linux-kernel - Re: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when guest doesn't have X86_FEATURE_PERFCTR

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Message-ID: <874kgubau4.fsf@vitty.brq.redhat.com>
Date:   Mon, 29 Mar 2021 10:52:51 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wei Huang <wei.huang2@....com>, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs
 when guest doesn't have X86_FEATURE_PERFCTR_CORE

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 23/03/21 09:45, Vitaly Kuznetsov wrote:
>> MSR_F15H_PERF_CTL0-5, MSR_F15H_PERF_CTR0-5 MSRs are only available when
>> X86_FEATURE_PERFCTR_CORE CPUID bit was exposed to the guest. KVM, however,
>> allows these MSRs unconditionally because kvm_pmu_is_valid_msr() ->
>> amd_msr_idx_to_pmc() check always passes and because kvm_pmu_set_msr() ->
>> amd_pmu_set_msr() doesn't fail.
>> 
>> In case of a counter (CTRn), no big harm is done as we only increase
>> internal PMC's value but in case of an eventsel (CTLn), we go deep into
>> perf internals with a non-existing counter.
>> 
>> Note, kvm_get_msr_common() just returns '0' when these MSRs don't exist
>> and this also seems to contradict architectural behavior which is #GP
>> (I did check one old Opteron host) but changing this status quo is a bit
>> scarier.
>
> Hmm, since these do have a cpuid bit it may not be that scary.

Well, if you're not scared I can send a patch)

-- 
Vitaly