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

[Amit Daniel Kachhap]

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