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

[Amit Daniel Kachhap]

Hi,
On 1/31/19 9:57 PM, James Morse wrote:
> Hi Amit,
>
> 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/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)
>
> Isn't that a VCPU flag? Shouldn't this be when each VCPU is created?
Yes it is a VCPU flag.
> > 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.
>
> ... what happens if KVM's user-space enables ptrauth on some vcpus, but not on
> others?
Yes seems to be issue. Let me check more on this if there are other ways 
of passing the userspace parameter such as in CREATE_VM type ioctl.
> You removed the id-register suppression in the previous patch, but it doesn't
> get hooked up to kvm_arm_vcpu_ptrauth_allowed() here. (you could add
> kvm_arm_vcpu_ptrauth_allowed() earlier, and default it to true to make it easier).
>
> Doesn't this mean that if the CPU supports pointer auth, but user-space doesn't
> specify this flag, the guest gets mysterious undef's whenever it tries to use
> the advertised feature?
Agree, ID registers should be masked  when userspace disables it.
> (whether we support big/little virtual-machines is probably a separate issue,
> but the id registers need to be consistent with our trap-and-undef behaviour)
>
>
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index c798d0c..4a6ec40 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -453,14 +453,15 @@ static inline bool kvm_arch_requires_vhe(void)
> >   
> >   void kvm_arm_vcpu_ptrauth_enable(struct kvm_vcpu *vcpu);
> >   void kvm_arm_vcpu_ptrauth_disable(struct kvm_vcpu *vcpu);
> > +bool kvm_arm_vcpu_ptrauth_allowed(struct kvm_vcpu *vcpu);
> >   
> >   static inline void kvm_arm_vcpu_ptrauth_reset(struct kvm_vcpu *vcpu)
> >   {
> >   	/* Disable ptrauth and use it in a lazy context via traps */
> > -	if (has_vhe() && kvm_supports_ptrauth())
> > +	if (has_vhe() && kvm_supports_ptrauth()
> > +			&& kvm_arm_vcpu_ptrauth_allowed(vcpu))
> >   		kvm_arm_vcpu_ptrauth_disable(vcpu);
> >   }
> > -
> >   void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu);
> >   
>
> > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > index 5b980e7..c0e5dcd 100644
> > --- a/arch/arm64/kvm/handle_exit.c
> > +++ b/arch/arm64/kvm/handle_exit.c
> > @@ -179,7 +179,8 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >    */
> >   void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu)
> >   {
> > -	if (has_vhe() && kvm_supports_ptrauth())
> > +	if (has_vhe() && kvm_supports_ptrauth()
> > +			&& kvm_arm_vcpu_ptrauth_allowed(vcpu))
>
> Duplication. If has_vhe() moved into kvm_supports_ptrauth(), and
> kvm_supports_ptrauth() was called from kvm_arm_vcpu_ptrauth_allowed() it would
> be clearer that use of this feature was becoming user-controlled policy.
>
> (We don't need to list the dependencies at every call site)
ok.
> > diff --git a/arch/arm64/kvm/hyp/ptrauth-sr.c b/arch/arm64/kvm/hyp/ptrauth-sr.c
> > index 0576c01..369624f 100644
> > --- a/arch/arm64/kvm/hyp/ptrauth-sr.c
> > +++ b/arch/arm64/kvm/hyp/ptrauth-sr.c
> > @@ -42,3 +42,16 @@ void __no_ptrauth __hyp_text __ptrauth_switch_to_host(struct kvm_vcpu *vcpu,
> >   	ptrauth_keys_store((struct ptrauth_keys *) &guest_ctxt->sys_regs[APIAKEYLO_EL1]);
> >   	ptrauth_keys_switch((struct ptrauth_keys *) &host_ctxt->sys_regs[APIAKEYLO_EL1]);
> >   }
> > +
> > +/**
> > + * kvm_arm_vcpu_ptrauth_allowed - checks if ptrauth feature is present in vcpu
>
> ('enabled by KVM's user-space' may be clearer. 'Present in vcpu' could be down
> to a cpufeature thing)
ok.
> > + *
> > + * @vcpu: The VCPU pointer
> > + *
> > + * This function will be used to enable/disable ptrauth in guest as configured
>
> ... but it just tests the bit ...
>
> > + * 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);
> > +}
>
>
> Thanks,
>
> James
//Amit