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

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Date:   Tue, 8 Feb 2022 12:33:31 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Tony Luck <tony.luck@...el.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Sean Christopherson <seanjc@...gle.com>,
        David Airlie <airlied@...ux.ie>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Daniel Vetter <daniel@...ll.ch>,
        Borislav Petkov <bp@...en8.de>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH RESEND 07/30] KVM: x86: nSVM: deal with L1 hypervisor that
 intercepts interrupts but lets L2 control them

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@...hat.com>
> ---
>   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?

Thanks,

Paolo