Re: [PATCH v3 14/15] KVM: arm64: Handle protected guests at 32 bits

[Oliver Upton <oupton@google.com>]

On Mon, Jul 19, 2021 at 9:04 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.
>
> Adapted from commit 22f553842b14 ("KVM: arm64: Handle Asymmetric
> AArch32 systems")
>
> Signed-off-by: Fuad Tabba <tabba@google.com>

Would it make sense to document how we handle misbehaved guests, in
case a particular VMM wants to clean up the mess afterwards?

--
Thanks,
Oliver

> ---
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 8431f1514280..f09343e15a80 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -23,6 +23,7 @@
>  #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>
> @@ -477,6 +478,29 @@ static inline bool fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
>                         write_sysreg_el2(read_sysreg_el2(SYS_ELR) - 4, SYS_ELR);
>         }
>
> +       /*
> +        * 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 doesn't 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.
> +        */
> +       if (unlikely(is_nvhe_hyp_code() &&
> +                    kvm_vm_is_protected(kern_hyp_va(vcpu->kvm)) &&
> +                    FIELD_GET(FEATURE(ID_AA64PFR0_EL0),
> +                              PVM_ID_AA64PFR0_ALLOW) <
> +                            ID_AA64PFR0_ELx_32BIT_64BIT &&
> +                    vcpu_mode_is_32bit(vcpu))) {
> +               /*
> +                * As we have caught the guest red-handed, decide that it isn't
> +                * fit for purpose anymore by making the vcpu invalid.
> +                */
> +               vcpu->arch.target = -1;
> +               *exit_code = ARM_EXCEPTION_IL;
> +               goto exit;
> +       }
> +
>         /*
>          * We're using the raw exception code in order to only process
>          * the trap if no SError is pending. We will come back to the
> --
> 2.32.0.402.g57bb445576-goog
>
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