Re: [PATCH RESEND 07/30] KVM: x86: nSVM: deal with L1 hypervisor that intercepts interrupts but lets L2 control them

[Maxim Levitsky]

On Tue, 2022-02-08 at 12:33 +0100, Paolo Bonzini wrote:
> On 2/7/22 16:54, Maxim Levitsky wrote:
> > Fix a corner case in which the L1 hypervisor intercepts
> > interrupts (INTERCEPT_INTR) and either doesn't set
> > 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
> > injecting the INTR vmexit.
> > 
> > Currently the KVM instead enters an endless loop of 'req_immediate_exit'.
> > 
> > Exactly the same issue also happens for SMIs and NMI.
> > Fix this as well.
> > 
> > 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
> > are ignored, or not set, in which case no VMexits are delivered.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/svm/svm.c | 17 +++++++++++++----
> >   1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 9a4e299ed5673..22e614008cf59 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -3372,11 +3372,13 @@ static int svm_nmi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> >   	if (svm->nested.nested_run_pending)
> >   		return -EBUSY;
> >   
> > +	if (svm_nmi_blocked(vcpu))
> > +		return 0;
> > +
> >   	/* An NMI must not be injected into L2 if it's supposed to VM-Exit.  */
> >   	if (for_injection && is_guest_mode(vcpu) && nested_exit_on_nmi(svm))
> >   		return -EBUSY;
> > -
> > -	return !svm_nmi_blocked(vcpu);
> > +	return 1;
> >   }
> >   
> >   static bool svm_get_nmi_mask(struct kvm_vcpu *vcpu)
> > @@ -3428,9 +3430,13 @@ 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);
> > +
> >   	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.
> > @@ -3438,7 +3444,7 @@ static int svm_interrupt_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> >   	if (for_injection && is_guest_mode(vcpu) && nested_exit_on_intr(svm))
> >   		return -EBUSY;
> >   
> > -	return !svm_interrupt_blocked(vcpu);
> > +	return 1;
> >   }
> >   
> >   static void svm_enable_irq_window(struct kvm_vcpu *vcpu)
> > @@ -4169,11 +4175,14 @@ static int svm_smi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
> >   	if (svm->nested.nested_run_pending)
> >   		return -EBUSY;
> >   
> > +	if (svm_smi_blocked(vcpu))
> > +		return 0;
> > +
> >   	/* An SMI must not be injected into L2 if it's supposed to VM-Exit.  */
> >   	if (for_injection && is_guest_mode(vcpu) && nested_exit_on_smi(svm))
> >   		return -EBUSY;
> >   
> > -	return !svm_smi_blocked(vcpu);
> > +	return 1;
> >   }
> >   
> >   static int svm_enter_smm(struct kvm_vcpu *vcpu, char *smstate)
> 
> Can you prepare a testcase for at least the interrupt case?


Yep, I already wrote a kvm unit tests for all the cases, and I will send them very soon.

Best regards,
	Maxim Levitsky
> 
> Thanks,
> 
> Paolo
>