Re: [PATCH v2 5/5] KVM: arm64: implement the TRNG hypervisor call

[Marc Zyngier <maz@misterjones.org>]

On 2020-11-05 12:56, Andre Przywara wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> 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 arch_get_unsigned_seed_long(), which is 
> implemented
> in terms of RNDRRS currently, and will be alternatively backed by a SMC
> call to the secure firmware using same interface after a future patch.
> If neither are available, the kernel's entropy pool is used instead.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  2 +
>  arch/arm64/kvm/Makefile           |  2 +-
>  arch/arm64/kvm/hypercalls.c       |  6 ++
>  arch/arm64/kvm/trng.c             | 91 +++++++++++++++++++++++++++++++
>  4 files changed, 100 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/kvm/trng.c
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h
> b/arch/arm64/include/asm/kvm_host.h
> index 781d029b8aa8..615932bacf76 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -652,4 +652,6 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu 
> *vcpu);
>  #define kvm_arm_vcpu_sve_finalized(vcpu) \
>  	((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
> 
> +int kvm_trng_call(struct kvm_vcpu *vcpu);
> +
>  #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index 1504c81fbf5d..a510037e3270 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -16,7 +16,7 @@ kvm-y := $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o
> $(KVM)/eventfd.o \
>  	 inject_fault.o regmap.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 \
> -	 aarch32.o arch_timer.o \
> +	 aarch32.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 \
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 25ea4ecb6449..ead21b98b620 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/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);
>  	}
> diff --git a/arch/arm64/kvm/trng.c b/arch/arm64/kvm/trng.c
> new file mode 100644
> index 000000000000..5a27b2d99977
> --- /dev/null
> +++ b/arch/arm64/kvm/trng.c
> @@ -0,0 +1,91 @@
> +// 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
> +
> +#define TRNG_SUCCESS			0UL

SMCCC_RET_SUCCESS

> +#define TRNG_NOT_SUPPORTED		((unsigned long)-1)

SMCCC_RET_NOT_SUPPORTED

> +#define TRNG_INVALID_PARAMETER		((unsigned long)-2)

*crap*. Why isn't that the same value as SMCCC_RET_INVALID_PARAMETER?
Is it too late to fix the spec?

> +#define TRNG_NO_ENTROPY			((unsigned long)-3)
> +
> +#define MAX_BITS32			96
> +#define MAX_BITS64			192

Nothing seems to be using these definitions.

> +
> +static const uuid_t arm_smc_trng_uuid __aligned(4) = UUID_INIT(
> +	0x023534a2, 0xe0bc, 0x45ec, 0x95, 0xdd, 0x33, 0x34, 0xc1, 0xcc, 0x31, 
> 0x89);

I object to the lack of Easter egg in this UUID. Or at least one I can
understand. ;-)

> +
> +static int kvm_trng_do_rnd(struct kvm_vcpu *vcpu, int size)
> +{
> +	u32 num_bits = smccc_get_arg1(vcpu);
> +	u64 bits[3];
> +	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, 64); i++)
> +		/* use the arm64 specific backend directly if one exists */
> +		if (!arch_get_random_seed_long((unsigned long *)&bits[i]))
> +			bits[i] = get_random_long();
> +
> +	if (num_bits % 64)
> +		bits[i - 1] &= U64_MAX >> (64 - (num_bits % 64));
> +
> +	while (i < ARRAY_SIZE(bits))
> +		bits[i++] = 0;

I just wasted 3 minutes trying to understand what this was doing, only 
to
realise this is clearing the [MAX_BITS:num_bits] range.

How about using a bitmap instead? It would result in the exact same data
structure, only much more readable (and no u64->unsigned long casts).

> +
> +	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;
> +}

Thanks,

         M.
-- 
Who you jivin' with that Cosmik Debris?