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

From: Sean Christopherson
Date: Wed Jun 09 2021 - 19:43:19 EST

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@xxxxxxxxxx>
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) ||
@@ -5498,13 +5496,10 @@ static int nested_vmx_eptp_switching(struct kvm_vcpu *vcpu,
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);