KVM: arm64: Implement the TRNG hypervisor call

Provide a hypervisor implementation of the ARM architected TRNG firmware
interface described in ARM spec DEN0098. All function IDs are implemented,
including both 32-bit and 64-bit versions of the TRNG_RND service, which
is the centerpiece of the API.

The API is backed by the kernel's entropy pool only, to avoid guests
draining more precious direct entropy sources.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
[Andre: minor fixes, drop arch_get_random() usage]
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210106103453.152275-6-andre.przywara@arm.com

arch/arm64/include/asm/kvm_host.h

@@ -771,4 +771,6 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
 #define kvm_vcpu_has_pmu(vcpu)					\
 	(test_bit(KVM_ARM_VCPU_PMU_V3, (vcpu)->arch.features))
 
+int kvm_trng_call(struct kvm_vcpu *vcpu);
+
 #endif /* __ARM64_KVM_HOST_H__ */

arch/arm64/kvm/Makefile

@@ -16,7 +16,7 @@ kvm-y := $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o $(KVM)/eventfd.o \
 	 inject_fault.o va_layout.o handle_exit.o \
 	 guest.o debug.o reset.o sys_regs.o \
 	 vgic-sys-reg-v3.o fpsimd.o pmu.o \
-	 arch_timer.o \
+	 arch_timer.o trng.o\
 	 vgic/vgic.o vgic/vgic-init.o \
 	 vgic/vgic-irqfd.o vgic/vgic-v2.o \
 	 vgic/vgic-v3.o vgic/vgic-v4.o \

arch/arm64/kvm/hypercalls.c

@@ -71,6 +71,12 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 		if (gpa != GPA_INVALID)
 			val = gpa;
 		break;
+	case ARM_SMCCC_TRNG_VERSION:
+	case ARM_SMCCC_TRNG_FEATURES:
+	case ARM_SMCCC_TRNG_GET_UUID:
+	case ARM_SMCCC_TRNG_RND32:
+	case ARM_SMCCC_TRNG_RND64:
+		return kvm_trng_call(vcpu);
 	default:
 		return kvm_psci_call(vcpu);
 	}

arch/arm64/kvm/trng.c

+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2020 Arm Ltd.
+
+#include <linux/arm-smccc.h>
+#include <linux/kvm_host.h>
+
+#include <asm/kvm_emulate.h>
+
+#include <kvm/arm_hypercalls.h>
+
+#define ARM_SMCCC_TRNG_VERSION_1_0	0x10000UL
+
+/* Those values are deliberately separate from the generic SMCCC definitions. */
+#define TRNG_SUCCESS			0UL
+#define TRNG_NOT_SUPPORTED		((unsigned long)-1)
+#define TRNG_INVALID_PARAMETER		((unsigned long)-2)
+#define TRNG_NO_ENTROPY			((unsigned long)-3)
+
+#define TRNG_MAX_BITS64			192
+
+static const uuid_t arm_smc_trng_uuid __aligned(4) = UUID_INIT(
+	0x0d21e000, 0x4384, 0x11eb, 0x80, 0x70, 0x52, 0x44, 0x55, 0x4e, 0x5a, 0x4c);
+
+static int kvm_trng_do_rnd(struct kvm_vcpu *vcpu, int size)
+{
+	DECLARE_BITMAP(bits, TRNG_MAX_BITS64);
+	u32 num_bits = smccc_get_arg1(vcpu);
+	int i;
+
+	if (num_bits > 3 * size) {
+		smccc_set_retval(vcpu, TRNG_INVALID_PARAMETER, 0, 0, 0);
+		return 1;
+	}
+
+	/* get as many bits as we need to fulfil the request */
+	for (i = 0; i < DIV_ROUND_UP(num_bits, BITS_PER_LONG); i++)
+		bits[i] = get_random_long();
+
+	bitmap_clear(bits, num_bits, TRNG_MAX_BITS64 - num_bits);
+
+	if (size == 32)
+		smccc_set_retval(vcpu, TRNG_SUCCESS, lower_32_bits(bits[1]),
+				 upper_32_bits(bits[0]), lower_32_bits(bits[0]));
+	else
+		smccc_set_retval(vcpu, TRNG_SUCCESS, bits[2], bits[1], bits[0]);
+
+	memzero_explicit(bits, sizeof(bits));
+	return 1;
+}
+
+int kvm_trng_call(struct kvm_vcpu *vcpu)
+{
+	const __le32 *u = (__le32 *)arm_smc_trng_uuid.b;
+	u32 func_id = smccc_get_function(vcpu);
+	unsigned long val = TRNG_NOT_SUPPORTED;
+	int size = 64;
+
+	switch (func_id) {
+	case ARM_SMCCC_TRNG_VERSION:
+		val = ARM_SMCCC_TRNG_VERSION_1_0;
+		break;
+	case ARM_SMCCC_TRNG_FEATURES:
+		switch (smccc_get_arg1(vcpu)) {
+		case ARM_SMCCC_TRNG_VERSION:
+		case ARM_SMCCC_TRNG_FEATURES:
+		case ARM_SMCCC_TRNG_GET_UUID:
+		case ARM_SMCCC_TRNG_RND32:
+		case ARM_SMCCC_TRNG_RND64:
+			val = TRNG_SUCCESS;
+		}
+		break;
+	case ARM_SMCCC_TRNG_GET_UUID:
+		smccc_set_retval(vcpu, le32_to_cpu(u[0]), le32_to_cpu(u[1]),
+				 le32_to_cpu(u[2]), le32_to_cpu(u[3]));
+		return 1;
+	case ARM_SMCCC_TRNG_RND32:
+		size = 32;
+		fallthrough;
+	case ARM_SMCCC_TRNG_RND64:
+		return kvm_trng_do_rnd(vcpu, size);
+	}
+
+	smccc_set_retval(vcpu, val, 0, 0, 0);
+	return 1;
+}