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

[Yu Zhang]

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@xxxxxxxxxx
> 
> 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():

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