KVM: nVMX: Update VMCS02 when L2 PAE PDPTE updates detected

When L2 uses PAE, L0 intercepts of L2 writes to CR0/CR3/CR4 call
load_pdptrs to read the possibly updated PDPTEs from the guest
physical address referenced by CR3.  It loads them into
vcpu->arch.walk_mmu->pdptrs and sets VCPU_EXREG_PDPTR in
vcpu->arch.regs_dirty.

At the subsequent assumed reentry into L2, the mmu will call
vmx_load_mmu_pgd which calls ept_load_pdptrs. ept_load_pdptrs sees
VCPU_EXREG_PDPTR set in vcpu->arch.regs_dirty and loads
VMCS02.GUEST_PDPTRn from vcpu->arch.walk_mmu->pdptrs[]. This all works
if the L2 CRn write intercept always resumes L2.

The resume path calls vmx_check_nested_events which checks for
exceptions, MTF, and expired VMX preemption timers. If
vmx_check_nested_events finds any of these conditions pending it will
reflect the corresponding exit into L1. Live migration at this point
would also cause a missed immediate reentry into L2.

After L1 exits, vmx_vcpu_run calls vmx_register_cache_reset which
clears VCPU_EXREG_PDPTR in vcpu->arch.regs_dirty.  When L2 next
resumes, ept_load_pdptrs finds VCPU_EXREG_PDPTR clear in
vcpu->arch.regs_dirty and does not load VMCS02.GUEST_PDPTRn from
vcpu->arch.walk_mmu->pdptrs[]. prepare_vmcs02 will then load
VMCS02.GUEST_PDPTRn from vmcs12->pdptr0/1/2/3 which contain the stale
values stored at last L2 exit. A repro of this bug showed L2 entering
triple fault immediately due to the bad VMCS02.GUEST_PDPTRn values.

When L2 is in PAE paging mode add a call to ept_load_pdptrs before
leaving L2. This will update VMCS02.GUEST_PDPTRn if they are dirty in
vcpu->arch.walk_mmu->pdptrs[].

Tested:
kvm-unit-tests with new directed test: vmx_mtf_pdpte_test.
Verified that test fails without the fix.

Also ran Google internal VMM with an Ubuntu 16.04 4.4.0-83 guest running a
custom hypervisor with a 32-bit Windows XP L2 guest using PAE. Prior to fix
would repro readily. Ran 14 simultaneous L2s for 140 iterations with no
failures.
Signed-off-by: Peter Shier <pshier@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20200820230545.2411347-1-pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4404,6 +4404,14 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
 		kvm_vcpu_flush_tlb_current(vcpu);
 
+	/*
+	 * VCPU_EXREG_PDPTR will be clobbered in arch/x86/kvm/vmx/vmx.h between
+	 * now and the new vmentry.  Ensure that the VMCS02 PDPTR fields are
+	 * up-to-date before switching to L1.
+	 */
+	if (enable_ept && is_pae_paging(vcpu))
+		vmx_ept_load_pdptrs(vcpu);
+
 	leave_guest_mode(vcpu);
 
 	if (nested_cpu_has_preemption_timer(vmcs12))

arch/x86/kvm/vmx/vmx.c

@@ -2971,7 +2971,7 @@ static void vmx_flush_tlb_guest(struct kvm_vcpu *vcpu)
 	vpid_sync_context(to_vmx(vcpu)->vpid);
 }
 
-static void ept_load_pdptrs(struct kvm_vcpu *vcpu)
+void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu)
 {
 	struct kvm_mmu *mmu = vcpu->arch.walk_mmu;
 
@@ -3114,7 +3114,7 @@ static void vmx_load_mmu_pgd(struct kvm_vcpu *vcpu, unsigned long pgd,
 			guest_cr3 = vcpu->arch.cr3;
 		else /* vmcs01.GUEST_CR3 is already up-to-date. */
 			update_guest_cr3 = false;
-		ept_load_pdptrs(vcpu);
+		vmx_ept_load_pdptrs(vcpu);
 	} else {
 		guest_cr3 = pgd;
 	}

arch/x86/kvm/vmx/vmx.h

@@ -356,6 +356,7 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp);
 int vmx_find_msr_index(struct vmx_msrs *m, u32 msr);
 int vmx_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
 			      struct x86_exception *e);
+void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu);
 
 #define POSTED_INTR_ON  0
 #define POSTED_INTR_SN  1