linux-kernel - Re: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi

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Message-ID: <Y3IFo9NrAcYalBzM@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Nov 2022 10:08:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Li, Xin3" <xin3.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq()
 for NMI/IRQ reinjection

On Mon, Nov 14, 2022 at 04:39:40AM +0000, Li, Xin3 wrote:

> > 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?

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