Re: [Question] int3 instruction generates a #UD in SEV VM

[Tom Lendacky]

On 8/2/23 10:04, Sean Christopherson wrote:
> On Wed, Aug 02, 2023, Tom Lendacky wrote:
> > On 8/2/23 09:25, Tom Lendacky wrote:
> > > On 8/2/23 09:01, Sean Christopherson wrote:
> > > > > You're right. The #UD is injected by KVM.
> > > > >
> > > > > The path I found is:
> > > > >       svm_vcpu_run
> > > > >           svm_complete_interrupts
> > > > >          kvm_requeue_exception // vector = 3
> > > > >              kvm_make_request
> > > > >
> > > > >       vcpu_enter_guest
> > > > >           kvm_check_and_inject_events
> > > > >          svm_inject_exception
> > > > >              svm_update_soft_interrupt_rip
> > > > >              __svm_skip_emulated_instruction
> > > > >                  x86_emulate_instruction
> > > > >                  svm_can_emulate_instruction
> > > > >                      kvm_queue_exception(vcpu, UD_VECTOR)
> > > > >
> > > > > Does this mean a #PF intercept occur when the guest try to deliver a
> > > > > #BP through the IDT? But why?
> > > >
> > > > I doubt it's a #PF.  A #NPF is much more likely, though it could be
> > > > something
> > > > else entirely, but I'm pretty sure that would require bugs in both
> > > > the host and
> > > > guest.
> > > >
> > > > What is the last exit recorded by trace_kvm_exit() before the #UD is
> > > > injected?
> > >
> > > I'm guessing it was a #NPF, too. Could it be related to the changes that
> > > went in around svm_update_soft_interrupt_rip()?
> > >
> > > 6ef88d6e36c2 ("KVM: SVM: Re-inject INT3/INTO instead of retrying the
> > > instruction")
> >
> > Sorry, that should have been:
> >
> > 7e5b5ef8dca3 ("KVM: SVM: Re-inject INTn instead of retrying the insn on "failure"")
> >
> > > Before this the !nrips check would prevent the call into
> > > svm_skip_emulated_instruction(). But now, there is a call to:
> > >
> > >     svm_update_soft_interrupt_rip()
> > >       __svm_skip_emulated_instruction()
> > >         kvm_emulate_instruction()
> > >           x86_emulate_instruction() (passed a NULL insn pointer)
> > >             kvm_can_emulate_insn() (passed a NULL insn pointer)
> > >               svm_can_emulate_instruction() (passed NULL insn pointer)
> > >
> > > Because it is an SEV guest, it ends up in the "if (unlikely(!insn))" path
> > > and injects the #UD.
>
> Yeah, my money is on that too.  I believe this is the least awful solution:
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d381ad424554..2eace114a934 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -385,6 +385,9 @@ static int __svm_skip_emulated_instruction(struct kvm_vcpu *vcpu,
>          }
>   
>          if (!svm->next_rip) {
> +               if (sev_guest(vcpu->kvm))
> +                       return 0;
> +
>                  if (unlikely(!commit_side_effects))
>                          old_rflags = svm->vmcb->save.rflags;
>   
> I'll send a formal patch (with a comment) if that solves the problem.
>
>
> Side topic, KVM should require nrips for SEV and beyond, I don't see how SEV can
> possibly work if KVM doesn't utilize nrips.  E.g. this
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 2eace114a934..43e500503d48 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5111,9 +5111,11 @@ static __init int svm_hardware_setup(void)
>   
>          svm_adjust_mmio_mask();
>   
> +       nrips = nrips && boot_cpu_has(X86_FEATURE_NRIPS);
> +
>          /*
>           * Note, SEV setup consumes npt_enabled and enable_mmio_caching (which
> -        * may be modified by svm_adjust_mmio_mask()).
> +        * may be modified by svm_adjust_mmio_mask()), as well as nrips.
>           */
>          sev_hardware_setup();

You moved the setting of nrips up, I'm assuming you then want to add a 
check in sev_hardware_setup() for nrips?

Thanks,
Tom

>   
> @@ -5125,11 +5127,6 @@ static __init int svm_hardware_setup(void)
>                          goto err;
>          }
>   
> -       if (nrips) {
> -               if (!boot_cpu_has(X86_FEATURE_NRIPS))
> -                       nrips = false;
> -       }
> -
>          enable_apicv = avic = avic && avic_hardware_setup();
>   
>          if (!enable_apicv) {