Re: [PATCH v3 6/8] KVM:VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Paolo Bonzini]

On 08/03/19 17:28, Sean Christopherson wrote:
> On Mon, Mar 04, 2019 at 08:36:55PM +0800, Yang Weijiang wrote:
>> On Mon, Mar 04, 2019 at 07:12:02PM -0800, Sean Christopherson wrote:
>>> On Mon, Mar 04, 2019 at 05:56:40PM +0800, Yang Weijiang wrote:
>>>> Cannot agree with you more!
>>>> This is some design limitation, but from my point of view, once vmm
>>>> exposes CET capability to guest via CPUID, it grants the guest kernel freedom to choose
>>>> which features to be enabled, we don't need to add extra constraints on
>>>> the usage.
>>>
>>> But if KVM allows SHSTK and IBT to be toggled independently then the VMM
>>> has only exposed SHSTK or IBT, not CET as whole.
>>>
>>> Even if SHSTK and IBT are bundled together the guest still has to opt-in
>>> to enabling each feature.  I don't see what we gain by pretending that
>>> SHSTK/IBT can be individually exposed to the guest, and on the flip side
>>> doing so creates a virtualization hole.
>> you almost convinced me ;-), maybe I'll make the feature as a bundle in
>> next release after check with kernel team. BTW, what do you mean by
>> saying "create a virtualization hole"? Is it what you stated in above
>> reply?
> 
> By "virtualization hole" I mean the guest would be able to use a feature
> that the virtual CPU model says isn't supported.

I think it's okay to leave the hole and leave it to userspace to forbid
enabling only one of the bits.

Paolo

> After rereading the XSS architecture, there's a marginally less crappy
> option for handling XRSTOR as we could use the XSS_EXIT_BITMAP to
> intercept XRSTOR if SHSTK != IBT and the guest is restoring CET state,
> e.g. to ensure the guest isn't setting IA32_PL*_SSP if !SHSTK and isn't
> setting bits that are effectively reserved in IA32_U_CET.
> 
> But practically speaking that'd be the same as intercepting XRSTORS
> unconditionally when the guest is using CET, i.e. it's still going to
> tank the performance of a guest that uses CET+XSAVES/XRSTORS.
>