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

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Date:   Fri, 15 Feb 2019 10:19:38 +0530
From:   Amit Daniel Kachhap <amit.kachhap@....com>
To:     Kristina Martsenko <kristina.martsenko@....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, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 4/5] arm64/kvm: add a userspace option to enable
 pointer authentication

Hi,

On 2/13/19 11:05 PM, Kristina Martsenko wrote:
> 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/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index b200c14..b6950df 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -346,6 +346,10 @@ static inline int kvm_arm_have_ssbd(void)
>>   static inline void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu) {}
>>   static inline void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu) {}
>>   static inline void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu) {}
>> +static inline bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu)
>> +{
>> +	return false;
>> +}
> 
> It seems like this is only ever called from arm64 code, so do we need an
> arch/arm/ definition?
Yes not required. Nice catch.
> 
>> +/**
>> + * kvm_arm_vcpu_ptrauth_allowed - checks if ptrauth feature is present in vcpu
>> + *
>> + * @vcpu: The VCPU pointer
>> + *
>> + * This function will be used to enable/disable ptrauth in guest as configured
>> + * 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);
>> +}
> 
> I'm not sure, but should there also be something like
> 
> if (test_bit(KVM_ARM_VCPU_PTRAUTH, vcpu->arch.features) &&
>      !kvm_supports_ptrauth())
> 	return -EINVAL;
> 
> in kvm_reset_vcpu?
Yes makes sense. I missed it and with Dave martin patch this may be done 
cleanly.

Thanks,
Amit D

> 
> Thanks,
> Kristina
>