RE: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection

Li, Xin3
2022-11-15T07:51:01Z

> > > But what about NMIs, afaict this is all horribly broken for NMIs.
> > >
> > > So the whole VMX thing latches the NMI (which stops NMI recursion),
> right?
> > >
> > > But then you drop out of noinstr code, which means any random
> > > exception can happen (kprobes #BP, hw_breakpoint #DB, or even #PF
> > > due to random nonsense like *SAN). This exception will do IRET and
> > > clear the NMI latch, all before you get to run any of the NMI code.
> >
> > What you said here implies that we have this problem in the existing code.
> > Because a fake iret stack is created to call the NMI handler in the
> > IDT NMI descriptor, which lastly executes the IRET instruction.
>
> I can't follow; of course the IDT handler terminates with IRET, it has to no?

With FRED, ERETS/ERETU replace IRET, and use bit 28 of the popped CS field
to control whether to unblock NMI. If bit 28 of the field (above the selector)
is 1, ERETS/ERETU unblocks NMIs.

>
> And yes, the current code appears to suffer the same defect.