Date:   Thu, 07 Apr 2022 10:06:24 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Andrew Jones <drjones@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v5 02/10] KVM: arm64: Setup a framework for hypercall bitmap firmware registers

Hi Raghavendra,

On Thu, 07 Apr 2022 02:15:57 +0100,
Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> 
> KVM regularly introduces new hypercall services to the guests without
> any consent from the userspace. This means, the guests can observe
> hypercall services in and out as they migrate across various host
> kernel versions. This could be a major problem if the guest
> discovered a hypercall, started using it, and after getting migrated
> to an older kernel realizes that it's no longer available. Depending
> on how the guest handles the change, there's a potential chance that
> the guest would just panic.
> 
> As a result, there's a need for the userspace to elect the services
> that it wishes the guest to discover. It can elect these services
> based on the kernels spread across its (migration) fleet. To remedy
> this, extend the existing firmware psuedo-registers, such as

nit: pseudo

> KVM_REG_ARM_PSCI_VERSION, but by creating a new COPROC register space
> for all the hypercall services available.
> 
> These firmware registers are categorized based on the service call
> owners, but unlike the existing firmware psuedo-registers, they hold

nit: pseudo again

> the features supported in the form of a bitmap.
> 
> During the VM initialization, the registers are set to upper-limit of
> the features supported by the corresponding registers. It's expected
> that the VMMs discover the features provided by each register via
> GET_ONE_REG, and writeback the desired values using SET_ONE_REG.

nit: write back

> KVM allows this modification only until the VM has started.
> 
> Some of the standard features are not mapped to any bits of the
> registers. But since they can recreate the original problem of
> making it available without userspace's consent, they need to
> be explicitly added to the hvc_func_default_allowed_list[]. Any
> function-id that's not enabled via the bitmap, or not listed in
> hvc_func_default_allowed_list[], will be returned as
> SMCCC_RET_NOT_SUPPORTED to the guest.
> 
> Older userspace code can simply ignore the feature and the
> hypercall services will be exposed unconditionally to the guests,
> thus ensuring backward compatibility.
> 
> In this patch, the framework adds the register only for ARM's standard
> secure services (owner value 4). Currently, this includes support only
> for ARM True Random Number Generator (TRNG) service, with bit-0 of the
> register representing mandatory features of v1.0. Other services are
> momentarily added in the upcoming patches.
> 
> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  12 ++++
>  arch/arm64/include/uapi/asm/kvm.h |   9 +++
>  arch/arm64/kvm/arm.c              |   1 +
>  arch/arm64/kvm/guest.c            |   8 ++-
>  arch/arm64/kvm/hypercalls.c       | 102 ++++++++++++++++++++++++++++++
>  include/kvm/arm_hypercalls.h      |   7 ++
>  include/kvm/arm_psci.h            |  12 ++++
>  7 files changed, 149 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index e3b25dc6c367..6e663383d7b4 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -101,6 +101,15 @@ struct kvm_s2_mmu {
>  struct kvm_arch_memory_slot {
>  };
>  
> +/**
> + * struct kvm_smccc_features: Descriptor the hypercall services exposed to the guests
> + *
> + * @std_bmap: Bitmap of standard secure service calls
> + */
> +struct kvm_smccc_features {
> +	u64 std_bmap;

Consider using 'unsigned long' for bitmaps.

> +};
> +
>  struct kvm_arch {
>  	struct kvm_s2_mmu mmu;
>  
> @@ -140,6 +149,9 @@ struct kvm_arch {
>  
>  	u8 pfr0_csv2;
>  	u8 pfr0_csv3;
> +
> +	/* Hypercall features firmware registers' descriptor */
> +	struct kvm_smccc_features smccc_feat;
>  };
>  
>  struct kvm_vcpu_fault_info {
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index c1b6ddc02d2f..56e4bc58a355 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -332,6 +332,15 @@ struct kvm_arm_copy_mte_tags {
>  #define KVM_ARM64_SVE_VLS_WORDS	\
>  	((KVM_ARM64_SVE_VQ_MAX - KVM_ARM64_SVE_VQ_MIN) / 64 + 1)
>  
> +/* Bitmap feature firmware registers */
> +#define KVM_REG_ARM_FW_FEAT_BMAP		(0x0016 << KVM_REG_ARM_COPROC_SHIFT)
> +#define KVM_REG_ARM_FW_FEAT_BMAP_REG(r)		(KVM_REG_ARM64 | KVM_REG_SIZE_U64 | \
> +						KVM_REG_ARM_FW_FEAT_BMAP |	\
> +						((r) & 0xffff))
> +
> +#define KVM_REG_ARM_STD_BMAP			KVM_REG_ARM_FW_FEAT_BMAP_REG(0)
> +#define KVM_REG_ARM_STD_BIT_TRNG_V1_0		BIT(0)

