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

[Fuad Tabba]

Hi Will,


On Thu, Aug 12, 2021 at 11:57 AM Will Deacon <will at kernel.org> wrote:
>
> On Mon, Jul 19, 2021 at 05:03:45PM +0100, Fuad Tabba 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 at google.com>
> > ---
> >  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;
> > +     }
>
> Would this be better off inside the nvhe-specific run loop? Seems like we
> could elide fixup_guest_exit() altogether if we've detect that we're in
> AArch32 state when we shouldn't be and it would keep the code off the shared
> path.

Yes, it makes more sense and would result in cleaner code to have it
there, especially in the future where there's likely to be a separate
run loop for protected VMs. I'll move it.

Thanks,
/fuad
> Will