[PATCH v2 1/8] KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

Write the desired L2 CR3 into vmcs02.GUEST_CR3 during nested VM-Enter
instead of deferring the VMWRITE until vmx_set_cr3().  If the VMWRITE
is deferred, then KVM can consume a stale vmcs02.GUEST_CR3 when it
refreshes vmcs12->guest_cr3 during nested_vmx_vmexit() if the emulated
VM-Exit occurs without actually entering L2, e.g. if the nested run
is squashed because nested VM-Enter (from L1) is putting L2 into HLT.

Note, the above scenario can occur regardless of whether L1 is
intercepting HLT, e.g. L1 can intercept HLT and then re-enter L2 with
vmcs.GUEST_ACTIVITY_STATE=HALTED.  But practically speaking, a VMM will
likely put a guest into HALTED if and only if it's not intercepting HLT.

In an ideal world where EPT *requires* unrestricted guest (and vice
versa), VMX could handle CR3 similar to how it handles RSP and RIP,
e.g. mark CR3 dirty and conditionally load it at vmx_vcpu_run().  But
the unrestricted guest silliness complicates the dirty tracking logic
to the point that explicitly handling vmcs02.GUEST_CR3 during nested
VM-Enter is a simpler overall implementation.

Cc: stable@xxxxxxxxxxxxxxx
Reported-and-tested-by: Reto Buerki <reet@xxxxxxxxxxx>
Tested-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 10 ++++++++++
 arch/x86/kvm/vmx/vmx.c    | 10 +++++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 41abc62c9a8a..b72a00b53e4a 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2418,6 +2418,16 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 				entry_failure_code))
 		return -EINVAL;
 
+	/*
+	 * Immediately write vmcs02.GUEST_CR3.  It will be propagated to vmcs12
+	 * on nested VM-Exit, which can occur without actually running L2 and
+	 * thus without hitting vmx_set_cr3(), e.g. if L1 is entering L2 with
+	 * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept the
+	 * transition to HLT instead of running L2.
+	 */
+	if (enable_ept)
+		vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
+
 	/* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
 	if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
 	    is_pae_paging(vcpu)) {
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d4575ffb3cec..7679c2a05a50 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2984,6 +2984,7 @@ u64 construct_eptp(struct kvm_vcpu *vcpu, unsigned long root_hpa)
 void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 {
 	struct kvm *kvm = vcpu->kvm;
+	bool update_guest_cr3 = true;
 	unsigned long guest_cr3;
 	u64 eptp;
 
@@ -3000,15 +3001,18 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 			spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
 		}
 
-		if (enable_unrestricted_guest || is_paging(vcpu) ||
-		    is_guest_mode(vcpu))
+		/* Loading vmcs02.GUEST_CR3 is handled by nested VM-Enter. */
+		if (is_guest_mode(vcpu))
+			update_guest_cr3 = false;
+		else if (enable_unrestricted_guest || is_paging(vcpu))
 			guest_cr3 = kvm_read_cr3(vcpu);
 		else
 			guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
 		ept_load_pdptrs(vcpu);
 	}
 
-	vmcs_writel(GUEST_CR3, guest_cr3);
+	if (update_guest_cr3)
+		vmcs_writel(GUEST_CR3, guest_cr3);
 }
 
 int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
-- 
2.22.0