I'm really in two minds about this. Having one bit per service is easy
from an implementation perspective, but is also means that this
disallow fine grained control over which hypercalls are actually
available. If tomorrow TRNG 1.1 adds a new hypercall and that KVM
implements both, how does the selection mechanism works? You will
need a version selector (a la PSCI), which defeats this API somehow
(and renders the name of the #define invalid).

I wonder if a more correct way to look at this is to enumerate the
hypercalls themselves (all 5 of them), though coming up with an
encoding is tricky (RNG32 and RNG64 would clash, for example).

Thoughts?

> +
>  /* Device Control API: ARM VGIC */
>  #define KVM_DEV_ARM_VGIC_GRP_ADDR	0
>  #define KVM_DEV_ARM_VGIC_GRP_DIST_REGS	1
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 523bc934fe2f..a37fadbd617e 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -156,6 +156,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  	kvm->arch.max_vcpus = kvm_arm_default_max_vcpus();
>  
>  	set_default_spectre(kvm);
> +	kvm_arm_init_hypercalls(kvm);
>  
>  	return ret;
>  out_free_stage2_pgd:
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 0d5cca56cbda..8c607199cad1 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -756,7 +756,9 @@ int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  
>  	switch (reg->id & KVM_REG_ARM_COPROC_MASK) {
>  	case KVM_REG_ARM_CORE:	return get_core_reg(vcpu, reg);
> -	case KVM_REG_ARM_FW:	return kvm_arm_get_fw_reg(vcpu, reg);
> +	case KVM_REG_ARM_FW:
> +	case KVM_REG_ARM_FW_FEAT_BMAP:
> +		return kvm_arm_get_fw_reg(vcpu, reg);
>  	case KVM_REG_ARM64_SVE:	return get_sve_reg(vcpu, reg);
>  	}
>  
> @@ -774,7 +776,9 @@ int kvm_arm_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  
>  	switch (reg->id & KVM_REG_ARM_COPROC_MASK) {
>  	case KVM_REG_ARM_CORE:	return set_core_reg(vcpu, reg);
> -	case KVM_REG_ARM_FW:	return kvm_arm_set_fw_reg(vcpu, reg);
> +	case KVM_REG_ARM_FW:
> +	case KVM_REG_ARM_FW_FEAT_BMAP:
> +		return kvm_arm_set_fw_reg(vcpu, reg);
>  	case KVM_REG_ARM64_SVE:	return set_sve_reg(vcpu, reg);
>  	}
>  
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index fa6d9378d8e7..cf04b5ee5f56 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -58,6 +58,53 @@ static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
>  	val[3] = lower_32_bits(cycles);
>  }
>  
> +/*
> + * List of function-ids that are not gated with the bitmapped feature
> + * firmware registers, and are to be allowed for servicing the call by default.
> + */
> +static const u32 hvc_func_default_allowed_list[] = {
> +	ARM_SMCCC_VERSION_FUNC_ID,
> +	ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
> +	ARM_SMCCC_HV_PV_TIME_FEATURES,
> +	ARM_SMCCC_HV_PV_TIME_ST,
> +	ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID,
> +	ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID,
> +	ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
> +};
> +
> +static bool kvm_hvc_call_default_allowed(struct kvm_vcpu *vcpu, u32 func_id)
> +{
> +	unsigned int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(hvc_func_default_allowed_list); i++)
> +		if (func_id == hvc_func_default_allowed_list[i])
> +			return true;

Huh, this really is ugly. This array is bound to become bigger over
time, meaning that the average hypercall time is going to increase. At
the very least, this should be turned into a switch/case statement, as
the compile is pretty good at building a search tree (better than this
naive loop, for a start), and we have those everywhere else.

