Re: [PATCH] hw/arm/virt: KVM: Enable PAuth when supported by the host

[Marc Zyngier <maz@kernel.org>]

Hi Andrew,

On Mon, 03 Jan 2022 13:46:01 +0000,
Andrew Jones <drjones@redhat.com> wrote:
> 
> Hi Marc,
> 
> On Tue, Dec 28, 2021 at 06:23:47PM +0000, Marc Zyngier wrote:
> > Add basic support for Pointer Authentication when running a KVM
> > guest and that the host supports it, loosely based on the SVE
> > support.
> > 
> > Although the feature is enabled by default when the host advertises
> > it, it is possible to disable it by setting the 'pauth=off' CPU
> > property.
> > 
> > Tested on an Apple M1 running 5.16-rc6.
> > 
> > Cc: Eric Auger <eric.auger@redhat.com>
> > Cc: Andrew Jones <drjones@redhat.com>
> > Cc: Richard Henderson <richard.henderson@linaro.org>
> > Cc: Peter Maydell <peter.maydell@linaro.org>
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  docs/system/arm/cpu-features.rst |  5 +++++
> >  target/arm/cpu.c                 |  1 +
> >  target/arm/cpu.h                 |  1 +
> >  target/arm/cpu64.c               | 36 ++++++++++++++++++++++++++++++++
> >  target/arm/kvm.c                 | 13 ++++++++++++
> >  target/arm/kvm64.c               | 10 +++++++++
> >  target/arm/kvm_arm.h             |  7 +++++++
> >  7 files changed, 73 insertions(+)
> > 
> > diff --git a/docs/system/arm/cpu-features.rst b/docs/system/arm/cpu-features.rst
> > index 584eb17097..c9e39546a5 100644
> > --- a/docs/system/arm/cpu-features.rst
> > +++ b/docs/system/arm/cpu-features.rst
> > @@ -211,6 +211,11 @@ the list of KVM VCPU features and their descriptions.
> >                             influence the guest scheduler behavior and/or be
> >                             exposed to the guest userspace.
> >  
> > +  pauth                    Enable or disable ``FEAT_Pauth``, pointer
> > +                           authentication.  By default, the feature is enabled
> > +                           with ``-cpu host`` if supported by both the host
> > +                           kernel and the hardware.
> > +
> 
> Thanks for considering a documentation update. In this case, though, I
> think we should delete the "TCG VCPU Features" pauth paragraph, rather
> than add a new "KVM VCPU Features" pauth paragraph. We don't need to
> document each CPU feature. We just document complex ones, like sve*,
> KVM specific ones (kvm-*), and TCG specific ones (now only pauth-impdef).

Sure, works for me. Do we need to keep a trace of the available
options? I'm not sure how a user is supposed to find out about those
(I always end-up grepping through the code base, and something tells
me I'm doing it wrong...). The QMP stuff flies way over my head.

