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

[Amit Daniel Kachhap <amit.kachhap@arm.com>]

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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