linux-kernel - Re: [PATCH v2 1/4] KVM: vmx/pmu: Add MSR_ARCH_LBR

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Date:   Mon, 1 Mar 2021 14:34:00 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH emulation
 for Arch LBR

On Wed, Feb 03, 2021, Like Xu wrote:
> @@ -348,10 +352,26 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>  	return true;
>  }
>  
> +/*
> + * Check if the requested depth values is supported
> + * based on the bits [0:7] of the guest cpuid.1c.eax.
> + */
> +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
> +	if (depth && best)

> +		return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));

I believe this will genereate undefined behavior if depth > 64.  Or if depth < 8.
And I believe this check also needs to enforce that depth is a multiple of 8.

   For each bit n set in this field, the IA32_LBR_DEPTH.DEPTH value 8*(n+1) is
   supported.

Thus it's impossible for 0-7, 9-15, etc... to be legal depths.


> +
> +	return false;
> +}
> +