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

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Message-ID: <Y5pn2fYf5eHu8yCb@google.com>
Date:   Thu, 15 Dec 2022 00:18:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH v2 0/4] KVM: nVMX: Fix 2nd exec controls override goofs

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