KVM: arm64: PSCI: Forbid 64bit functions for 32bit guests

Implementing (and even advertising) 64bit PSCI functions to 32bit
guests is at least a bit odd, if not altogether violating the
spec which says ("5.2.1 Register usage in arguments and return values"):

"Adherence to the SMC Calling Conventions implies that any AArch32
caller of an SMC64 function will get a return code of 0xFFFFFFFF(int32).
This matches the NOT_SUPPORTED error code used in PSCI"

Tighten the implementation by pretending these functions are not
there for 32bit guests.

Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

virt/kvm/arm/psci.c

@@ -198,13 +198,32 @@ static void kvm_psci_narrow_to_32bit(struct kvm_vcpu *vcpu)
 		vcpu_set_reg(vcpu, i, lower_32_bits(vcpu_get_reg(vcpu, i)));
 }
 
+static unsigned long kvm_psci_check_allowed_function(struct kvm_vcpu *vcpu, u32 fn)
+{
+	switch(fn) {
+	case PSCI_0_2_FN64_CPU_SUSPEND:
+	case PSCI_0_2_FN64_CPU_ON:
+	case PSCI_0_2_FN64_AFFINITY_INFO:
+		/* Disallow these functions for 32bit guests */
+		if (vcpu_mode_is_32bit(vcpu))
+			return PSCI_RET_NOT_SUPPORTED;
+		break;
+	}
+
+	return 0;
+}
+
 static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
 	u32 psci_fn = smccc_get_function(vcpu);
 	unsigned long val;
 	int ret = 1;
 
+	val = kvm_psci_check_allowed_function(vcpu, psci_fn);
+	if (val)
+		goto out;
+
 	switch (psci_fn) {
 	case PSCI_0_2_FN_PSCI_VERSION:
 		/*
@@ -272,6 +291,7 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
 		break;
 	}
 
+out:
 	smccc_set_retval(vcpu, val, 0, 0, 0);
 	return ret;
 }
@@ -289,6 +309,10 @@ static int kvm_psci_1_0_call(struct kvm_vcpu *vcpu)
 		break;
 	case PSCI_1_0_FN_PSCI_FEATURES:
 		feature = smccc_get_arg1(vcpu);
+		val = kvm_psci_check_allowed_function(vcpu, feature);
+		if (val)
+			break;
+
 		switch(feature) {
 		case PSCI_0_2_FN_PSCI_VERSION:
 		case PSCI_0_2_FN_CPU_SUSPEND: