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

James Morse
2019-01-31T16:27:23Z

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.

> 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)

Isn't that a VCPU flag? Shouldn't this be when each VCPU is created?

> 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.

... what happens if KVM's user-space enables ptrauth on some vcpus, but not on
others?

You removed the id-register suppression in the previous patch, but it doesn't
get hooked up to kvm_arm_vcpu_ptrauth_allowed() here. (you could add
kvm_arm_vcpu_ptrauth_allowed() earlier, and default it to true to make it easier).

Doesn't this mean that if the CPU supports pointer auth, but user-space doesn't
specify this flag, the guest gets mysterious undef's whenever it tries to use
the advertised feature?

(whether we support big/little virtual-machines is probably a separate issue,
but the id registers need to be consistent with our trap-and-undef behaviour)

> 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
> @@ -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);
>

> 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))

Duplication. If has_vhe() moved into kvm_supports_ptrauth(), and
kvm_supports_ptrauth() was called from kvm_arm_vcpu_ptrauth_allowed() it would
be clearer that use of this feature was becoming user-controlled policy.

(We don't need to list the dependencies at every call site)

> 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

('enabled by KVM's user-space' may be clearer. 'Present in vcpu' could be down
to a cpufeature thing)

> + *
> + * @vcpu: The VCPU pointer
> + *
> + * This function will be used to enable/disable ptrauth in guest as configured

... but it just tests the bit ...

> + * by the KVM userspace API.
> + */
> +bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu)
> +{
> + return test_bit(KVM_ARM_VCPU_PTRAUTH, vcpu->arch.features);
> +}

Thanks,

James