Re: [PATCH v4 15/15] KVM: arm64: Handle protected guests at 32 bits

[Fuad Tabba <tabba@google.com>]

Hi Oliver,

On Thu, Aug 19, 2021 at 9:10 AM Oliver Upton <oupton@google.com> wrote:
>
> Hi Fuad,
>
> On Tue, Aug 17, 2021 at 1:12 AM 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 | 37 ++++++++++++++++++++++++++++++++
> >  1 file changed, 37 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> > index 398e62098898..0c24b7f473bf 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> > @@ -20,6 +20,7 @@
> >  #include <asm/kprobes.h>
> >  #include <asm/kvm_asm.h>
> >  #include <asm/kvm_emulate.h>
> > +#include <asm/kvm_fixed_config.h>
> >  #include <asm/kvm_hyp.h>
> >  #include <asm/kvm_mmu.h>
> >  #include <asm/fpsimd.h>
> > @@ -195,6 +196,39 @@ exit_handle_fn kvm_get_nvhe_exit_handler(struct kvm_vcpu *vcpu)
> >                 return NULL;
> >  }
> >
> > +/*
> > + * Some guests (e.g., protected VMs) might not be allowed to run in AArch32. The
> > + * check below is based on the one in kvm_arch_vcpu_ioctl_run().
> > + * 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.
> > + *
> > + * Returns true if the check passed and the guest run loop can continue, or
> > + * false if the guest should exit to the host.
> > + */
> > +static bool check_aarch32_guest(struct kvm_vcpu *vcpu, u64 *exit_code)
>
> This does a bit more than just check & return, so maybe call it
> handle_aarch32_guest()?
>
> > +{
> > +       if (kvm_vm_is_protected(kern_hyp_va(vcpu->kvm)) &&
>
> maybe initialize a local with a hyp pointer to the kvm structure.

Will do.

> > +           vcpu_mode_is_32bit(vcpu) &&
> > +           FIELD_GET(ARM64_FEATURE_MASK(ID_AA64PFR0_EL0),
> > +                                        PVM_ID_AA64PFR0_RESTRICT_UNSIGNED) <
> > +               ID_AA64PFR0_ELx_32BIT_64BIT) {
>
> It may be more readable to initialize a local variable with this
> feature check, i.e:
>
> bool aarch32_allowed = FIELD_GET(...) == ID_AA64PFR0_ELx_32BIT_64BIT;
>
> and then:
>
>   if (kvm_vm_is_protected(kvm) && vcpu_mode_is_32bit(vcpu) &&
> !aarch32_allowed) {

I agree.

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)
> >  {
> > @@ -255,6 +289,9 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> >                 /* Jump in the fire! */
> >                 exit_code = __guest_enter(vcpu);
> >
> > +               if (unlikely(!check_aarch32_guest(vcpu, &exit_code)))
> > +                       break;
> > +
> >                 /* And we're baaack! */
> >         } while (fixup_guest_exit(vcpu, &exit_code));
> >
> > --
> > 2.33.0.rc1.237.g0d66db33f3-goog
> >

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