> >  TCG VCPU Features
> >  =================
> >  
> > diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> > index a211804fd3..68b09cbc6a 100644
> > --- a/target/arm/cpu.c
> > +++ b/target/arm/cpu.c
> > @@ -2091,6 +2091,7 @@ static void arm_host_initfn(Object *obj)
> >      kvm_arm_set_cpu_features_from_host(cpu);
> >      if (arm_feature(&cpu->env, ARM_FEATURE_AARCH64)) {
> >          aarch64_add_sve_properties(obj);
> > +        aarch64_add_pauth_properties(obj);
> >      }
> >  #else
> >      hvf_arm_set_cpu_features_from_host(cpu);
> > diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> > index e33f37b70a..c6a4d50e82 100644
> > --- a/target/arm/cpu.h
> > +++ b/target/arm/cpu.h
> > @@ -1076,6 +1076,7 @@ void aarch64_sve_narrow_vq(CPUARMState *env, unsigned vq);
> >  void aarch64_sve_change_el(CPUARMState *env, int old_el,
> >                             int new_el, bool el0_a64);
> >  void aarch64_add_sve_properties(Object *obj);
> > +void aarch64_add_pauth_properties(Object *obj);
> >  
> >  /*
> >   * SVE registers are encoded in KVM's memory in an endianness-invariant format.
> > diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
> > index 15245a60a8..305c0e19fe 100644
> > --- a/target/arm/cpu64.c
> > +++ b/target/arm/cpu64.c
> > @@ -625,6 +625,42 @@ void aarch64_add_sve_properties(Object *obj)
> >  #endif
> >  }
> >  
> > +static bool cpu_arm_get_pauth(Object *obj, Error **errp)
> > +{
> > +    ARMCPU *cpu = ARM_CPU(obj);
> > +    return cpu_isar_feature(aa64_pauth, cpu);
> > +}
> > +
> > +static void cpu_arm_set_pauth(Object *obj, bool value, Error **errp)
> > +{
> > +    ARMCPU *cpu = ARM_CPU(obj);
> > +    uint64_t t;
> > +
> > +    if (value) {
> > +        if (!kvm_arm_pauth_supported()) {
> > +            error_setg(errp, "'pauth' feature not supported by KVM on this host");
> > +        }
> > +
> > +        return;
> > +    }
> > +
> > +    /*
> > +     * If the host supports PAuth, we only end-up here if the user has
> > +     * disabled the support, and value is false.
> > +     */
> > +    t = cpu->isar.id_aa64isar1;
> > +    t = FIELD_DP64(t, ID_AA64ISAR1, APA, value);
> > +    t = FIELD_DP64(t, ID_AA64ISAR1, GPA, value);
> > +    t = FIELD_DP64(t, ID_AA64ISAR1, API, value);
> > +    t = FIELD_DP64(t, ID_AA64ISAR1, GPI, value);
> > +    cpu->isar.id_aa64isar1 = t;
> > +}
> > +
> > +void aarch64_add_pauth_properties(Object *obj)
> > +{
> > +    object_property_add_bool(obj, "pauth", cpu_arm_get_pauth, cpu_arm_set_pauth);
> > +}
> 
> I think we should try to merge as much as possible between the TCG and KVM
> pauth property handling. I think we just need to move the
> qdev_property_add_static(DEVICE(obj), &arm_cpu_pauth_property) call into
> KVM/TCG common code and then modify arm_cpu_pauth_finalize() to
> handle checking KVM for support when KVM is enabled and also to not
> look at the impdef property.

Happy to merge things more, though using qdev_property_add_static()
feels a bit odd here (I have to forcefully replicate the probed state
into the cpu->prop_pauth property in order to have a sensible default
on KVM).

Anyway, I'll post something with this hack, and we add another coat of
paint to the bike shed! ;-)

> 
> > +
> >  void arm_cpu_pauth_finalize(ARMCPU *cpu, Error **errp)
> >  {
> >      int arch_val = 0, impdef_val = 0;
> > diff --git a/target/arm/kvm.c b/target/arm/kvm.c
> > index bbf1ce7ba3..71e2f46ce8 100644
> > --- a/target/arm/kvm.c
> > +++ b/target/arm/kvm.c
> > @@ -84,6 +84,7 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
> >      if (vmfd < 0) {
> >          goto err;
> >      }
> > +
> >      cpufd = ioctl(vmfd, KVM_CREATE_VCPU, 0);
> >      if (cpufd < 0) {
> >          goto err;
> > @@ -94,6 +95,18 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
> >          goto finish;
> >      }
> >  
> > +    /*
> > +     * Ask for Pointer Authentication if supported. We can't play the
> > +     * SVE trick of synthetising the ID reg as KVM won't tell us
> > +     * whether we have the architected or IMPDEF version of PAuth, so
> > +     * we have to use the actual ID regs.
> > +     */
> > +    if (ioctl(vmfd, KVM_CHECK_EXTENSION, KVM_CAP_ARM_PTRAUTH_ADDRESS) > 0 &&
> > +        ioctl(vmfd, KVM_CHECK_EXTENSION, KVM_CAP_ARM_PTRAUTH_GENERIC) > 0) {
> > +        init->features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
> > +                              1 << KVM_ARM_VCPU_PTRAUTH_GENERIC);
> > +    }
> > +
> 
> I think kvm_init() is called prior to kvm_arm_get_host_cpu_features(),
> which means we can do this instead
> 
> diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
> index e790d6c9a573..d1512035ac5b 100644
> --- a/target/arm/kvm64.c
> +++ b/target/arm/kvm64.c
> @@ -521,6 +521,17 @@ bool kvm_arm_get_host_cpu_features(ARMHostCPUFeatures *ahcf)
>       */
>      struct kvm_vcpu_init init = { .target = -1, };
>  
> +   /*
> +    * Ask for Pointer Authentication if supported. We can't play the
> +    * SVE trick of synthetising the ID reg as KVM won't tell us
> +    * whether we have the architected or IMPDEF version of PAuth, so
> +    * we have to use the actual ID regs.
> +    */
> +    if (kvm_arm_pauth_supported()) {
> +        init->features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
> +                              1 << KVM_ARM_VCPU_PTRAUTH_GENERIC);
> +    }
> +
>      if (!kvm_arm_create_scratch_host_vcpu(cpus_to_try, fdarray, &init)) {
>          return false;
>      }
> 
> Assuming I'm right about the call order, then I think I'd like that more.

