Subject: [PATCH 2/2] KVM: arm64: PSCI: Forbid 64bit functions for 32bit guests

From: Marc Zyngier <[email-masked]>
Date: Wed, 1 Apr 2020 17:58:16 +0100

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.

Signed-off-by: Marc Zyngier <[email-masked]>
---
 virt/kvm/arm/psci.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/virt/kvm/arm/psci.c b/virt/kvm/arm/psci.c
index 69ff4a51ceb5..122795cdd984 100644
--- a/virt/kvm/arm/psci.c
+++ b/virt/kvm/arm/psci.c
@@ -199,6 +199,21 @@ static void kvm_psci_narrow_to_32bit(struct kvm_vcpu *vcpu)
 		vcpu_set_reg(vcpu, i, (u32)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;
@@ -206,6 +221,10 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *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:
 		/*
@@ -273,6 +292,7 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
 		break;
 	}
 
+out:
 	smccc_set_retval(vcpu, val, 0, 0, 0);
 	return ret;
 }
@@ -290,6 +310,10 @@ 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:
-- 
2.25.0

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