Re: [PATCH v5 4/5] arm64/kvm: add a userspace option to enable pointer authentication

[Julien Thierry]

Hi Amit,

On 28/01/2019 06:58, Amit Daniel Kachhap wrote:
> This feature will allow the KVM guest to allow the handling of
> pointer authentication instructions or to treat them as undefined
> if not set. It uses the existing vcpu API KVM_ARM_VCPU_INIT to
> supply this parameter instead of creating a new API.
> 
> A new register is not created to pass this parameter via
> SET/GET_ONE_REG interface as just a flag (KVM_ARM_VCPU_PTRAUTH)
> supplied is enough to enable this feature.
> 

I had a bit of trouble parsing this last sentence. I'd suggest rewording
it as "No additional register is created for the SET/GET_ONE_REG API,
instead just pass this parameter as a flag".

> 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: Kristina Martsenko <kristina.martsenko@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> Cc: Ramana Radhakrishnan <ramana.radhakrishnan@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> ---
>  Documentation/arm64/pointer-authentication.txt |  9 +++++----
>  Documentation/virtual/kvm/api.txt              |  4 ++++
>  arch/arm/include/asm/kvm_host.h                |  4 ++++
>  arch/arm64/include/asm/kvm_host.h              |  7 ++++---
>  arch/arm64/include/uapi/asm/kvm.h              |  1 +
>  arch/arm64/kvm/handle_exit.c                   |  3 ++-
>  arch/arm64/kvm/hyp/ptrauth-sr.c                | 13 +++++++++++++
>  arch/arm64/kvm/reset.c                         |  3 +++
>  include/uapi/linux/kvm.h                       |  1 +
>  9 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
> index a25cd21..0529a7d 100644
> --- a/Documentation/arm64/pointer-authentication.txt
> +++ b/Documentation/arm64/pointer-authentication.txt
> @@ -82,7 +82,8 @@ pointers).
>  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 virtual machine is
> +created by passing a flag (KVM_ARM_VCPU_PTRAUTH) requesting this feature
> +to be enabled. Without this flag, pointer authentication is not enabled
> +in KVM guests and attempted use of the feature will result in an UNDEFINED
> +exception being injected into the guest.
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index 356156f..1e646fb 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -2642,6 +2642,10 @@ 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: Emulate Pointer authentication for the CPU.
> +	  Depends on KVM_CAP_ARM_PTRAUTH and only on arm64 architecture. If
> +	  set, then the KVM guest allows the execution of pointer authentication
> +	  instructions or treats them as undefined if not set.

I'd suggest "then the KVM guest allows the execution of pointer
authentication instructions. Otherwise, KVM treats these instructions as
undefined."

>  
>  
>  4.83 KVM_ARM_PREFERRED_TARGET
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index b200c14..b6950df 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -346,6 +346,10 @@ static inline int kvm_arm_have_ssbd(void)
>  static inline void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu) {}
>  static inline void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu) {}
>  static inline void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu) {}
> +static inline bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
>  
>  #define __KVM_HAVE_ARCH_VM_ALLOC
>  struct kvm *kvm_arch_alloc_vm(void);
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index c798d0c..4a6ec40 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -43,7 +43,7 @@
>  
>  #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
>  
> -#define KVM_VCPU_MAX_FEATURES 4
> +#define KVM_VCPU_MAX_FEATURES 5
>  
>  #define KVM_REQ_SLEEP \
>  	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> @@ -453,14 +453,15 @@ static inline bool kvm_arch_requires_vhe(void)
>  
>  void kvm_arm_vcpu_ptrauth_enable(struct kvm_vcpu *vcpu);
>  void kvm_arm_vcpu_ptrauth_disable(struct kvm_vcpu *vcpu);
> +bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu);
>  
>  static inline void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu)
>  {
>  	/* Disable ptrauth and use it in a lazy context via traps */
> -	if (has_vhe() && kvm_supports_ptrauth())
> +	if (has_vhe() && kvm_supports_ptrauth()
> +			&& kvm_arm_vcpu_ptrauth_allowed(vcpu))
>  		kvm_arm_vcpu_ptrauth_disable(vcpu);
>  }
> -
>  void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu);
>  
>  static inline void kvm_arch_hardware_unsetup(void) {}
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 97c3478..5f82ca1 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -102,6 +102,7 @@ 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		4 /* VCPU uses address authentication */
>  
>  struct kvm_vcpu_init {
>  	__u32 target;
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 5b980e7..c0e5dcd 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -179,7 +179,8 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
>   */
>  void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu)
>  {
> -	if (has_vhe() && kvm_supports_ptrauth())
> +	if (has_vhe() && kvm_supports_ptrauth()
> +			&& kvm_arm_vcpu_ptrauth_allowed(vcpu))
>  		kvm_arm_vcpu_ptrauth_enable(vcpu);
>  	else
>  		kvm_inject_undefined(vcpu);
> diff --git a/arch/arm64/kvm/hyp/ptrauth-sr.c b/arch/arm64/kvm/hyp/ptrauth-sr.c
> index 0576c01..369624f 100644
> --- a/arch/arm64/kvm/hyp/ptrauth-sr.c
> +++ b/arch/arm64/kvm/hyp/ptrauth-sr.c
> @@ -42,3 +42,16 @@ void __no_ptrauth __hyp_text __ptrauth_switch_to_host(struct kvm_vcpu *vcpu,
>  	ptrauth_keys_store((struct ptrauth_keys *) &guest_ctxt->sys_regs[APIAKEYLO_EL1]);
>  	ptrauth_keys_switch((struct ptrauth_keys *) &host_ctxt->sys_regs[APIAKEYLO_EL1]);
>  }
> +
> +/**
> + * kvm_arm_vcpu_ptrauth_allowed - checks if ptrauth feature is present in vcpu
> + *
> + * @vcpu: The VCPU pointer
> + *
> + * This function will be used to enable/disable ptrauth in guest as configured
> + * by the KVM userspace API.
> + */
> +bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu)

Nit: I find the "allowed" a bit strange. Maybe something like
"kvm_arm_vcpu_has_ptrauth()" ?

> +{
> +	return test_bit(KVM_ARM_VCPU_PTRAUTH, vcpu->arch.features);
> +}
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index b72a3dd..987e0c3c 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -91,6 +91,9 @@ int kvm_arch_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_ARM_VM_IPA_SIZE:
>  		r = kvm_ipa_limit;
>  		break;
> +	case KVM_CAP_ARM_PTRAUTH:
> +		r = kvm_supports_ptrauth();

Even if we don't have VHE?

Cheers,

-- 
Julien Thierry