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 21EBBC25B08 for ; Fri, 5 Aug 2022 23:05:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241647AbiHEXFo (ORCPT ); Fri, 5 Aug 2022 19:05:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59736 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241540AbiHEXF3 (ORCPT ); Fri, 5 Aug 2022 19:05:29 -0400 Received: from mail-pl1-x64a.google.com (mail-pl1-x64a.google.com [IPv6:2607:f8b0:4864:20::64a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 99E5A1057C for ; Fri, 5 Aug 2022 16:05:27 -0700 (PDT) Received: by mail-pl1-x64a.google.com with SMTP id o12-20020a170902d4cc00b0016e81c62cc8so2329537plg.13 for ; Fri, 05 Aug 2022 16:05:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:reply-to:from:to:cc; bh=WLuSlvIJZlcnazsELn/bSEkU+G0vGBR7V6zPFbRjYl0=; b=lf0DhdSASK1h/vy4nEBEmemTayzmtWDsnzYMSZwpBW9myRUV58cgyU8eR0d0pC7jTg 2mepqFjjY6zRmUMexsuN1ybaJtRpI+dCTQ1O1smEHwP03qsrKRWnlfG+6rLsgIS37O3F 3m9r2tO1Gm4Rrzt4pKZaG0E53Z5YYde4MiY6FylL498tj1bzL+FYjGGdtEA6UngKbnCU xvW/iwSC7BT7Hxj1A/WVBFfAgEijmvt5sfhQjHrNuGu+2mIubdVpPQyoijWZjF2im8Tj sLAqQAQNOaVcPGkw5MQ+AYfkErKbepIjtd3H77szTo6fZ8ZDMIKAjAH3vr4Us5q/RGwe xjRw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc; bh=WLuSlvIJZlcnazsELn/bSEkU+G0vGBR7V6zPFbRjYl0=; b=023VpTT21TcKtuGZnGe0NRD/v03EoVSvmGx8INeC1uD7uPRzaYGeV5jsFsceNEUm2v 5xLrFRiH8Dmri+tBnpt0BN4l59/3jPwc2Gohx50dwT0+TABO9t967cW2IwgnOrV2rT3g lPB0EiLqincxnBxO6WSiU6sRvuRUqwGEcMKvYOrL8mmAMd4oBAGbMiw8V56Ylre7AiN7 AmbMexjdel7G/fzrRlZlpG7zERrTyMs+ZjI/x9DrBnkyzn84wewhX6k7jejma1HrzdGc pN5SJPGe0bGQjP3UWCh+ami0JQ5NBqtDQhSKejaWXFo/IDNZraBnO6YA8O3l/YeJeWRH UgkA== X-Gm-Message-State: ACgBeo2/Hs2K0k+SGEhpjwX0pGtXDUaWO6gePeQHHlCQFuXg7sEdVxTA XtQyUFbs5W9bm30rTX1gOF9sXeMKjBc= X-Google-Smtp-Source: AA6agR432WEnrCUzqIvmpkhvNwROdM1p7cqZaz4OmRA8YYmCaKpLSXfFcLCkyxy4lyZWKWcPig9qP215a3s= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a17:90b:4cc5:b0:1f5:395:6c71 with SMTP id nd5-20020a17090b4cc500b001f503956c71mr18623544pjb.132.1659740727212; Fri, 05 Aug 2022 16:05:27 -0700 (PDT) Reply-To: Sean Christopherson Date: Fri, 5 Aug 2022 23:05:09 +0000 In-Reply-To: <20220805230513.148869-1-seanjc@google.com> Message-Id: <20220805230513.148869-5-seanjc@google.com> Mime-Version: 1.0 References: <20220805230513.148869-1-seanjc@google.com> X-Mailer: git-send-email 2.37.1.559.g78731f0fdb-goog Subject: [PATCH v3 4/8] 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, David Matlack , Yan Zhao , Mingwei Zhang , 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 --- 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 53d0dafa68ff..345b6b22ab68 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -3123,7 +3123,7 @@ 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 7314d27d57a4..52186b795bce 100644 --- a/arch/x86/kvm/mmu/spte.c +++ b/arch/x86/kvm/mmu/spte.c @@ -147,6 +147,18 @@ 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; -- 2.37.1.559.g78731f0fdb-goog