Re: [PATCH] KVM: VMX: Add quirk to allow L1 to set FREEZE_IN_SMM in vmcs12

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 04, 2026, Jim Mattson wrote:
> On Tue, Feb 3, 2026 at 6:00 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, Jan 22, 2026, Jim Mattson wrote:
> > > On Tue, Jan 13, 2026 at 7:47 PM Jim Mattson <jmattson@google.com> wrote:
> > > > On Tue, Jan 13, 2026 at 4:42 PM Sean Christopherson <seanjc@google.com> wrote:
> > > > >
> > > > > On Tue, Jan 13, 2026, Jim Mattson wrote:
> > > > > > Add KVM_X86_QUIRK_VMCS12_FREEZE_IN_SMM to allow L1 to set
> > > > > > IA32_DEBUGCTL.FREEZE_IN_SMM in vmcs12 when using nested VMX.  Prior to
> > > > > > commit 6b1dd26544d0 ("KVM: VMX: Preserve host's
> > > > > > DEBUGCTLMSR_FREEZE_IN_SMM while running the guest"), L1 could set
> > > > > > FREEZE_IN_SMM in vmcs12 to freeze PMCs during physical SMM coincident
> > > > > > with L2's execution.  The quirk is enabled by default for backwards
> > > > > > compatibility; userspace can disable it via KVM_CAP_DISABLE_QUIRKS2 if
> > > > > > consistency with WRMSR(IA32_DEBUGCTL) is desired.
> > > > >
> > > > > It's probably worth calling out that KVM will still drop FREEZE_IN_SMM in vmcs02
> > > > >
> > > > >         if (vmx->nested.nested_run_pending &&
> > > > >             (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS)) {
> > > > >                 kvm_set_dr(vcpu, 7, vmcs12->guest_dr7);
> > > > >                 vmx_guest_debugctl_write(vcpu, vmcs12->guest_ia32_debugctl &
> > > > >                                                vmx_get_supported_debugctl(vcpu, false)); <====
> > > > >         } else {
> > > > >                 kvm_set_dr(vcpu, 7, vcpu->arch.dr7);
> > > > >                 vmx_guest_debugctl_write(vcpu, vmx->nested.pre_vmenter_debugctl);
> > > > >         }
> > > > >
> > > > > both from a correctness standpoint and so that users aren't mislead into thinking
> > > > > the quirk lets L1 control of FREEZE_IN_SMM while running L2.
> > > >
> > > > Yes, it's probably worth pointing out that the VM is now subject to
> > > > the whims of the L0 administrators.
> > > >
> > > > While that makes some sense for the legacy vPMU, where KVM is just
> > > > another client of host perf, perhaps the decision should be revisited
> > > > in the case of the MPT vPMU, where KVM owns the PMU while the vCPU is
> > > > in VMX non-root operation.
> >
> > Eh, running guests with FREEZE_IN_SMM=0 seems absolutely crazy from a security
> > perspective.  If an admin wants to disable FREEZE_IN_SMM, they get to keep the
> > pieces.  And KVM definitely isn't going to override the admin, e.g. to allow the
> > guest to profile host SMM.
> 
> I'm not sure what you mean by "they get to keep the pieces." What is
> the security problem with allowing L1 to freeze *guest-owned* PMCs
> during SMM?

To give L1 the option to freeze PMCs, KVM would also need to give L1 the option
to *not* freeze PMCs.  At that point, the guest can use its PMCs to profile host
SMM code.  Maybe even leverage a PMI to attack a poorly written SMM handler.

In other words, unless I'm missing something, the only reasonable option is to
run the guest with FREEZE_IN_SMM=1, which means ignoring the guest's wishes.
Or I guess another way to look at it: you can have any color car you want, as
long as it's black :-)