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

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Date:   Mon, 18 May 2020 23:02:02 -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 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.