linux-kernel - Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

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Date:   Tue, 19 May 2020 00:55:23 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: emulate reserved nops from 0f/18 to 0f/1f

On Tue, May 19, 2020 at 09:43:23AM +0200, Paolo Bonzini wrote:
> On 19/05/20 08:02, Sean Christopherson wrote:
> > On Mon, May 18, 2020 at 07:37:08PM +0200, Paolo Bonzini wrote:
> >> On 18/05/20 18:07, Sean Christopherson wrote:
> >>> On Fri, May 15, 2020 at 12:19:19PM -0400, Paolo Bonzini wrote:
> >>>> Instructions starting with 0f18 up to 0f1f are reserved nops, except those
> >>>> that were assigned to MPX.
> >>> Well, they're probably reserved NOPs again :-D.
> >>
> >> So are you suggesting adding them back to the list as well?
> > 
> > Doesn't KVM still support MPX?
> > 
> >>>> These include the endbr markers used by CET.
> >>> And RDSPP.  Wouldn't it make sense to treat RDSPP as a #UD even though it's
> >>> a NOP if CET is disabled?  The logic being that a sane guest will execute
> >>> RDSSP iff CET is enabled, and in that case it'd be better to inject a #UD
> >>> than to silently break the guest.
> >>
> >> We cannot assume that guests will bother checking CPUID before invoking
> >> RDSPP.  This is especially true userspace, which needs to check if CET
> >> is enable for itself and can only use RDSPP to do so.
> > 
> > Ugh, yeah, just read through the CET enabling thread that showed code snippets
> > that do exactly this.
> > 
> > I assume it would be best to make SHSTK dependent on unrestricted guest?
> > Emulating RDSPP by reading vmcs.GUEST_SSP seems pointless as it will become
> > statle apart on the first emulated CALL/RET.
> 
> Running arbitrary code under the emulator is problematic anyway with
> CET, since you won't be checking ENDBR markers or updating the state
> machine.  So perhaps in addition to what you say we should have a mode
> where, unless unrestricted guest is disabled, the emulator only accepts
> I/O, MOV and ALU instructions.

Doh, I forgot all about those pesky ENDBR markers.  I think a slimmed down
emulator makes sense?

Tangentially related, isn't the whole fastop thing doomed once CET kernel
support lands?