Re: [PATCH] KVM: SVM: fix cr8 intercept window

["Radim Krčmář" <rkrcmar@redhat.com>]

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@vger.kernel.org>
> > Signed-off-by: Radim Krčmář <rkrcmar@redhat.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.

Definitely should, and we will set KVM_REQ_EVENT through kvm_set_cr8()
if we lower the TPR.
(I checked that the bug isn't in apic_update_ppr().)

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

It is intended, I suppose this is because we run with V_INTR_MASKING, so
writes to CR8 only affect V_TPR register; guest then raises it once more
and APIC incorrectly gives us low priority interrupt.