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

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> From: Dapeng Mi <dapeng1.mi@linux.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@intel.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.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.

> +	}
> +
>  	/*
>  	 * 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.