Re: [PATCH v8 6/9] KVM: arm64: Add vcpu feature flags to control ptrauth accessibility

From: Dave Martin
Date: Fri Apr 05 2019 - 07:02:52 EST

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On Tue, Apr 02, 2019 at 07:57:14AM +0530, Amit Daniel Kachhap wrote:
> Since Pointer authentication will be enabled or disabled on a
> per-vcpu basis, vcpu feature flags are added in order to know which
> vcpus have it enabled from userspace.
>
> This features will allow the KVM guest to allow the handling of
> pointer authentication instructions or to treat them as undefined
> if not set.
>
> The helper macro added checks the feature flag along with other
> conditions such as VHE mode present and system support for
> pointer address/generic authentication.

Can this patch be put after the context switch patch instead?

Here, we accept a request from userspace to enable ptrauth, but it will
mysteriously fail to work. I worked around a similar issue by defining
KVM_ARM64_GUEST_HAS_SVE early in the SVE series, but putting the logic
to set this flag in vcpu->arch.flags later on (see also comments about
this below).

> Necessary documentations are added to reflect the changes done.
>
> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> ---
>
> Changes since v7:
> * Moved the check for userspace features in this patch [James Morse].
> * Moved the vcpu feature flags Documentation in this patch [James Morse].
>
> Documentation/arm64/pointer-authentication.txt | 13 +++++++++----
> Documentation/virtual/kvm/api.txt | 4 ++++
> arch/arm64/include/asm/kvm_host.h | 8 +++++++-
> arch/arm64/include/uapi/asm/kvm.h | 2 ++
> arch/arm64/kvm/reset.c | 7 +++++++
> 5 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
> index 5baca42..b164886 100644
> --- a/Documentation/arm64/pointer-authentication.txt
> +++ b/Documentation/arm64/pointer-authentication.txt
> @@ -87,7 +87,12 @@ used to get and set the keys for a thread.
> Virtualization
> --------------
>
> -Pointer authentication is not currently supported in KVM guests. KVM
> -will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
> -the feature will result in an UNDEFINED exception being injected into
> -the guest.
> +Pointer authentication is enabled in KVM guest when each virtual cpu is
> +initialised by passing flags KVM_ARM_VCPU_PTRAUTH_[ADDRESS/GENERIC] and
> +requesting these two separate cpu features to be enabled. The current KVM
> +guest implementation works by enabling both features together, so both these
> +userspace flags are checked together before enabling pointer authentication.
> +The separate userspace flag will allow to have no userspace ABI changes when
> +both features are implemented in an isolated way in future.

Nit: we might make this change, but we don't promise that it will happsen.

So, maybe write:

"[...] have no userspace ABI changes if support is added in the future
to allow these two features to be enabled independently of one another."

> +
> +Pointer Authentication is supported in KVM guest only in VHE mode.
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 7de9eee..aaa048d 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2659,6 +2659,10 @@ Possible features:
> Depends on KVM_CAP_ARM_PSCI_0_2.
> - KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU.
> Depends on KVM_CAP_ARM_PMU_V3.
> + - KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication
> + for the CPU and supported only on arm64 architecture.

We should probably add:

Must be requested if KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested.

> + - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
> + for the CPU and supported only on arm64 architecture.

Similarly:

Must be requested if KVM_ARM_VCPU_PTRAUTH_ADDRESS is also requested.

(Or otherwise explain that both features must enabled together or not at
all.)

> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index e3ccd7b..9dd2918 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -45,7 +45,7 @@
>
> #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
>
> -#define KVM_VCPU_MAX_FEATURES 4
> +#define KVM_VCPU_MAX_FEATURES 6
>
> #define KVM_REQ_SLEEP \
> KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> @@ -491,6 +491,12 @@ static inline bool kvm_arch_requires_vhe(void)
> return false;
> }
>
> +#define vcpu_has_ptrauth(vcpu) (has_vhe() && \
> + system_supports_address_auth() && \
> + system_supports_generic_auth() && \
> + test_bit(KVM_ARM_VCPU_PTRAUTH_ADDRESS, vcpu->arch.features) && \
> + test_bit(KVM_ARM_VCPU_PTRAUTH_GENERIC, vcpu->arch.features))

We're checking 5 things here, which we don't necessarily want to do
every time.

Is this used on any hot path?

This kind of thing is one reason why I added vcpu->arch.flags: we can
make the policy decision about whether to set the flag in
kvm_reset_vcpu(), then afterwards we only need to check the flag.

> +
> static inline void kvm_arch_hardware_unsetup(void) {}
> static inline void kvm_arch_sync_events(struct kvm *kvm) {}
> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 97c3478..8806f71 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -102,6 +102,8 @@ struct kvm_regs {
> #define KVM_ARM_VCPU_EL1_32BIT 1 /* CPU running a 32bit VM */
> #define KVM_ARM_VCPU_PSCI_0_2 2 /* CPU uses PSCI v0.2 */
> #define KVM_ARM_VCPU_PMU_V3 3 /* Support guest PMUv3 */
> +#define KVM_ARM_VCPU_PTRAUTH_ADDRESS 4 /* VCPU uses address authentication */
> +#define KVM_ARM_VCPU_PTRAUTH_GENERIC 5 /* VCPU uses generic authentication */
>
> struct kvm_vcpu_init {
> __u32 target;
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index f16a5f8..717afed 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -128,6 +128,13 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> if (loaded)
> kvm_arch_vcpu_put(vcpu);
>
> + if (test_bit(KVM_ARM_VCPU_PTRAUTH_ADDRESS, vcpu->arch.features) ||
> + test_bit(KVM_ARM_VCPU_PTRAUTH_GENERIC, vcpu->arch.features)) {
> + /* Verify that KVM startup matches the conditions for ptrauth */
> + if (!vcpu_has_ptrauth(vcpu))
> + goto out;
> + }
> +

This looks like it works, but I find the way vcpu->arch.features is used
in two different ways at the same time a bit confusing.

Cheers
---Dave

Next message: Dave Martin: "Re: [PATCH v8 8/9] KVM: arm64: Add capability to advertise ptrauth for guest"
Previous message: Dave Martin: "Re: [PATCH v8 5/9] KVM: arm/arm64: preserve host MDCR_EL2 value"
In reply to: Amit Daniel Kachhap: "[PATCH v8 6/9] KVM: arm64: Add vcpu feature flags to control ptrauth accessibility"
Next in thread: Amit Daniel Kachhap: "[PATCH v8 5/9] KVM: arm/arm64: preserve host MDCR_EL2 value"
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