linux-kernel - Re: [PATCH v6 4/6] arm64/kvm: add a userspace option to enable pointer authentication

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Date:   Thu, 28 Feb 2019 14:55:02 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Jones <drjones@...hat.com>,
        Dave Martin <Dave.Martin@....com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Kristina Martsenko <kristina.martsenko@....com>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: Re: [PATCH v6 4/6] arm64/kvm: add a userspace option to enable
 pointer authentication

Hi,

On 2/21/19 6:04 PM, Mark Rutland wrote:
> On Tue, Feb 19, 2019 at 02:54:29PM +0530, 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.
>>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Christoffer Dall <christoffer.dall@....com>
>> Cc: kvmarm@...ts.cs.columbia.edu
>> ---
>>   Documentation/arm64/pointer-authentication.txt |  9 +++++----
>>   Documentation/virtual/kvm/api.txt              |  4 ++++
>>   arch/arm64/include/asm/kvm_host.h              |  3 ++-
>>   arch/arm64/include/uapi/asm/kvm.h              |  1 +
>>   arch/arm64/kvm/handle_exit.c                   |  2 +-
>>   arch/arm64/kvm/hyp/ptrauth-sr.c                | 16 +++++++++++++++-
>>   arch/arm64/kvm/reset.c                         |  3 +++
>>   arch/arm64/kvm/sys_regs.c                      | 26 +++++++++++++-------------
>>   include/uapi/linux/kvm.h                       |  1 +
>>   9 files changed, 45 insertions(+), 20 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. Otherwise, KVM treats these instructions as undefined.
> 
> I think that we should have separate flags for address auth and generic
> auth, to match the ID register split.
> 
> For now, we can have KVM only support the case where both are set, but
> it gives us freedom to support either in isolation if we have to in
> future, without an ABI break.
yes agree with you about having two address and generic ptrauth flags. 
Will add in next iteration.

Thanks,
Amit D
> 
> Thanks,
> Mark.
>