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SubjectRe: [PATCH v10 3/5] KVM: arm64: Add userspace flag to enable pointer authentication
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Hi,

On 4/23/19 9:15 PM, Dave Martin wrote:
> On Tue, Apr 23, 2019 at 10:12:36AM +0530, Amit Daniel Kachhap wrote:
>> Now that the building blocks of pointer authentication are present, lets
>> add userspace flags KVM_ARM_VCPU_PTRAUTH_ADDRESS and
>> KVM_ARM_VCPU_PTRAUTH_GENERIC. These flags will enable pointer
>> authentication for the KVM guest on a per-vcpu basis through the ioctl
>> KVM_ARM_VCPU_INIT.
>>
>> This features will allow the KVM guest to allow the handling of
>> pointer authentication instructions or to treat them as undefined
>> if not set.
>>
>> Necessary documentations are added to reflect the changes done.
>>
>> Reviewed-by: Dave Martin <Dave.Martin@arm.com>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: kvmarm@lists.cs.columbia.edu
>> ---
>> Changed since v9:
>> * Fixed tab alignment at few places [Dave Martin].
>> * Split the system capability checks [Dave Martin].
>>
>> Documentation/arm64/pointer-authentication.txt | 22 +++++++++++++++++----
>> Documentation/virtual/kvm/api.txt | 10 ++++++++++
>> arch/arm64/include/asm/kvm_host.h | 2 +-
>> arch/arm64/include/uapi/asm/kvm.h | 2 ++
>> arch/arm64/kvm/reset.c | 27 ++++++++++++++++++++++++++
>> 5 files changed, 58 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
>> index 5baca42..fc71b33 100644
>> --- a/Documentation/arm64/pointer-authentication.txt
>> +++ b/Documentation/arm64/pointer-authentication.txt
>> @@ -87,7 +87,21 @@ 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 before enabling pointer authentication.
>> +The separate userspace flag will allow to have no userspace ABI changes
>> +if support is added in the future to allow these two features to be
>> +enabled independently of one another.
>> +
>> +As Arm Architecture specifies that Pointer Authentication feature is
>> +implemented along with the VHE feature so KVM arm64 ptrauth code relies
>> +on VHE mode to be present.
>> +
>> +Additionally, when these vcpu feature flags are not set then KVM will
>> +filter out the Pointer Authentication system key registers from
>> +KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
>> +register. Any attempt to use the Pointer Authentication instructions 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 e410a9f..32afe7f 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -2761,6 +2761,16 @@ Possible features:
>> - 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 arm64 only.
>> + Both KVM_ARM_VCPU_PTRAUTH_ADDRESS and KVM_ARM_VCPU_PTRAUTH_GENERIC
>> + must be requested or neither must be requested.
>> +
>> + - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication
>> + for arm64 only.
>> + Both KVM_ARM_VCPU_PTRAUTH_ADDRESS and KVM_ARM_VCPU_PTRAUTH_GENERIC
>> + must be requested or neither must be requested.
>> +
>
> This looks pretty clear now.
>
>> - KVM_ARM_VCPU_SVE: Enables SVE for the CPU (arm64 only).
>> Depends on KVM_CAP_ARM_SVE.
>> Requires KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_SVE):
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 7eebea7..f772ac2 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -49,7 +49,7 @@
>>
>> #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
>>
>> -#define KVM_VCPU_MAX_FEATURES 5
>> +#define KVM_VCPU_MAX_FEATURES 7
>>
>> #define KVM_REQ_SLEEP \
>> KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>> index edd2db8..7b7ac0f 100644
>> --- a/arch/arm64/include/uapi/asm/kvm.h
>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>> @@ -104,6 +104,8 @@ struct kvm_regs {
>> #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_SVE 4 /* enable SVE for this CPU */
>> +#define KVM_ARM_VCPU_PTRAUTH_ADDRESS 5 /* VCPU uses address authentication */
>> +#define KVM_ARM_VCPU_PTRAUTH_GENERIC 6 /* 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 3402543..028d0c6 100644
>> --- a/arch/arm64/kvm/reset.c
>> +++ b/arch/arm64/kvm/reset.c
>> @@ -221,6 +221,27 @@ static void kvm_vcpu_reset_sve(struct kvm_vcpu *vcpu)
>> memset(vcpu->arch.sve_state, 0, vcpu_sve_state_size(vcpu));
>> }
>>
>> +static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
>> +{
>> + /* Support ptrauth only if the system supports these capabilities. */
>> + if (!has_vhe())
>> + return -EINVAL;
>> +
>> + if (!system_supports_address_auth() ||
>> + !system_supports_generic_auth())
>> + return -EINVAL;
>
> Since pointer auth implies v8.3 and v8.3 implies v8.1 and v8.1 implies VHE:
>
> ((system_supports_address_auth() || system_supports_generic_auth()) &&
> !has_vhe()) implies that the hardware is broken or the kernel is buggy.
>
> So, it probably makes sense to write
>
> if (!system_supports_address_auth() ||
> !system_supports_generic_auth())
> return -EINVAL;
>
> if (WARN_ON(!has_vhe()))
> return -EINVAL;

Yes agree with you. Now with config VHE restriction set it makes more
sense that this situation will only occur now in buggy h/w case.
>
> This is not essential, and doesn't affect the ABI -- so we could apply
> it on top later on.
>
> [...]
>
> FWIW:
>
> Reviewed-by: Dave Martin <Dave.Martin@arm.com> (for the updates)
Thanks for the review effort.

Thanks,
Amit D
>
> Cheers
> ---Dave
>

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