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

[Oliver Upton <oupton@google.com>]

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.

> +           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) {

> +               /*
> +                * 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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