Re: [PATCH v2 06/21] KVM: x86/pmu: WARN and bug the VM if PMU is refreshed after vCPU has run

[Like Xu]

On 10/2/2023 8:31 am, Sean Christopherson wrote:
> Now that KVM disallows changing feature MSRs, i.e. PERF_CAPABILITIES,
> after running a vCPU, WARN and bug the VM if the PMU is refreshed after
> the vCPU has run.
>
> Note, KVM has disallowed CPUID updates after running a vCPU since commit
> feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN"), i.e.
> PERF_CAPABILITIES was the only remaining way to trigger a PMU refresh
> after KVM_RUN.

A malicious user space could have saved the vcpu state and then deleted
and recreated a new vcpu w/ previous state so that it would have a chance
to re-set the features msr.

The key to this issue may be focused on the KVM_CREATE_VM interface.

How about the contract that when the first vcpu is created and "after
KVM_RUN of any vcpu", the values of all feature msrs for all vcpus on
the same guest cannot be changed, even if the (likely) first ever ran
vcpu is deleted ?

> Cc: Like Xu <like.xu.linux@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/pmu.c |  3 +++
>   arch/x86/kvm/x86.c | 10 +++++++++-
>   2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 612e6c70ce2e..7e974c4e61b0 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -589,6 +589,9 @@ int kvm_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>    */
>   void kvm_pmu_refresh(struct kvm_vcpu *vcpu)
>   {
> +	if (KVM_BUG_ON(kvm_vcpu_has_run(vcpu), vcpu->kvm))
> +		return;
> +
>   	static_call(kvm_x86_pmu_refresh)(vcpu);
>   }
>   
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 186cb6a81643..1b14632a94a3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3626,9 +3626,17 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		if (data & ~kvm_caps.supported_perf_cap)
>   			return 1;
>   
> +		/*
> +		 * Note, this is not just a performance optimization!  KVM
> +		 * disallows changing feature MSRs after the vCPU has run; PMU
> +		 * refresh will bug the VM if called after the vCPU has run.
> +		 */
> +		if (vcpu->arch.perf_capabilities == data)
> +			break;
> +
>   		vcpu->arch.perf_capabilities = data;
>   		kvm_pmu_refresh(vcpu);
> -		return 0;
> +		break;
>   	case MSR_EFER:
>   		return set_efer(vcpu, msr_info);
>   	case MSR_K7_HWCR: