Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 31, 2022, Maciej S. Szmigiero wrote:
> On 30.03.2022 23:59, Sean Christopherson wrote:
> > On Thu, Mar 10, 2022, Maciej S. Szmigiero wrote:
> > > @@ -3627,6 +3632,14 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
> > >   	if (!(exitintinfo & SVM_EXITINTINFO_VALID))
> > >   		return;
> > > +	/* L1 -> L2 event re-injection needs a different handling */
> > > +	if (is_guest_mode(vcpu) &&
> > > +	    exit_during_event_injection(svm, svm->nested.ctl.event_inj,
> > > +					svm->nested.ctl.event_inj_err)) {
> > > +		nested_svm_maybe_reinject(vcpu);
> > 
> > Why is this manually re-injecting?  More specifically, why does the below (out of
> > sight in the diff) code that re-queues the exception/interrupt not work?  The
> > re-queued event should be picked up by nested_save_pending_event_to_vmcb12() and
> > propagatred to vmcb12.
> 
> A L1 -> L2 injected event should either be re-injected until successfully
> injected into L2 or propagated to VMCB12 if there is a nested VMEXIT
> during its delivery.
> 
> svm_complete_interrupts() does not do such re-injection in some cases
> (soft interrupts, soft exceptions, #VC) - it is trying to resort to
> emulation instead, which is incorrect in this case.
> 
> I think it's better to split out this L1 -> L2 nested case to a
> separate function in nested.c rather than to fill
> svm_complete_interrupts() in already very large svm.c with "if" blocks
> here and there.

Ah, I see it now.  WTF.

Ugh, commit 66fd3f7f901f ("KVM: Do not re-execute INTn instruction.") fixed VMX,
but left SVM broken.

Re-executing the INTn is wrong, the instruction has already completed decode and
execution.  E.g. if there's there's a code breakpoint on the INTn, rewinding will
cause a spurious #DB.

KVM's INT3 shenanigans are bonkers, but I guess there's no better option given
that the APM says "Software interrupts cannot be properly injected if the processor
does not support the NextRIP field.".  What a mess.

Anyways, for the common nrips=true case, I strongly prefer that we properly fix
the non-nested case and re-inject software interrupts, which should in turn
naturally fix this nested case.  And for nrips=false, my vote is to either punt
and document it as a "KVM erratum", or straight up make nested require nrips.

Note, that also requires updating svm_queue_exception(), which assumes it will
only be handed hardware exceptions, i.e. hardcodes type EXEPT.  That's blatantly
wrong, e.g. if userspace injects a software exception via KVM_SET_VCPU_EVENTS.