linux-kernel - Re: [PATCH MANUALSEL 5.15 2/2] KVM: x86: nSVM: deal with L1 hypervisor that intercepts interrupts but lets L2 control them

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Date:   Tue, 1 Mar 2022 18:10:24 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.15 2/2] KVM: x86: nSVM: deal with L1
 hypervisor that intercepts interrupts but lets L2 control them

On 2/22/22 15:05, Sasha Levin wrote:
> From: Maxim Levitsky <mlevitsk@...hat.com>
> 
> [ Upstream commit 2b0ecccb55310a4b8ad5d59c703cf8c821be6260 ]
> 
> 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>
> Message-Id: <20220207155447.840194-8-mlevitsk@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   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 f05aa7290267d..aeee6df523af9 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3465,11 +3465,13 @@ 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)
> @@ -3521,9 +3523,13 @@ 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.
> @@ -3531,7 +3537,7 @@ 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)
> @@ -4286,11 +4292,14 @@ 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)


Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo