Re: [PATCH v4 10/12] KVM: x86: never write to memory from kvm_vcpu_check_block()

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On Tue, 2023-12-12 at 07:28 -0800, Sean Christopherson wrote:
> On Sun, Dec 10, 2023, Jim Mattson wrote:
> > On Thu, Dec 7, 2023 at 8:21 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > Doh.  We got the less obvious cases and missed the obvious one.
> > > 
> > > Ugh, and we also missed a related mess in kvm_guest_apic_has_interrupt().  That
> > > thing should really be folded into vmx_has_nested_events().
> > > 
> > > Good gravy.  And vmx_interrupt_blocked() does the wrong thing because that
> > > specifically checks if L1 interrupts are blocked.
> > > 
> > > Compile tested only, and definitely needs to be chunked into multiple patches,
> > > but I think something like this mess?
> > 
> > The proposed patch does not fix the problem. In fact, it messes things
> > up so much that I don't get any test results back.
> Drat.
> > Google has an internal K-U-T test that demonstrates the problem. I
> > will post it soon.
> Received, I'll dig in soonish, though "soonish" might unfortunately might mean
> 2024.


So this is what I think:

KVM does have kvm_guest_apic_has_interrupt() for this exact purpose,
to check if nested APICv has a pending interrupt before halting.

However the problem is bigger - with APICv we have in essence 2 pending interrupt
bitmaps - the PIR and the IRR, and to know if the guest has a pending interrupt
one has in theory to copy PIR to IRR, then see if the max is larger then the current PPR.

Since we don't want to write to guest memory, and the IRR here resides in the guest memory,
I guess we have to do a 'dry-run' version of 'vmx_complete_nested_posted_interrupt' and call
it from  kvm_guest_apic_has_interrupt().

What do you think? I can prepare a patch for this.

Can you share a reproducer or write a new one that can be shared?

Best regards,
	Maxim Levitsky

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