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

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Date:   Mon, 13 Dec 2021 12:34:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 1/5] KVM: nSVM: deal with L1 hypervisor that intercepts
 interrupts but lets L2 control EFLAGS.IF

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@...hat.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 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;

Paolo