linux-kernel - Re: [PATCH] KVM: x86: Check for nested events if there is an injectable interrupt

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Date:	Tue, 08 Jul 2014 07:50:58 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>, kvm@...r.kernel.org
CC:	linux-kernel@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH] KVM: x86: Check for nested events if there is an injectable
 interrupt

Il 08/07/2014 06:30, Bandan Das ha scritto:
>
> With commit b6b8a1451fc40412c57d1 that introduced
> vmx_check_nested_events, checks for injectable interrupts happen
> at different points in time for L1 and L2 that could potentially
> cause a race. The regression occurs because KVM_REQ_EVENT is always
> set when nested_run_pending is set even if there's no pending interrupt.
> Consequently, there could be a small window when check_nested_events
> returns without exiting to L1, but an interrupt comes through soon
> after and it incorrectly, gets injected to L2 by inject_pending_event
> Fix this by adding a call to check for nested events too when a check
> for injectable interrupt returns true
>
> Signed-off-by: Bandan Das <bsd@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 73537ec..56327a6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5907,6 +5907,19 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>  			kvm_x86_ops->set_nmi(vcpu);
>  		}
>  	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
> +		/*
> +		 * TODO/FIXME: We are calling check_nested_events again
> +		 * here to avoid a race condition. We should really be
> +		 * setting KVM_REQ_EVENT only on certain events
> +		 * and not unconditionally.
> +		 * See https://lkml.org/lkml/2014/7/2/60 for discussion
> +		 * about this proposal and current concerns
> +		 */
> +		if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) {
> +			r = kvm_x86_ops->check_nested_events(vcpu, req_int_win);
> +			if (r != 0)
> +				return r;
> +		}
>  		if (kvm_x86_ops->interrupt_allowed(vcpu)) {
>  			kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
>  					    false);
>

I think this should be done for NMI as well.

Jan, what do you think?  Can you run Jailhouse through this patch?

Paolo
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