linux-kernel - Re: [PATCH] KVM: SVM: fix cr8 intercept window

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Date:	Tue, 11 Mar 2014 22:05:14 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com, gleb@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: fix cr8 intercept window

On Tue, Mar 11, 2014 at 07:11:18PM +0100, Radim Krčmář wrote:
> We always disable cr8 intercept in its handler, but only re-enable it
> if handling KVM_REQ_EVENT, so there can be a window where we do not
> intercept cr8 writes, which allows an interrupt to disrupt a higher
> priority task.
> 
> Fix this by disabling intercepts in the same function that re-enables
> them when needed. This fixes BSOD in Windows 2008.
> 
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/x86/kvm/svm.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 64d9bb9..f676c18 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3003,10 +3003,8 @@ static int cr8_write_interception(struct vcpu_svm *svm)
>  	u8 cr8_prev = kvm_get_cr8(&svm->vcpu);
>  	/* instruction emulation calls kvm_set_cr8() */
>  	r = cr_interception(svm);
> -	if (irqchip_in_kernel(svm->vcpu.kvm)) {
> -		clr_cr_intercept(svm, INTERCEPT_CR8_WRITE);
> +	if (irqchip_in_kernel(svm->vcpu.kvm))
>  		return r;
> -	}
>  	if (cr8_prev <= kvm_get_cr8(&svm->vcpu))
>  		return r;
>  	kvm_run->exit_reason = KVM_EXIT_SET_TPR;
> @@ -3568,6 +3566,8 @@ static void update_cr8_intercept(struct kvm_vcpu *vcpu, int tpr, int irr)
>  	if (is_guest_mode(vcpu) && (vcpu->arch.hflags & HF_VINTR_MASK))
>  		return;
>  
> +	clr_cr_intercept(svm, INTERCEPT_CR8_WRITE);
> +
>  	if (irr == -1)
>  		return;

Shouldnt IRR be injected if TPR < IRR ? (via KVM_REQ_EVENT).

1) IRR has interrupt 10.
2) TPR now 9 due to CR8 write.
3) 10 should be injected.

Also not clearing the intercept can cause continuous CR8 writes to 
exit until KVM_REQ_EVENT ? 

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