Re: [PATCH v7 7/9] KVM: x86/pmu: Restrict RDPMC passthrough to known CPUs

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 7/28/2026 3:21 AM, Zide Chen wrote:
> RDPMC type encodings are vendor-defined and could theoretically be
> extended on future CPUs. An unknown RDPMC type could leak host
> PMU information to the guest through RDPMC passthrough.
>
> Rather than assuming future CPUs are safe, explicitly permit only
> reviewed CPU models and require RDPMC interception everywhere else.
> This ensures RDPMC passthrough is enabled only on CPUs whose RDPMC
> encodings are audited and supported by KVM.
>
> Suggested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Suggested-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Zide Chen <zide.chen@intel.com>
> ---
> v7: new patch.
> ---
>  arch/x86/kvm/pmu.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 51c6d00a485f..092809bd757d 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -77,6 +77,21 @@ static const struct x86_cpu_id vmx_pebs_pdist_cpu[] = {
>  	{}
>  };
>  
> +/*
> + * CPUs whose RDPMC encodings have been audited for KVM RDPMC
> + * passthrough support.
> + */
> +static const struct x86_cpu_id kvm_rdpmc_known_cpus[] = {
> +	X86_MATCH_VFM(INTEL_ATOM_DARKMONT_X, NULL),
> +	X86_MATCH_VFM(INTEL_LUNARLAKE_M, NULL),
> +	X86_MATCH_VFM(INTEL_NOVALAKE, NULL),
> +	X86_MATCH_VFM(INTEL_NOVALAKE_L, NULL),
> +	X86_MATCH_VFM(INTEL_PANTHERLAKE_L, NULL),
> +	X86_MATCH_VFM(INTEL_PANTHERLAKE_R, NULL),
> +	X86_MATCH_VFM(INTEL_WILDCATLAKE_L, NULL),
> +	{}
> +};
> +
>  /* NOTE:
>   * - Each perf counter is defined as "struct kvm_pmc";
>   * - There are two types of perf counters: general purpose (gp) and fixed.
> @@ -807,6 +822,20 @@ bool kvm_need_perf_global_ctrl_intercept(struct kvm_vcpu *vcpu)
>  }
>  EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_need_perf_global_ctrl_intercept);
>  
> +static bool kvm_rdpmc_encoding_supported(void)
> +{
> +	/* KVM understands all RDPMC encodings prior to PMU v6. */
> +	if (kvm_host_pmu.version < 6)

I'm not sure if this works for AMD platforms. Jim said current AMD
platforms already have security holes and I believe the PMU version of AMD
platforms are less than 6.

Need AMD guys to confirm. @Sandipan, @Manali.


> +		return true;
> +
> +	/*
> +	 * Future PMU v6 implementations and future PMU versions require RDPMC
> +	 * interception until their RDPMC encodings are audited and supported
> +	 * by KVM.
> +	 */
> +	return x86_match_cpu(kvm_rdpmc_known_cpus);

x86_match_cpu() returns structure x86_cpu_id pointer or NULL instead of a
boolean variable. Better explicitly covert it to a boolean variable.

return x86_match_cpu(kvm_rdpmc_known_cpus) != NULL;



> +}
> +
>  bool kvm_need_rdpmc_intercept(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> @@ -818,6 +847,9 @@ bool kvm_need_rdpmc_intercept(struct kvm_vcpu *vcpu)
>  	if (enable_vmware_backdoor)
>  		return true;
>  
> +	if (!kvm_rdpmc_encoding_supported())
> +		return true;
> +
>  	return kvm_need_any_pmc_intercept(vcpu) ||
>  	       pmu->counter_bitmask[KVM_PMC_GP] != (BIT_ULL(kvm_host_pmu.bit_width_gp) - 1) ||
>  	       pmu->counter_bitmask[KVM_PMC_FIXED] != (BIT_ULL(kvm_host_pmu.bit_width_fixed) - 1);