Re: [PATCH v4 1/3] KVM: nVMX: Sync L2 guest CET states between L1/L2

Yang Weijiang
2021-03-24T13:39:19Z

On Tue, Mar 23, 2021 at 03:56:30PM +0000, Sean Christopherson wrote:
> On Tue, Mar 23, 2021, Yang Weijiang wrote:
> > On Tue, Mar 16, 2021 at 05:03:47PM +0800, Yang Weijiang wrote:
> >
> > Hi, Sean,
> > Could you respond my below rely? I'm not sure how to proceed, thanks!
> >
> > > On Mon, Mar 15, 2021 at 09:45:11AM -0700, Sean Christopherson wrote:
> > > > On Mon, Mar 15, 2021, Yang Weijiang wrote:
>
> ...
>
> > > > > @@ -2556,6 +2563,15 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> > > > > if (kvm_mpx_supported() && (!vmx->nested.nested_run_pending ||
> > > > > !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
> > > > > vmcs_write64(GUEST_BNDCFGS, vmx->nested.vmcs01_guest_bndcfgs);
> > > > > +
> > > > > + if (kvm_cet_supported() && (!vmx->nested.nested_run_pending ||
> > > > > + !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE))) {
> > > >
> > > > Not your code per se, since this pattern comes from BNDCFGS and DEBUGCTL, but I
> > > > don't see how loading vmcs01 state in this combo is correct:
> > > >
> > > > a. kvm_xxx_supported() == 1
> > > > b. nested_run_pending == false
> > > > c. vm_entry_controls.load_xxx_state == true
> > > >
> > > > nested_vmx_enter_non_root_mode() only snapshots vmcs01 if
> > > > vm_entry_controls.load_xxx_state == false, which means the above combo is
> > > > loading stale values (or more likely, zeros).
> > > >
> > > > I _think_ nested_vmx_enter_non_root_mode() just needs to snapshot vmcs01 if
> > > > nested_run_pending=false. For migration, if userspace restores MSRs after
> > > > KVM_SET_NESTED_STATE, then what's done here is likely irrelevant. If userspace
> > > > restores MSRs before nested state, then vmcs01 will hold the desired value since
> > > > setting MSRs would have written the value into vmcs01.
> > >
> > > Then the code nested_vmx_enter_non_root_mode() would look like:
> > >
> > > if (kvm_cet_supported() && !vmx->nested.nested_run_pending &&
> > > !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {
> > > ...
> > > }
> > >
> > > I have another concern now, if vm_entry_controls.load_cet_state == false, and L1
> > > updated vmcs fields, so the latest states are in vmcs12, but they cannot
> > > be synced to vmcs02 because in prepare_vmcs02_rare():
> > >
> > > if (kvm_cet_supported() && vmx->nested.nested_run_pending &&
> > > (vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {
> > > ...
> > > }
> > >
> > > so L2 got stale status. IMO, L1 guest sets vm_entry_controls.load_cet_state == false
> > > should be rare case. We can even igore this case :-)
>
> Yes, that's an L1 bug if it expects L2 state to come from vmcs12 in that case.
> Architecturally, the vcms12 value won't be visible to L2 until L1 enables the
> VM-Entry control, at which point KVM would detect the refreshed vmcs12 and sync
> the "rare" fields.

Thanks, Sean!
So I'll change code as below:

if (kvm_cet_supported() && !vmx->nested.nested_run_pending &&
!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_CET_STATE)) {
...
}
>
> > > > I suspect no one has reported this issue because guests simply don't use MPX,
> > > > and up until the recent LBR stuff, KVM effectively zeroed out DEBUGCTL for the
> > > > guest.
> > > >
> > > So for MPX and DEBUGCTL, is it worth some separate fix patch?
>
> Yes, assuming my analysis is correct. That doesn't necessarily need to be your
> responsibility, though patches are of course welcome :-)
>
> Jim, Paolo, any thoughts?
>
OK, let me wait for Jim and Paolo's comments on this...

> > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > > index 45622e9c4449..4184ff601120 100644
> > > > --- a/arch/x86/kvm/vmx/nested.c
> > > > +++ b/arch/x86/kvm/vmx/nested.c
> > > > @@ -3298,10 +3298,11 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> > > > if (likely(!evaluate_pending_interrupts) && kvm_vcpu_apicv_active(vcpu))
> > > > evaluate_pending_interrupts |= vmx_has_apicv_interrupt(vcpu);
> > > >
> > > > - if (!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
> > > > + if (!vmx->nested.nested_run_pending ||
> > > > + !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
> > > > vmx->nested.vmcs01_debugctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
> > > > - if (kvm_mpx_supported() &&
> > > > - !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS))
> > > > + if (kvm_mpx_supported() && (!vmx->nested.nested_run_pending ||
> > > > + !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_BNDCFGS)))
> > > > vmx->nested.vmcs01_guest_bndcfgs = vmcs_read64(GUEST_BNDCFGS);
> > > >
> > > > /*
> > > >
> > > >
> > > > Side topic, all of this code is broken for SMM emulation. SMI+RSM don't do a
> > > > full VM-Exit -> VM-Entry; the CPU forcefully exits non-root, but most state that
> > > > is loaded from the VMCS is left untouched. It's the SMI handler's responsibility
> > > > to not enable features, e.g. to not set CR4.CET. For sane use cases, this
> > > > probably doesn't matter as vmcs12 will be configured to context switch state,
> > > > but if L1 is doing anything out of the ordinary, SMI+RSM will corrupt state.
> > > >
> > > > E.g. if L1 enables MPX in the guest, does not intercept L2 writes to BNDCFGS,
> > > > and does not load BNDCFGS on VM-Entry, then SMI+RSM would corrupt BNDCFGS since
> > > > the SMI "exit" would clear BNDCFGS, and the RSM "entry" would load zero. This
> > > > is 100% contrived, and probably doesn't impact real world use cases, but it
> > > > still bugs me :-)
> > >
> > > Exactly, should it be fixed by separate patch or leave it as is?
>
> Definitely leave it for now, properly fixing the SMI+RSM code goes far beyond
> basic CET support.

Sure.