Re: [PATCH v7 2/9] KVM: arm64: Setup a framework for hypercall bitmap firmware registers

Marc Zyngier
2022-05-03T13:28:19Z

On Tue, 03 May 2022 00:38:46 +0100,
Raghavendra Rao Ananta <rananta@google.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 pseudo-registers, such as
> 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 pseudo-registers, they hold
> 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 write back the desired values using SET_ONE_REG.
> 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 case-list in
> kvm_hvc_call_default_allowed(). Any function-id that's not enabled
> via the bitmap, or not listed in kvm_hvc_call_default_allowed, 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@google.com>
> Reviewed-by: Gavin Shan <gshan@redhat.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 | 106 ++++++++++++++++++++++++++++++
> arch/arm64/kvm/psci.c | 13 ++++
> include/kvm/arm_hypercalls.h | 6 ++
> include/kvm/arm_psci.h | 2 +-
> 8 files changed, 154 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 94a27a7520f4..fef597af0beb 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 of the hypercall services exposed to the guests
> + *
> + * @std_bmap: Bitmap of standard secure service calls
> + */
> +struct kvm_smccc_features {
> + unsigned long std_bmap;
> +};
> +
> struct kvm_arch {
> struct kvm_s2_mmu mmu;
>
> @@ -150,6 +159,9 @@ 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..0b79d2dc6ffd 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 0
> +
> /* 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..5c53fd1f5690 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -58,6 +58,48 @@ static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
> val[3] = lower_32_bits(cycles);
> }
>
> +static bool kvm_arm_fw_reg_feat_enabled(unsigned long *reg_bmap, unsigned long feat_bit)
> +{
> + return test_bit(feat_bit, reg_bmap);
> +}

Please use test_bit(std_bmap, bit) directly. It is just as readable as
kvm_arm_fw_reg_feat_enabled.

> +
> +static bool kvm_hvc_call_default_allowed(u32 func_id)
> +{
> + switch (func_id) {
> + /*
> + * 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.
> + */
> + case ARM_SMCCC_VERSION_FUNC_ID:
> + case ARM_SMCCC_ARCH_FEATURES_FUNC_ID:
> + case ARM_SMCCC_HV_PV_TIME_FEATURES:
> + case ARM_SMCCC_HV_PV_TIME_ST:
> + case ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID:
> + case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
> + case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
> + return true;
> + default:
> + return kvm_psci_func_id_is_valid(func_id);
> + }
> +}
> +
> +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(func_id);
> + }
> +}
> +
> int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
> {
> u32 func_id = smccc_get_function(vcpu);
> @@ -65,6 +107,9 @@ 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 +200,7 @@ 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 +210,16 @@ 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 +291,7 @@ 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 +304,9 @@ 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 +317,52 @@ 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;
> + unsigned long *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;
> +
> + /*
> + * Return -EBUSY if the VM (any vCPU) has already started running.
> + *
> + * To avoid unnecessary serialization of kvm->lock, check for
> + * KVM_ARCH_FLAG_HAS_RAN_ONCE, or if there's an update to the
> + * current bitmap's value, before grabbing the lock.
> + */
> + if (test_bit(KVM_ARCH_FLAG_HAS_RAN_ONCE, &kvm->arch.flags))
> + return -EBUSY;
> +
> + if (val == *fw_reg_bmap)
> + return 0;

Honestly, I don't think we need this fast path. It only makes the
error case fast, and it is the one we really don't care about. All
that can go.

> +
> + mutex_lock(&kvm->lock);
> +
> + if (test_bit(KVM_ARCH_FLAG_HAS_RAN_ONCE, &kvm->arch.flags)) {
> + ret = -EBUSY;
> + goto out;
> + }
> +
> + WRITE_ONCE(*fw_reg_bmap, val);
> +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 +441,8 @@ 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/arch/arm64/kvm/psci.c b/arch/arm64/kvm/psci.c
> index 4bb76eb09248..94111f02674f 100644
> --- a/arch/arm64/kvm/psci.c
> +++ b/arch/arm64/kvm/psci.c
> @@ -437,3 +437,16 @@ int kvm_psci_call(struct kvm_vcpu *vcpu)
> return -EINVAL;
> }
> }
> +
> +bool kvm_psci_func_id_is_valid(u32 func_id)
> +{
> + 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)

The second term is pretty much useless: func_id is a u32, and
FUNC_NUM() only extracts the bottom 16 bits. So this is always an
unsigned quantity, and the test means nothing. A good compiler should
tell you that (but I guess our compilers are all terrible).

> + return true;
> +
> + /* KVM's PSCI 0.1 doesn't comply with SMCCC, and has its own function-id base and range */
> + if (func_id >= KVM_PSCI_FN(0) && func_id <= KVM_PSCI_FN(3))
> + return true;
> +
> + return false;
> +}

Also, why is this stuck in psci.c, while the *only* user seems to be
in hypercall.h?

> diff --git a/include/kvm/arm_hypercalls.h b/include/kvm/arm_hypercalls.h
> index 5d38628a8d04..c832e8411609 100644
> --- a/include/kvm/arm_hypercalls.h
> +++ b/include/kvm/arm_hypercalls.h
> @@ -6,6 +6,11 @@
>
> #include <asm/kvm_emulate.h>
>
> +/* Last valid bit of the bitmapped firmware registers */
> +#define KVM_REG_ARM_STD_BMAP_BIT_MAX 0

Does this bit represent KVM_REG_ARM_STD_BIT_TRNG_V1_0? If so, this is
very error prone. You really want to keep the definitions together so
that you can update the max when adding something else.

> +
> +#define KVM_ARM_SMCCC_STD_FEATURES GENMASK(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 +47,7 @@ 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..1f9ec5f2e513 100644
> --- a/include/kvm/arm_psci.h
> +++ b/include/kvm/arm_psci.h
> @@ -36,7 +36,7 @@ static inline int kvm_psci_version(struct kvm_vcpu *vcpu)
> return KVM_ARM_PSCI_0_1;
> }
>
> -
> int kvm_psci_call(struct kvm_vcpu *vcpu);
> +bool kvm_psci_func_id_is_valid(u32 func_id);

As I mentioned above, I don't think this should be globally visible.

>
> #endif /* __KVM_ARM_PSCI_H__ */
> --
> 2.36.0.464.gb9c8b46e94-goog
>
>

Thanks,

M.

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