> +
> +	return kvm_psci_func_id_is_valid(vcpu, func_id);
> +}
> +
> +static bool kvm_arm_fw_reg_feat_enabled(u64 reg_bmap, u64 feat_bit)
> +{
> +	return reg_bmap & feat_bit;
> +}

We really don't need to reimplement test_bit().

> +
> +static bool kvm_hvc_call_allowed(struct kvm_vcpu *vcpu, u32 func_id)
> +{
> +	struct kvm_smccc_features *smccc_feat = &vcpu->kvm->arch.smccc_feat;
> +
> +	switch (func_id) {
> +	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_arm_fw_reg_feat_enabled(smccc_feat->std_bmap,
> +						KVM_REG_ARM_STD_BIT_TRNG_V1_0);
> +	default:
> +		return kvm_hvc_call_default_allowed(vcpu, func_id);
> +	}
> +}
> +
>  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  {
>  	u32 func_id = smccc_get_function(vcpu);
> @@ -65,6 +112,9 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  	u32 feature;
>  	gpa_t gpa;
>  
> +	if (!kvm_hvc_call_allowed(vcpu, func_id))
> +		goto out;
> +
>  	switch (func_id) {
>  	case ARM_SMCCC_VERSION_FUNC_ID:
>  		val[0] = ARM_SMCCC_VERSION_1_1;
> @@ -155,6 +205,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  		return kvm_psci_call(vcpu);
>  	}
>  
> +out:
>  	smccc_set_retval(vcpu, val[0], val[1], val[2], val[3]);
>  	return 1;
>  }
> @@ -164,8 +215,16 @@ static const u64 kvm_arm_fw_reg_ids[] = {
>  	KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1,
>  	KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2,
>  	KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_3,
> +	KVM_REG_ARM_STD_BMAP,
>  };
>  
> +void kvm_arm_init_hypercalls(struct kvm *kvm)
> +{
> +	struct kvm_smccc_features *smccc_feat = &kvm->arch.smccc_feat;
> +
> +	smccc_feat->std_bmap = KVM_ARM_SMCCC_STD_FEATURES;
> +}
> +
>  int kvm_arm_get_fw_num_regs(struct kvm_vcpu *vcpu)
>  {
>  	return ARRAY_SIZE(kvm_arm_fw_reg_ids);
> @@ -237,6 +296,7 @@ static int get_kernel_wa_level(u64 regid)
>  
>  int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  {
> +	struct kvm_smccc_features *smccc_feat = &vcpu->kvm->arch.smccc_feat;
>  	void __user *uaddr = (void __user *)(long)reg->addr;
>  	u64 val;
>  
> @@ -249,6 +309,9 @@ int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  	case KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_3:
>  		val = get_kernel_wa_level(reg->id) & KVM_REG_FEATURE_LEVEL_MASK;
>  		break;
> +	case KVM_REG_ARM_STD_BMAP:
> +		val = READ_ONCE(smccc_feat->std_bmap);
> +		break;
>  	default:
>  		return -ENOENT;
>  	}
> @@ -259,6 +322,43 @@ int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  	return 0;
>  }
>  
> +static int kvm_arm_set_fw_reg_bmap(struct kvm_vcpu *vcpu, u64 reg_id, u64 val)
> +{
> +	int ret = 0;
> +	struct kvm *kvm = vcpu->kvm;
> +	struct kvm_smccc_features *smccc_feat = &kvm->arch.smccc_feat;
> +	u64 *fw_reg_bmap, fw_reg_features;
> +
> +	switch (reg_id) {
> +	case KVM_REG_ARM_STD_BMAP:
> +		fw_reg_bmap = &smccc_feat->std_bmap;
> +		fw_reg_features = KVM_ARM_SMCCC_STD_FEATURES;
> +		break;
> +	default:
> +		return -ENOENT;
> +	}
> +
> +	/* Check for unsupported bit */
> +	if (val & ~fw_reg_features)
> +		return -EINVAL;
> +
> +	mutex_lock(&kvm->lock);
> +
> +	/*
> +	 * If the VM (any vCPU) has already started running, return success
> +	 * if there's no change in the value. Else, return -EBUSY.

