arm64: KVM: Report SMCCC_ARCH_WORKAROUND_1 BP hardening support

From: Marc Zyngier <marc.zyngier@arm.com>

commit 6167ec5c upstream.

A new feature of SMCCC 1.1 is that it offers firmware-based CPU
workarounds. In particular, SMCCC_ARCH_WORKAROUND_1 provides
BP hardening for CVE-2017-5715.

If the host has some mitigation for this issue, report that
we deal with it using SMCCC_ARCH_WORKAROUND_1, as we apply the
host workaround on every guest exit.
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[v4.9: account for files moved to virt/ upstream]
Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]
Tested-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm/include/asm/kvm_host.h

@@ -318,4 +318,10 @@ static inline int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
 	return -ENXIO;
 }
 
+static inline bool kvm_arm_harden_branch_predictor(void)
+{
+	/* No way to detect it yet, pretend it is not there. */
+	return false;
+}
+
 #endif /* __ARM_KVM_HOST_H__ */

arch/arm/kvm/psci.c

@@ -403,13 +403,20 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 {
 	u32 func_id = smccc_get_function(vcpu);
 	u32 val = PSCI_RET_NOT_SUPPORTED;
+	u32 feature;
 
 	switch (func_id) {
 	case ARM_SMCCC_VERSION_FUNC_ID:
 		val = ARM_SMCCC_VERSION_1_1;
 		break;
 	case ARM_SMCCC_ARCH_FEATURES_FUNC_ID:
-		/* Nothing supported yet */
+		feature = smccc_get_arg1(vcpu);
+		switch(feature) {
+		case ARM_SMCCC_ARCH_WORKAROUND_1:
+			if (kvm_arm_harden_branch_predictor())
+				val = 0;
+			break;
+		}
 		break;
 	default:
 		return kvm_psci_call(vcpu);

arch/arm64/include/asm/kvm_host.h

@@ -393,4 +393,9 @@ static inline void __cpu_init_stage2(void)
 		  "PARange is %d bits, unsupported configuration!", parange);
 }
 
+static inline bool kvm_arm_harden_branch_predictor(void)
+{
+	return cpus_have_cap(ARM64_HARDEN_BRANCH_PREDICTOR);
+}
+
 #endif /* __ARM64_KVM_HOST_H__ */

include/linux/arm-smccc.h

@@ -73,6 +73,11 @@
 			   ARM_SMCCC_SMC_32,				\
 			   0, 1)
 
+#define ARM_SMCCC_ARCH_WORKAROUND_1					\
+	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
+			   ARM_SMCCC_SMC_32,				\
+			   0, 0x8000)
+
 #ifndef __ASSEMBLY__
 
 #include <linux/linkage.h>