 07:42:58 -0400 Received: from mga03.intel.com ([134.134.136.65]:19326 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725268AbgCLLm6 (ORCPT ); Thu, 12 Mar 2020 07:42:58 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 12 Mar 2020 04:42:57 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,544,1574150400"; d="scan'208";a="389577574" Received: from xiaoyaol-mobl.ccr.corp.intel.com (HELO [10.249.169.134]) ([10.249.169.134]) by orsmga004.jf.intel.com with ESMTP; 12 Mar 2020 04:42:54 -0700 Subject: Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write To: Sean Christopherson , Paolo Bonzini Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Andy Lutomirski , x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, David Laight References: <20200203151608.28053-1-xiaoyao.li@intel.com> <20200203151608.28053-4-xiaoyao.li@intel.com> <95d29a81-62d5-f5b6-0eb6-9d002c0bba23@redhat.com> <878sl945tj.fsf@nanos.tec.linutronix.de> <20200227001117.GX9940@linux.intel.com> From: Xiaoyao Li Message-ID: <18333d32-9ec4-4aee-8c58-b2f44bb8e83d@intel.com> Date: Thu, 12 Mar 2020 19:42:53 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200227001117.GX9940@linux.intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/27/2020 8:11 AM, Sean Christopherson wrote: > On Tue, Feb 11, 2020 at 02:34:18PM +0100, Paolo Bonzini wrote: >> On 11/02/20 14:22, Thomas Gleixner wrote: >>> Paolo Bonzini writes: >>>> On 03/02/20 16:16, Xiaoyao Li wrote: >>>>> A sane guest should never tigger emulation on a split-lock access, but >>>>> it cannot prevent malicous guest from doing this. So just emulating the >>>>> access as a write if it's a split-lock access to avoid malicous guest >>>>> polluting the kernel log. >>>> >>>> Saying that anything doing a split lock access is malicious makes little >>>> sense. >>> >>> Correct, but we also have to accept, that split lock access can be used >>> in a malicious way, aka. DoS. >> >> Indeed, a more accurate emulation such as temporarily disabling >> split-lock detection in the emulator would allow the guest to use split >> lock access as a vehicle for DoS, but that's not what the commit message >> says. If it were only about polluting the kernel log, there's >> printk_ratelimited for that. (In fact, if we went for incorrect >> emulation as in this patch, a rate-limited pr_warn would be a good idea). >> >> It is much more convincing to say that since this is pretty much a >> theoretical case, we can assume that it is only done with the purpose of >> DoS-ing the host or something like that, and therefore we kill the guest. > > The problem with "kill the guest", and the reason I'd prefer to emulate the > split-lock as a write, is that killing the guest in this case is annoyingly > difficult. > > Returning X86EMUL_UNHANDLEABLE / EMULATION_FAILED gets KVM to > handle_emulation_failure(), but handle_emulation_failure() will only "kill" > the guest if emulation failed in L1 CPL==0. For all other modes, it will > inject a #UD and resume the guest. KVM also injects a #UD for L1 CPL==0, > but that's the least annoying thing. > > Adding a new emulation type isn't an option because this code can be > triggered through normal emulation. A new return type could be added for > split-lock, but that's code I'd really not add, both from an Intel > perspective and a KVM maintenance perspective. And, we'd still have the > conundrum of what to do if/when split-lock #AC is exposed to L1, e.g. in > that case, KVM should inject an #AC into L1, not kill the guest. Again, > totally doable, but ugly and IMO an unnecessary maintenance burden. > > I completely agree that poorly emulating the instruction from the (likely) > malicious guest is a hack, but it's a simple and easy to maintain hack. Paolo, What's your opinion about above? >>>> Split lock detection is essentially a debugging feature, there's a >>>> reason why the MSR is called "TEST_CTL". So you don't want to make the >>> >>> The fact that it ended up in MSR_TEST_CTL does not say anything. That's >>> where they it ended up to be as it was hastily cobbled together for >>> whatever reason. >> >> Or perhaps it was there all the time in test silicon or something like >> that... That would be a very plausible reason for all the quirks behind it. >> >> Paolo >>