Re: [PATCH v2 0/4] KVM: nVMX: Fix 2nd exec controls override goofs

Sean Christopherson
2022-12-15T00:19:53Z

On Wed, Dec 14, 2022, Yu Zhang wrote:
> On Tue, Dec 13, 2022 at 06:23:02AM +0000, Sean Christopherson wrote:
> > Fix bugs in KVM's (mis)handling of secondary execution controls.
> >
> > KVM overrides the secondary execution control VMX MSR during KVM_SET_CPUID.
> > Similar to the somewhat recent reverts
> >
> > 8805875aa473 ("Revert "KVM: nVMX: Do not expose MPX VMX controls when guest MPX disabled"")
> > 9389d5774aca ("Revert "KVM: nVMX: Expose load IA32_PERF_GLOBAL_CTRL VM-{Entry,Exit} control"")
> >
> > undo misguided KVM behavior where KVM overrides allowed-1 settings in the
> > secondary execution controls in response to changes to the guest's CPUID
> > model. To avoid breaking userspace that doesn't take ownership of the
> > VMX MSRs, go hands off if and only if userpace sets the MSR in question.
> >
> > Before fixing that, fix another bug it was hiding where the umwait/tpause
> > control was being exposed to L1 for nVMX only after KVM_SET_CPUID, and
> > harden KVM against similar bugs in the future.
> >
> > v2: Fix the ENABLE_USR_WAIT_PAUSE bug too. [Aaron]
> >
> > v1: https://lore.kernel.org/all/20221110005706.1064832-1-seanjc@google.com
> >
> > Sean Christopherson (4):
> > KVM: nVMX: Properly expose ENABLE_USR_WAIT_PAUSE control to L1
> > KVM: nVMX: Don't stuff secondary execution control if it's not
> > supported
> > KVM: nVMX: Don't muck with allowed sec exec controls on CPUID changes
> > KVM: selftests: Test KVM's handling of VMX's sec exec MSR on
> > KVM_SET_CPUID
>
> BTW, we may need another patch to remove the obsolete comments in
> nested_vmx_setup_ctls_msrs():

Ouch, indeed. Want to send a proper patch? Or provide your SoB and I'll write
a changelog?

The comment was added by commit 80154d77c922 ("KVM: VMX: cache secondary exec controls"),
but arguably the below is the appropriate Fixes, as it's the commit that fixed the
existing cases where KVM didn't enumerate supported-but-conditional controls.

Fixes: 6defc591846d ("KVM: nVMX: include conditional controls in /dev/kvm KVM_GET_MSRS")

> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index b6f4411b613e..42ceddcafd3e 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -6854,11 +6854,6 @@ void nested_vmx_setup_ctls_msrs(struct vmcs_config *vmcs_conf, u32 ept_caps)
> msrs->procbased_ctls_low &=
> ~(CPU_BASED_CR3_LOAD_EXITING | CPU_BASED_CR3_STORE_EXITING);
>
> - /*
> - * secondary cpu-based controls. Do not include those that
> - * depend on CPUID bits, they are added later by
> - * vmx_vcpu_after_set_cpuid.
> - */
> msrs->secondary_ctls_low = 0;
>
> msrs->secondary_ctls_high = vmcs_conf->cpu_based_2nd_exec_ctrl;
>
> B.R.
> Yu