linux-kernel - Re: [PATCH v4 15/38] KVM: x86/pmu: Check PMU cpuid configuration from user space

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Message-ID: <d54e8654-d761-47aa-b9f7-10d8c44a07ec@linux.intel.com>
Date: Thu, 15 May 2025 11:00:16 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com,
 Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Yongwei Ma <yongwei.ma@...el.com>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Jim Mattson <jmattson@...gle.com>, Sandipan Das <sandipan.das@....com>,
 Zide Chen <zide.chen@...el.com>, Eranian Stephane <eranian@...gle.com>,
 Shukla Manali <Manali.Shukla@....com>,
 Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 15/38] KVM: x86/pmu: Check PMU cpuid configuration from
 user space


On 5/15/2025 8:12 AM, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>> From: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>>
>> Check user space's PMU cpuid configuration and filter the invalid
>> configuration.
>>
>> Either legacy perf-based vPMU or mediated vPMU needs kernel to support
>> local APIC, otherwise PMI has no way to be injected into guest. If
>> kernel doesn't support local APIC, reject user space to enable PMU
>> cpuid.
>>
>> User space configured PMU version must be no larger than KVM supported
>> maximum pmu version for mediated vPMU, otherwise guest may manipulate
>> some unsupported or unallowed PMU MSRs, this is dangerous and harmful.
>>
>> If the pmu version is larger than 1 but smaller than 5, CPUID.AH.ECX
>> must be 0 as well which is required by SDM.
>>
>> Suggested-by: Zide Chen <zide.chen@...el.com>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
>> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
>> ---
>>  arch/x86/kvm/cpuid.c | 15 +++++++++++++++
>>  arch/x86/kvm/pmu.c   |  7 +++++--
>>  arch/x86/kvm/pmu.h   |  1 +
>>  3 files changed, 21 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 8eb3a88707f2..f849ced9deba 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -179,6 +179,21 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
>>  			return -EINVAL;
>>  	}
>>  
>> +	best = kvm_find_cpuid_entry(vcpu, 0xa);
>> +	if (vcpu->kvm->arch.enable_pmu && best) {
>> +		union cpuid10_eax eax;
>> +
>> +		eax.full = best->eax;
>> +		if (enable_mediated_pmu &&
>> +		    eax.split.version_id > kvm_pmu_cap.version)
>> +			return -EINVAL;
>> +		if (eax.split.version_id > 0 && !vcpu_pmu_can_enable(vcpu))
>> +			return -EINVAL;
>> +		if (eax.split.version_id > 1 && eax.split.version_id < 5 &&
>> +		    best->ecx != 0)
>> +			return -EINVAL;
> NAK, unless there is a really, *really* strong need for this.  I do not want to
> get in the business of vetting the vCPU model presented to the guest.  If KVM
> needs to constrain things for its own safety, then by all means, but AFAICT these
> are nothing more than sanity checks on userspace.

Ok.


>
>> +	}
>> +
>>  	/*
>>  	 * Exposing dynamic xfeatures to the guest requires additional
>>  	 * enabling in the FPU, e.g. to expand the guest XSAVE state size.
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
>> index 4f455afe4009..92c742ead663 100644
>> --- a/arch/x86/kvm/pmu.c
>> +++ b/arch/x86/kvm/pmu.c
>> @@ -743,6 +743,10 @@ static void kvm_pmu_reset(struct kvm_vcpu *vcpu)
>>  	kvm_pmu_call(reset)(vcpu);
>>  }
>>  
>> +inline bool vcpu_pmu_can_enable(struct kvm_vcpu *vcpu)
>> +{
>> +	return vcpu->kvm->arch.enable_pmu && lapic_in_kernel(vcpu);
> Again, the APIC check belongs in the VM enablement path, not here.  Hmm, that
> may require more thought with respect to enabling the PMU by default.

Sure.