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

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Date:	Thu, 13 Mar 2014 15:52:06 +0200
From:	Gleb Natapov <gleb@...nel.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Radim Krčmář <rkrcmar@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: fix cr8 intercept window

On Wed, Mar 12, 2014 at 06:20:01PM +0100, Paolo Bonzini wrote:
> Il 12/03/2014 11:40, Radim Krčmář ha scritto:
> >2014-03-11 22:05-0300, Marcelo Tosatti:
> >>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;
> >>>-	}
> 
> I think that the old code here makes little sense, and for two reasons:
> 
I agree that old code is wrong and the patch looks correct, but I only
see how the bug may cause pending IRR to not be delivered in time,
not how interrupt can disrupt a higher priority task.

--
			Gleb.
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