linux-kernel - Re: [PATCH 3/7] KVM: x86/pmu: Avoid setting BIT_ULL(-1) to pmu->host_cross_mapped

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Message-ID: <YtihtuxO/uefpAqJ@google.com>
Date:   Thu, 21 Jul 2022 00:45:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Like Xu <likexu@...cent.com>
Subject: Re: [PATCH 3/7] KVM: x86/pmu: Avoid setting BIT_ULL(-1) to
 pmu->host_cross_mapped_mask

On Wed, Jul 13, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> In the extreme case of host counters multiplexing and contention, the
> perf_event requested by the guest's pebs counter is not allocated to any
> actual physical counter, in which case hw.idx is bookkept as -1,
> resulting in an out-of-bounds access to host_cross_mapped_mask.
> 
> Fixes: 854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 53ccba896e77..1588627974fa 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -783,20 +783,19 @@ static void intel_pmu_cleanup(struct kvm_vcpu *vcpu)
>  void intel_pmu_cross_mapped_check(struct kvm_pmu *pmu)
>  {
>  	struct kvm_pmc *pmc = NULL;
> -	int bit;
> +	int bit, hw_idx;
>  
>  	for_each_set_bit(bit, (unsigned long *)&pmu->global_ctrl,
>  			 X86_PMC_IDX_MAX) {
>  		pmc = intel_pmc_idx_to_pmc(pmu, bit);
>  
>  		if (!pmc || !pmc_speculative_in_use(pmc) ||
> -		    !intel_pmc_is_enabled(pmc))
> +		    !intel_pmc_is_enabled(pmc) || !pmc->perf_event)
>  			continue;
>  
> -		if (pmc->perf_event && pmc->idx != pmc->perf_event->hw.idx) {
> -			pmu->host_cross_mapped_mask |=
> -				BIT_ULL(pmc->perf_event->hw.idx);
> -		}
> +		hw_idx = pmc->perf_event->hw.idx;
> +		if (hw_idx != pmc->idx && hw_idx != -1)

How about "hw_idx > 0" so that KVM is a little less dependent on perf's exact
behavior?  A comment here would be nice too.

> +			pmu->host_cross_mapped_mask |= BIT_ULL(hw_idx);
>  	}
>  }
>  
> -- 
> 2.37.0
>