Re: [RFC PATCH v2 6/6] KVM: SVM: Add CET features to supported_xss

[Edgecombe, Rick P]

On Fri, 2023-06-09 at 10:34 -0500, John Allen wrote:
> > Is setting XFEATURE_MASK_CET_KERNEL here ok? The host kernel will
> > not
> > support XFEATURE_MASK_CET_KERNEL. I guess after this there is a
> > small
> > window of time where host IA32_XSS could have non-host supported
> > supervisor state.
> > 
> > Sort of separately, how does SVM work with respect to saving and
> > restoring guest supervisor CET state (I mean the CET_S stuff)?
> 
> Apart from a minor exception involving SEV-ES, we are piggybacking on
> the state saving/restoring in Yang Weijiang's x86/VMX series. So by
> inspection, it looks like guest supervisor support is broken as the
> supervisor XSAVES state and MSRs are not included in that series. I
> currently don't have a way to test this case, but I think there are
> operating systems that support it. I'll work on getting a guest set
> up
> that can actually test this and hopefully have working guest
> supervisor
> support in the next version of the series.

Hmm, interesting. VMX has some separate non-xsaves thing to save and
restore the guests supervisor CET state, so Weijiang's series doesn't
use the xsaves supervisor CET support. Also, since the host might have
CR4.CET set for its own reasons, if the host handled an exit with the
the guests MSR_IA32_S_CET set it could suddenly be subjected to CET
enforcement that it doesn't expect. Waiting to restore it until
returning to the guest is too late.

At least that's the reasoning on the VMX side as I understand it. If
you need to add XFEATURE_CET_KERNEL generally, we'll have to think
about how it works with host IBT. Probably easiest to leave it disabled
on the Intel side.