linux-kernel - [PATCH 03/15] KVM: nVMX: Don't clobber nested MMU's A/D status on EPTP switch

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Message-Id: <20210609234235.1244004-4-seanjc@google.com>
Date:   Wed,  9 Jun 2021 16:42:23 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Junaid Shahid <junaids@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>
Subject: [PATCH 03/15] KVM: nVMX: Don't clobber nested MMU's A/D status on
 EPTP switch

Drop bogus logic that incorrectly clobbers the accessed/dirty enabling
status of the nested MMU on an EPTP switch.  When nested EPT is enabled,
walk_mmu points at L2's _legacy_ page tables, not L1's EPT for L2.

This is likely a benign bug, as mmu->ept_ad is never consumed (since the
MMU is not a nested EPT MMU), and stuffing mmu_role.base.ad_disabled will
never propagate into future shadow pages since the nested MMU isn't used
to map anything, just to walk L2's page tables.

Note, KVM also does a full MMU reload, i.e. the guest_mmu will be
recreated using the new EPTP, and thus any change in A/D enabling will be
properly recognized in the relevant MMU.

Fixes: 41ab93727467 ("KVM: nVMX: Emulate EPTP switching for the L1 hypervisor")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/nested.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index c3624109ffeb..e102a5c10a83 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5488,8 +5488,6 @@ static int nested_vmx_eptp_switching(struct kvm_vcpu *vcpu,
 {
 	u32 index = kvm_rcx_read(vcpu);
 	u64 new_eptp;
-	bool accessed_dirty;
-	struct kvm_mmu *mmu = vcpu->arch.walk_mmu;
 
 	if (!nested_cpu_has_eptp_switching(vmcs12) ||
 	    !nested_cpu_has_ept(vmcs12))
@@ -5498,13 +5496,10 @@ static int nested_vmx_eptp_switching(struct kvm_vcpu *vcpu,
 	if (index >= VMFUNC_EPTP_ENTRIES)
 		return 1;
 
-
 	if (kvm_vcpu_read_guest_page(vcpu, vmcs12->eptp_list_address >> PAGE_SHIFT,
 				     &new_eptp, index * 8, 8))
 		return 1;
 
-	accessed_dirty = !!(new_eptp & VMX_EPTP_AD_ENABLE_BIT);
-
 	/*
 	 * If the (L2) guest does a vmfunc to the currently
 	 * active ept pointer, we don't have to do anything else
@@ -5513,8 +5508,6 @@ static int nested_vmx_eptp_switching(struct kvm_vcpu *vcpu,
 		if (!nested_vmx_check_eptp(vcpu, new_eptp))
 			return 1;
 
-		mmu->ept_ad = accessed_dirty;
-		mmu->mmu_role.base.ad_disabled = !accessed_dirty;
 		vmcs12->ept_pointer = new_eptp;
 
 		kvm_make_request(KVM_REQ_MMU_RELOAD, vcpu);
-- 
2.32.0.rc1.229.g3e70b5a671-goog