Re: [PATCH v6 12/12] KVM: arm64: Handle protected guests at 32 bits

[Fuad Tabba <tabba@google.com>]

Hi Marc,

On Tue, Oct 5, 2021 at 9:48 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Wed, 22 Sep 2021 13:47:04 +0100,
> Fuad Tabba <tabba@google.com> wrote:
> >
> > Protected KVM does not support protected AArch32 guests. However,
> > it is possible for the guest to force run AArch32, potentially
> > causing problems. Add an extra check so that if the hypervisor
> > catches the guest doing that, it can prevent the guest from
> > running again by resetting vcpu->arch.target and returning
> > ARM_EXCEPTION_IL.
> >
> > If this were to happen, The VMM can try and fix it by re-
> > initializing the vcpu with KVM_ARM_VCPU_INIT, however, this is
> > likely not possible for protected VMs.
> >
> > Adapted from commit 22f553842b14 ("KVM: arm64: Handle Asymmetric
> > AArch32 systems")
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/switch.c | 40 ++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> > index 2bf5952f651b..d66226e49013 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> > @@ -235,6 +235,43 @@ static const exit_handler_fn *kvm_get_exit_handler_array(struct kvm *kvm)
> >       return hyp_exit_handlers;
> >  }
> >
> > +/*
> > + * Some guests (e.g., protected VMs) might not be allowed to run in AArch32.
> > + * The ARMv8 architecture does not give the hypervisor a mechanism to prevent a
> > + * guest from dropping to AArch32 EL0 if implemented by the CPU. If the
> > + * hypervisor spots a guest in such a state ensure it is handled, and don't
> > + * trust the host to spot or fix it.  The check below is based on the one in
> > + * kvm_arch_vcpu_ioctl_run().
> > + *
> > + * Returns false if the guest ran in AArch32 when it shouldn't have, and
> > + * thus should exit to the host, or true if a the guest run loop can continue.
> > + */
> > +static bool handle_aarch32_guest(struct kvm_vcpu *vcpu, u64 *exit_code)
> > +{
> > +     struct kvm *kvm = (struct kvm *) kern_hyp_va(vcpu->kvm);
>
> There is no need for an extra cast. kern_hyp_va() already provides a
> cast to the type of the parameter.

Will drop it.

> > +     bool is_aarch32_allowed =
> > +             FIELD_GET(ARM64_FEATURE_MASK(ID_AA64PFR0_EL0),
> > +                       get_pvm_id_aa64pfr0(vcpu)) >=
> > +                             ID_AA64PFR0_ELx_32BIT_64BIT;
> > +
> > +
> > +     if (kvm_vm_is_protected(kvm) &&
> > +         vcpu_mode_is_32bit(vcpu) &&
> > +         !is_aarch32_allowed) {
>
> Do we really need to go through this is_aarch32_allowed check?
> Protected VMs don't have AArch32, and we don't have the infrastructure
> to handle 32bit at all. For non-protected VMs, we already have what we
> need at EL1. So the extra check only adds complexity.

No. I could change it to a build-time assertion just to make sure that
AArch32 is not allowed instead.

Thanks,
/fuad

> > +             /*
> > +              * As we have caught the guest red-handed, decide that it isn't
> > +              * fit for purpose anymore by making the vcpu invalid. The VMM
> > +              * can try and fix it by re-initializing the vcpu with
> > +              * KVM_ARM_VCPU_INIT, however, this is likely not possible for
> > +              * protected VMs.
> > +              */
> > +             vcpu->arch.target = -1;
> > +             *exit_code = ARM_EXCEPTION_IL;
> > +             return false;
> > +     }
> > +
> > +     return true;
> > +}
> > +
> >  /* Switch to the guest for legacy non-VHE systems */
> >  int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >  {
> > @@ -297,6 +334,9 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >               /* Jump in the fire! */
> >               exit_code = __guest_enter(vcpu);
> >
> > +             if (unlikely(!handle_aarch32_guest(vcpu, &exit_code)))
> > +                     break;
> > +
> >               /* And we're baaack! */
> >       } while (fixup_guest_exit(vcpu, &exit_code));
> >
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.
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