KVM: arm64: Restrict supported capabilities for protected VMs

For practical reasons as well as security related ones, not all
capabilities are supported for protected VMs in pKVM.

Add a function that restricts the capabilities for protected VMs.
This behaves as an allow-list to ensure that future capabilities
are checked for compatibility and security before being allowed
for protected VMs.
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-30-tabba@google.comSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/arm.c

@@ -69,6 +69,31 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
 	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
 }
 
+/*
+ * This functions as an allow-list of protected VM capabilities.
+ * Features not explicitly allowed by this function are denied.
+ */
+static bool pkvm_ext_allowed(struct kvm *kvm, long ext)
+{
+	switch (ext) {
+	case KVM_CAP_IRQCHIP:
+	case KVM_CAP_ARM_PSCI:
+	case KVM_CAP_ARM_PSCI_0_2:
+	case KVM_CAP_NR_VCPUS:
+	case KVM_CAP_MAX_VCPUS:
+	case KVM_CAP_MAX_VCPU_ID:
+	case KVM_CAP_MSI_DEVID:
+	case KVM_CAP_ARM_VM_IPA_SIZE:
+	case KVM_CAP_ARM_PMU_V3:
+	case KVM_CAP_ARM_SVE:
+	case KVM_CAP_ARM_PTRAUTH_ADDRESS:
+	case KVM_CAP_ARM_PTRAUTH_GENERIC:
+		return true;
+	default:
+		return false;
+	}
+}
+
 int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			    struct kvm_enable_cap *cap)
 {
@@ -77,6 +102,9 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 	if (cap->flags)
 		return -EINVAL;
 
+	if (kvm_vm_is_protected(kvm) && !pkvm_ext_allowed(kvm, cap->cap))
+		return -EINVAL;
+
 	switch (cap->cap) {
 	case KVM_CAP_ARM_NISV_TO_USER:
 		r = 0;
@@ -215,6 +243,10 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 {
 	int r;
+
+	if (kvm && kvm_vm_is_protected(kvm) && !pkvm_ext_allowed(kvm, ext))
+		return 0;
+
 	switch (ext) {
 	case KVM_CAP_IRQCHIP:
 		r = vgic_present;