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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 448AEC4332F for ; Wed, 19 Oct 2022 16:56:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231139AbiJSQ4g (ORCPT ); Wed, 19 Oct 2022 12:56:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37102 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230226AbiJSQ43 (ORCPT ); Wed, 19 Oct 2022 12:56:29 -0400 Received: from mail-pg1-x549.google.com (mail-pg1-x549.google.com [IPv6:2607:f8b0:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 915AC1C711F for ; Wed, 19 Oct 2022 09:56:27 -0700 (PDT) Received: by mail-pg1-x549.google.com with SMTP id f186-20020a636ac3000000b0044adaa7d347so9955121pgc.14 for ; Wed, 19 Oct 2022 09:56:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=gy24ztDzsu2pVLCZMStQCiAA6/olN6OB6IFeEvw2nwc=; b=RkT5JmVHOx1xLJuKyPUTQ/GHWwPLL/Ofaa9fMFcWafnho5RGXPgWMJuq6JtFQIVRU5 3MTRHaV+92xzf++NNAnl+DR9b5UgIZvfci41IYKi7CDUqVB2+NLKldXiZn18g62Ly3eN GP5xfcS2y6ft5gvqGdsKQufjnDmEap6e+Ggqnq1KWQ1ZMLIDN/eNajNku93QRdQyQSGg cXnlJOFm94o6frDwyz1i/mRrPIKOOgCJ3heiRrvlB8tXVHnRqxX8wwG5+Mf53qjVtc8D i/CqLQ/EdC68sMeiadZQajE7mvYEdOHQmt2H8C20u9lUD8opffszIpqLdrdMCs9Gvj+9 isNw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=gy24ztDzsu2pVLCZMStQCiAA6/olN6OB6IFeEvw2nwc=; b=eAfb4B3GN0Hke9gaRQ8rqb1/XR02p2zfyfkdTps1Oz91Pm7D+OpKQDHzd3DKs4g61t 9/QNTKQ064KajGfvRPAWznT/C2eREuE+UHFfMRvQE/A6zSmSmR9UikOVpfY5N2T7DLHl cIdzJ5KVZhMNRBW8B4X4SGFw8/ETq2kTmG8Jm640cGrNYihUODl2hbdLezTV6AbgmuUu Qecb3oN8lVjG9UAEwsS92mKbTEj6Ed+QE02vnbWqi4DsMSm88QfYVp5vio/eObIOGFfe CwUsJHVL5NqG2PL5DoqZCl0GpgbMD++P0aBvvC9V4f5P0ZvriwBP/JA2X5EFePGzASpK lH8g== X-Gm-Message-State: ACrzQf2DoKkIYc32nOdGMYNaQgHDSvW7EsBKbU5Xadt/o5W+CWuBlZOu slPMo1pl61B5GJu+Iz1J04V/AeMVU+0= X-Google-Smtp-Source: AMsMyM7z+/HoT4H7P+QTJw0UCU7RU2z3T7Pf6h3t9Z69xlXBk9B/YfAGWls9cWCXFmfCrFwseqxLeAzoXxI= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:902:f78a:b0:184:f2e2:a5fa with SMTP id q10-20020a170902f78a00b00184f2e2a5famr9560092pln.161.1666198587158; Wed, 19 Oct 2022 09:56:27 -0700 (PDT) Reply-To: Sean Christopherson Date: Wed, 19 Oct 2022 16:56:13 +0000 In-Reply-To: <20221019165618.927057-1-seanjc@google.com> Mime-Version: 1.0 References: <20221019165618.927057-1-seanjc@google.com> X-Mailer: git-send-email 2.38.0.413.g74048e4d9e-goog Message-ID: <20221019165618.927057-4-seanjc@google.com> Subject: [PATCH v6 3/8] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs From: Sean Christopherson To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Mingwei Zhang , David Matlack , Yan Zhao , Ben Gardon Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Account and track NX huge pages for nonpaging MMUs so that a future enhancement to precisely check if a shadow page can't be replaced by a NX huge page doesn't get false positives. Without correct tracking, KVM can get stuck in a loop if an instruction is fetching and writing data on the same huge page, e.g. KVM installs a small executable page on the fetch fault, replaces it with an NX huge page on the write fault, and faults again on the fetch. Alternatively, and perhaps ideally, KVM would simply not enforce the workaround for nonpaging MMUs. The guest has no page tables to abuse and KVM is guaranteed to switch to a different MMU on CR0.PG being toggled so there's no security or performance concerns. However, getting make_spte() to play nice now and in the future is unnecessarily complex. In the current code base, make_spte() can enforce the mitigation if TDP is enabled or the MMU is indirect, but make_spte() may not always have a vCPU/MMU to work with, e.g. if KVM were to support in-line huge page promotion when disabling dirty logging. Without a vCPU/MMU, KVM could either pass in the correct information and/or derive it from the shadow page, but the former is ugly and the latter subtly non-trivial due to the possibility of direct shadow pages in indirect MMUs. Given that using shadow paging with an unpaged guest is far from top priority _and_ has been subjected to the workaround since its inception, keep it simple and just fix the accounting glitch. Signed-off-by: Sean Christopherson Reviewed-by: David Matlack Reviewed-by: Mingwei Zhang --- arch/x86/kvm/mmu/mmu.c | 2 +- arch/x86/kvm/mmu/spte.c | 12 ++++++++++++ 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 5dd98cdc5283..99086a684dd2 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -3143,7 +3143,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) continue; link_shadow_page(vcpu, it.sptep, sp); - if (fault->is_tdp && fault->huge_page_disallowed) + if (fault->huge_page_disallowed) account_nx_huge_page(vcpu->kvm, sp, fault->req_level >= it.level); } diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c index 2e08b2a45361..c0fd7e049b4e 100644 --- a/arch/x86/kvm/mmu/spte.c +++ b/arch/x86/kvm/mmu/spte.c @@ -161,6 +161,18 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, if (!prefetch) spte |= spte_shadow_accessed_mask(spte); + /* + * For simplicity, enforce the NX huge page mitigation even if not + * strictly necessary. KVM could ignore the mitigation if paging is + * disabled in the guest, as the guest doesn't have an page tables to + * abuse. But to safely ignore the mitigation, KVM would have to + * ensure a new MMU is loaded (or all shadow pages zapped) when CR0.PG + * is toggled on, and that's a net negative for performance when TDP is + * enabled. When TDP is disabled, KVM will always switch to a new MMU + * when CR0.PG is toggled, but leveraging that to ignore the mitigation + * would tie make_spte() further to vCPU/MMU state, and add complexity + * just to optimize a mode that is anything but performance critical. + */ if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) && is_nx_huge_page_enabled(vcpu->kvm)) { pte_access &= ~ACC_EXEC_MASK; -- 2.38.0.413.g74048e4d9e-goog