KVM: nVMX: Verify VMCS shadowing VMCS link pointer

Intel SDM considers these checks to be part of
"Checks on Guest Non-Register State".

Note that it is legal for vmcs->vmcs_link_pointer to be -1ull
when VMCS shadowing is enabled. In this case, any VMREAD/VMWRITE to
shadowed-field sets the ALU flags for VMfailInvalid (i.e. CF=1).
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -11755,6 +11755,33 @@ static int check_vmentry_prereqs(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 	return 0;
 }
 
+static int nested_vmx_check_vmcs_link_ptr(struct kvm_vcpu *vcpu,
+					  struct vmcs12 *vmcs12)
+{
+	int r;
+	struct page *page;
+	struct vmcs12 *shadow;
+
+	if (vmcs12->vmcs_link_pointer == -1ull)
+		return 0;
+
+	if (!page_address_valid(vcpu, vmcs12->vmcs_link_pointer))
+		return -EINVAL;
+
+	page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->vmcs_link_pointer);
+	if (is_error_page(page))
+		return -EINVAL;
+
+	r = 0;
+	shadow = kmap(page);
+	if (shadow->hdr.revision_id != VMCS12_REVISION ||
+	    shadow->hdr.shadow_vmcs != nested_cpu_has_shadow_vmcs(vmcs12))
+		r = -EINVAL;
+	kunmap(page);
+	kvm_release_page_clean(page);
+	return r;
+}
+
 static int check_vmentry_postreqs(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 				  u32 *exit_qual)
 {
@@ -11766,8 +11793,7 @@ static int check_vmentry_postreqs(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 	    !nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4))
 		return 1;
 
-	if (!nested_cpu_has_shadow_vmcs(vmcs12) &&
-	    vmcs12->vmcs_link_pointer != -1ull) {
+	if (nested_vmx_check_vmcs_link_ptr(vcpu, vmcs12)) {
 		*exit_qual = ENTRY_FAIL_VMCS_LINK_PTR;
 		return 1;
 	}