Re: [PATCH 1/2] KVM: vmx/pmu: Fix dummy check if lbr_desc->event is created

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 24, 2021, Xu, Like wrote:
> On 2021/2/24 1:15, Sean Christopherson wrote:
> > On Tue, Feb 23, 2021, Like Xu wrote:
> > > If lbr_desc->event is successfully created, the intel_pmu_create_
> > > guest_lbr_event() will return 0, otherwise it will return -ENOENT,
> > > and then jump to LBR msrs dummy handling.
> > > 
> > > Fixes: 1b5ac3226a1a ("KVM: vmx/pmu: Pass-through LBR msrs when the guest LBR event is ACTIVE")
> > > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > > ---
> > >   arch/x86/kvm/vmx/pmu_intel.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > > index d1df618cb7de..d6a5fe19ff09 100644
> > > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > > @@ -320,7 +320,7 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
> > >   	if (!intel_pmu_is_valid_lbr_msr(vcpu, index))
> > >   		return false;
> > > -	if (!lbr_desc->event && !intel_pmu_create_guest_lbr_event(vcpu))
> > > +	if (!lbr_desc->event && intel_pmu_create_guest_lbr_event(vcpu))
> > >   		goto dummy;

...
 
> > AFAICT, event contention would lead to a #GP crash in the host due to
> > lbr_desc->event being dereferenced, no?
> 
> a #GP crash in the host ？Can you share more understanding about it ?

The original code is will dereference a null lbr_desc->event if
intel_pmu_create_guest_lbr_event() fails.

	if (!lbr_desc->event && intel_pmu_create_guest_lbr_event(vcpu))  <- falls through
		goto dummy;

	/*
	 * Disable irq to ensure the LBR feature doesn't get reclaimed by the
	 * host at the time the value is read from the msr, and this avoids the
	 * host LBR value to be leaked to the guest. If LBR has been reclaimed,
	 * return 0 on guest reads.
	 */
	local_irq_disable();
	if (lbr_desc->event->state == PERF_EVENT_STATE_ACTIVE) { <--------- kaboom
		if (read)
			rdmsrl(index, msr_info->data);
		else
			wrmsrl(index, msr_info->data);
		__set_bit(INTEL_PMC_IDX_FIXED_VLBR, vcpu_to_pmu(vcpu)->pmc_in_use);
		local_irq_enable();
		return true;
	}