Messages in this thread | | | Date | Wed, 15 Feb 2023 19:53:27 +0800 | Subject | Re: [PATCH v2 06/21] KVM: x86/pmu: WARN and bug the VM if PMU is refreshed after vCPU has run | From | Like Xu <> |
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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@gmail.com> > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > 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:
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