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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 12E94C10F25 for ; Sat, 7 Mar 2020 15:47:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E5B482073C for ; Sat, 7 Mar 2020 15:47:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726206AbgCGPrI (ORCPT ); Sat, 7 Mar 2020 10:47:08 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:55627 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726073AbgCGPrI (ORCPT ); Sat, 7 Mar 2020 10:47:08 -0500 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jAbfL-0006eZ-M6; Sat, 07 Mar 2020 16:47:04 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 7F7D0104088; Sat, 7 Mar 2020 16:47:01 +0100 (CET) From: Thomas Gleixner To: Andy Lutomirski , Andy Lutomirski Cc: LKML , X86 ML , kvm list , Paolo Bonzini , stable Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS In-Reply-To: References: Date: Sat, 07 Mar 2020 16:47:01 +0100 Message-ID: <87ftek9ngq.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Andy Lutomirski writes: > On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski wrote: >> + /* >> + * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current >> + * KVM paravirt ABI, the following scenario is possible: >> + * >> + * #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT) >> + * NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT >> + * NMI accesses user memory, e.g. due to perf >> + * #PF: normal page fault >> + * #PF reads CR2 and apf_reason -- apf_reason should be 0 >> + * >> + * outer #PF reads CR2 and apf_reason -- apf_reason should be >> + * KVM_PV_REASON_PAGE_NOT_PRESENT >> + * >> + * There is no possible way that both reads of CR2 and >> + * apf_reason get the correct values. Fixing this would >> + * require paravirt ABI changes. >> + */ >> + > > Upon re-reading my own comment, I think the problem is real, but I > don't think my patch fixes it. The outer #PF could just as easily > have come from user mode. We may actually need the NMI code (and > perhaps MCE and maybe #DB too) to save, clear, and restore apf_reason. > If we do this, then maybe CPL0 async PFs are actually okay, but the > semantics are so poorly defined that I'm not very confident about > that. I think even with the current mode this is fixable on the host side when it keeps track of the state. The host knows exactly when it injects a async PF and it can store CR2 and reason of that async PF in flight. On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0 then it knows that the guest has read CR2 and apf_reason. All good nothing to worry about. If not it needs to be careful. As long as the apf_reason of the last async #PF is not cleared by the guest no new async #PF can be injected. That's already correct because in that case IF==0 which prevents a nested async #PF. If MCE, NMI trigger a real pagefault then the #PF injection needs to clear apf_reason and set the correct CR2. When that #PF returns then the old CR2 and apf_reason need to be restored. I tried to figure out whether any of this logic exists in the KVM code, but I got completely lost in that code. Maybe I try later today again. Thanks, tglx