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=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 2E32CC43461 for ; Fri, 11 Sep 2020 16:06:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F3461206CA for ; Fri, 11 Sep 2020 16:06:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726074AbgIKQFk (ORCPT ); Fri, 11 Sep 2020 12:05:40 -0400 Received: from mga14.intel.com ([192.55.52.115]:51191 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726184AbgIKQFV (ORCPT ); Fri, 11 Sep 2020 12:05:21 -0400 IronPort-SDR: kys1aNcFhmZQPIqPTrk5KaDVrHUYPSrSXKkYEtlyYKFzAq8h38KpklnrtuP71OKO184uPDrl3H /7s7Baih+6Gw== X-IronPort-AV: E=McAfee;i="6000,8403,9741"; a="158071546" X-IronPort-AV: E=Sophos;i="5.76,416,1592895600"; d="scan'208";a="158071546" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Sep 2020 09:05:01 -0700 IronPort-SDR: J+uNvfXjsVKPaULLRObScXcXxC0aGVPmjVazhcEikrkeaQMnXn5rWg+TG/LI7fzWiTyQbwEHmM 2PCEvdVhi3qg== X-IronPort-AV: E=Sophos;i="5.76,416,1592895600"; d="scan'208";a="300969658" Received: from sjchrist-ice.jf.intel.com (HELO sjchrist-ice) ([10.54.31.34]) by orsmga003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 11 Sep 2020 09:04:58 -0700 Date: Fri, 11 Sep 2020 09:04:56 -0700 From: Sean Christopherson To: Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , "Dr . David Alan Gilbert" , linux-kernel@vger.kernel.org Subject: Re: [PATCH] KVM: x86: always allow writing '0' to MSR_KVM_ASYNC_PF_EN Message-ID: <20200911160455.GB4344@sjchrist-ice> References: <20200911093147.484565-1-vkuznets@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200911093147.484565-1-vkuznets@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Sep 11, 2020 at 11:31:47AM +0200, Vitaly Kuznetsov wrote: > Even without in-kernel LAPIC we should allow writing '0' to > MSR_KVM_ASYNC_PF_EN as we're not enabling the mechanism. In > particular, QEMU with 'kernel-irqchip=off' fails to start > a guest with > > qemu-system-x86_64: error: failed to set MSR 0x4b564d02 to 0x0 > > Fixes: 9d3c447c72fb2 ("KVM: X86: Fix async pf caused null-ptr-deref") > Reported-by: Dr. David Alan Gilbert > Signed-off-by: Vitaly Kuznetsov > --- > arch/x86/kvm/x86.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index d39d6cf1d473..44a86f7f2397 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2730,9 +2730,6 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) > if (data & 0x30) > return 1; > > - if (!lapic_in_kernel(vcpu)) > - return 1; > - > vcpu->arch.apf.msr_en_val = data; > > if (!kvm_pv_async_pf_enabled(vcpu)) { > @@ -2741,6 +2738,9 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) > return 0; > } > > + if (!lapic_in_kernel(vcpu)) This doesn't actually verify that @data == 0. kvm_pv_async_pf_enabled() returns true iff KVM_ASYNC_PF_ENABLED and KVM_ASYNC_PF_DELIVERY_AS_INT are set, e.g. this would allow setting one and not the other. This also allows userspace to set vcpu->arch.apf.msr_en_val to an unsupported value, i.e. @data has already been propagated to the vcpu and isn't unwound. Why not just pivot on @data when lapic_in_kernel() is false? vcpu->arch.apic is immutable so there's no need to update apf.msr_en_val in either direction. diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 539ea1cd6020..36969d5ec291 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2735,7 +2735,7 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) return 1; if (!lapic_in_kernel(vcpu)) - return 1; + return data ? 1 : 0; vcpu->arch.apf.msr_en_val = data; > + return 1; > + > if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa, > sizeof(u64))) > return 1; > -- > 2.25.4 >