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

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Date:   Mon, 4 Mar 2019 16:26:34 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     James Morse <james.morse@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     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, Mark Rutland <mark.rutland@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: Re: [PATCH v6 4/6] arm64/kvm: add a userspace option to enable
 pointer authentication

Hi James,

On 2/27/19 12:03 AM, James Morse wrote:
> Hi Amit,
> 
> On 19/02/2019 09:24, 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.
> 
> and an attempt to restore the id register with the other version would fail.
> 
> 
>> 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)
> 
> (This is still mixing VM and VCPU)
> 
> 
>> + requesting this feature to be enabled.
> 
> .. on each vcpu?
> 
> 
>> +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.
> 
> 'guests' here suggests its a VM property. If you set it on some VCPU but not others KVM
> will generate undefs instead of enabling the feature. (which is the right thing to do)
> 
> I think it needs to be clear this is a per-vcpu property.
ok.
> 
> 
>> 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 */
> 
> Just address authentication? I agree with Mark we should have two bits to match what gets
> exposed to EL0. One would then be address, the other generic.
ok.
> 
> 
>> diff --git a/arch/arm64/kvm/hyp/ptrauth-sr.c b/arch/arm64/kvm/hyp/ptrauth-sr.c
>> index 528ee6e..6846a23 100644
>> --- a/arch/arm64/kvm/hyp/ptrauth-sr.c
>> +++ b/arch/arm64/kvm/hyp/ptrauth-sr.c
>> @@ -93,9 +93,23 @@ void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu)
> 
>> +/**
>> + * kvm_arm_vcpu_ptrauth_allowed - checks if ptrauth feature is allowed by user
>> + *
>> + * @vcpu: The VCPU pointer
>> + *
>> + * This function will be used to check userspace option to have ptrauth or not
>> + * in the guest kernel.
>> + */
>> +bool kvm_arm_vcpu_ptrauth_allowed(const struct kvm_vcpu *vcpu)
>> +{
>> +	return kvm_supports_ptrauth() &&
>> +		test_bit(KVM_ARM_VCPU_PTRAUTH, vcpu->arch.features);
>> +}
> 
> This isn't used from world-switch, could it be moved to guest.c?
yes sure.
> 
> 
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 12529df..f7bcc60 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -1055,7 +1055,7 @@ static bool access_cntp_cval(struct kvm_vcpu *vcpu,
>>   }
>>   
>>   /* Read a sanitised cpufeature ID register by sys_reg_desc */
>> -static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
>> +static u64 read_id_reg(struct kvm_vcpu *vcpu, struct sys_reg_desc const *r, bool raz)
> 
> (It might be easier on the reviewer to move these mechanical changes to an earlier patch)
Yes with including some of Dave SVE patches this wont be required.

Thanks,
Amit D
> 
> 
> Looks good,
> 
> Thanks,
> 
> James
>