Re: [PATCH v2 1/5] KVM: nSVM: deal with L1 hypervisor that intercepts interrupts but lets L2 control EFLAGS.IF

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2021-12-13 at 12:34 +0100, Paolo Bonzini wrote:
> On 12/13/21 11:46, Maxim Levitsky wrote:
> > Fix a corner case in which L1 hypervisor intercepts interrupts (INTERCEPT_INTR)
> > and either doesn't use virtual interrupt masking (V_INTR_MASKING) or
> > enters a nested guest with EFLAGS.IF disabled prior to the entry.
> > 
> > In this case, despite the fact that L1 intercepts the interrupts,
> > KVM still needs to set up an interrupt window to wait before it
> > can deliver INTR vmexit.
> > 
> > Currently instead, the KVM enters an endless loop of 'req_immediate_exit'.
> > 
> > Note that on VMX this case is impossible as there is only
> > 'vmexit on external interrupts' execution control which either set,
> > in which case both host and guest's EFLAGS.IF
> > is ignored, or clear, in which case no VMexit is delivered.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >   arch/x86/kvm/svm/svm.c | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index e57e6857e0630..c9668a3b51011 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -3372,17 +3372,21 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
> >   static int svm_interrupt_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> >   {
> >   	struct vcpu_svm *svm = to_svm(vcpu);
> > +	bool blocked;
> > +
> >   	if (svm->nested.nested_run_pending)
> >   		return -EBUSY;
> >   
> > +	blocked = svm_interrupt_blocked(vcpu);
> > +
> >   	/*
> >   	 * An IRQ must not be injected into L2 if it's supposed to VM-Exit,
> >   	 * e.g. if the IRQ arrived asynchronously after checking nested events.
> >   	 */
> >   	if (for_injection && is_guest_mode(vcpu) && nested_exit_on_intr(svm))
> > -		return -EBUSY;
> > -
> > -	return !svm_interrupt_blocked(vcpu);
> > +		return !blocked ? -EBUSY : 0;
> > +	else
> > +		return !blocked;
> >   }
> >   
> >   static void svm_enable_irq_window(struct kvm_vcpu *vcpu)
> > 
> 
> Right, another case is when CLGI is not trapped and the guest therefore
> runs with GIF=0.  I think that means that a similar change has to be
> done in all the *_allowed functions.

I think that SVM sets real GIF to 1 on VMentry regardless if it is trapped or not.

However if not trapped, and neither EFLAGS.IF is trapped, one could enter a guest
that has EFLAGS.IF == 0, then the guest could disable GIF, enable EFLAGS.IF,
and then enable GIF, but then GIF enablement should trigger out interrupt window
VINTR as well.


> 
> I would write it as
> 
>    	if (svm->nested.nested_run_pending)
>    		return -EBUSY;
>    
> 	if (svm_interrupt_blocked(vcpu))
> 		return 0;
> 
>    	/*
>    	 * An IRQ must not be injected into L2 if it's supposed to VM-Exit,
>    	 * e.g. if the IRQ arrived asynchronously after checking nested events.
>    	 */
>    	if (for_injection && is_guest_mode(vcpu) && nested_exit_on_intr(svm))
> 		return -EBUSY;
> 	return 1;

Thanks! I was worried to not break the non nested case but looking again at the code,
it is logically equivalent. 


Thanks for the review,
	Best regards,
		Maxim Levitsky

> 
> Paolo
>