No, this should *always* fail if a vcpu has started. Otherwise, you
start allowing hard to spot races.

> +	 */
> +	if (test_bit(KVM_ARCH_FLAG_HAS_RAN_ONCE, &kvm->arch.flags)) {
> +		ret = *fw_reg_bmap != val ? -EBUSY : 0;
> +		goto out;
> +	}
> +
> +	WRITE_ONCE(*fw_reg_bmap, val);

I'm not sure what this WRITE_ONCE guards against. Do you expect
concurrent reads at this stage?

> +out:
> +	mutex_unlock(&kvm->lock);
> +	return ret;
> +}
> +
>  int kvm_arm_set_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  {
>  	void __user *uaddr = (void __user *)(long)reg->addr;
> @@ -337,6 +437,8 @@ int kvm_arm_set_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>  			return -EINVAL;
>  
>  		return 0;
> +	case KVM_REG_ARM_STD_BMAP:
> +		return kvm_arm_set_fw_reg_bmap(vcpu, reg->id, val);
>  	default:
>  		return -ENOENT;
>  	}
> diff --git a/include/kvm/arm_hypercalls.h b/include/kvm/arm_hypercalls.h
> index 5d38628a8d04..fd3ff350ee9d 100644
> --- a/include/kvm/arm_hypercalls.h
> +++ b/include/kvm/arm_hypercalls.h
> @@ -6,6 +6,12 @@
>  
>  #include <asm/kvm_emulate.h>
>  
> +/* Last valid bits of the bitmapped firmware registers */
> +#define KVM_REG_ARM_STD_BMAP_BIT_MAX		0
> +
> +#define KVM_ARM_SMCCC_STD_FEATURES \
> +	GENMASK_ULL(KVM_REG_ARM_STD_BMAP_BIT_MAX, 0)
> +
>  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu);
>  
>  static inline u32 smccc_get_function(struct kvm_vcpu *vcpu)
> @@ -42,6 +48,7 @@ static inline void smccc_set_retval(struct kvm_vcpu *vcpu,
>  
>  struct kvm_one_reg;
>  
> +void kvm_arm_init_hypercalls(struct kvm *kvm);
>  int kvm_arm_get_fw_num_regs(struct kvm_vcpu *vcpu);
>  int kvm_arm_copy_fw_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
>  int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg);
> diff --git a/include/kvm/arm_psci.h b/include/kvm/arm_psci.h
> index 6e55b9283789..d7a87367de56 100644
> --- a/include/kvm/arm_psci.h
> +++ b/include/kvm/arm_psci.h
> @@ -36,6 +36,18 @@ static inline int kvm_psci_version(struct kvm_vcpu *vcpu)
>  	return KVM_ARM_PSCI_0_1;
>  }
>  
> +static inline bool kvm_psci_func_id_is_valid(struct kvm_vcpu *vcpu, u32 func_id)
> +{
> +	/* PSCI 0.1 doesn't comply with the standard SMCCC */
> +	if (kvm_psci_version(vcpu) == KVM_ARM_PSCI_0_1)
> +		return (func_id == KVM_PSCI_FN_CPU_OFF || func_id == KVM_PSCI_FN_CPU_ON);
> +
> +	if (ARM_SMCCC_OWNER_NUM(func_id) == ARM_SMCCC_OWNER_STANDARD &&
> +		ARM_SMCCC_FUNC_NUM(func_id) >= 0 && ARM_SMCCC_FUNC_NUM(func_id) <= 0x1f)
> +		return true;
> +
> +	return false;
> +}

Why the inline function? Do you expect this to be shared with
something else? If not, I'd rather you move it into the caller.

>  
>  int kvm_psci_call(struct kvm_vcpu *vcpu);
>  

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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