Yup, works nicely, and allows for some further cleanups.

>
> 
> >      if (init->target == -1) {
> >          struct kvm_vcpu_init preferred;
> >  
> > diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
> > index e790d6c9a5..95b6902ca0 100644
> > --- a/target/arm/kvm64.c
> > +++ b/target/arm/kvm64.c
> > @@ -725,6 +725,12 @@ bool kvm_arm_sve_supported(void)
> >      return kvm_check_extension(kvm_state, KVM_CAP_ARM_SVE);
> >  }
> >  
> > +bool kvm_arm_pauth_supported(void)
> > +{
> > +    return (kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_ADDRESS) &&
> > +            kvm_check_extension(kvm_state, KVM_CAP_ARM_PTRAUTH_GENERIC));
> > +}
> > +
> >  bool kvm_arm_steal_time_supported(void)
> >  {
> >      return kvm_check_extension(kvm_state, KVM_CAP_STEAL_TIME);
> > @@ -865,6 +871,10 @@ int kvm_arch_init_vcpu(CPUState *cs)
> >          assert(kvm_arm_sve_supported());
> >          cpu->kvm_init_features[0] |= 1 << KVM_ARM_VCPU_SVE;
> >      }
> > +    if (cpu_isar_feature(aa64_pauth, cpu)) {
> > +	    cpu->kvm_init_features[0] |= (1 << KVM_ARM_VCPU_PTRAUTH_ADDRESS |
> > +					  1 << KVM_ARM_VCPU_PTRAUTH_GENERIC);
> > +    }
> >  
> >      /* Do KVM_ARM_VCPU_INIT ioctl */
> >      ret = kvm_arm_vcpu_init(cs);
> > diff --git a/target/arm/kvm_arm.h b/target/arm/kvm_arm.h
> > index b7f78b5215..c26acf7866 100644
> > --- a/target/arm/kvm_arm.h
> > +++ b/target/arm/kvm_arm.h
> > @@ -306,6 +306,13 @@ bool kvm_arm_pmu_supported(void);
> >   */
> >  bool kvm_arm_sve_supported(void);
> >  
> > +/**
> > + * kvm_arm_pauth_supported:
> > + *
> > + * Returns true if KVM can enable Pointer Authentication and false otherwise.
> > + */
> > +bool kvm_arm_pauth_supported(void);
> > +
> 
> If we merge more of the pauth property handling with the TCG code, then
> we'll also need a stub in the !CONFIG_KVM section for compiling without
> kvm support.

Actually, this can go altogether, as it can now be made static in
kvm64.c and not be visible outside of the KVM code at all.

Thanks a lot for the review and the guidance!

	M.

-- 
Without deviation from the norm, progress is not possible.