linux-kernel - RE: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC

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Date:   Sun, 28 Jan 2018 21:41:24 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:     KarimAllah Ahmed <karahmed@...zon.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "daniel.kiper@...cle.com" <daniel.kiper@...cle.com>
Subject: RE: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL

> 
> On Sun, 2018-01-28 at 12:40 -0800, Andy Lutomirski wrote:
> >
> > Do you mean that the host would intercept the guest WRMSR and do
> > WRMSR itself?  I would suggest that doing so is inconsistent with the
> > docs.  As specified, doing WRMSR to write 1 to IBRS does *not*
> > protect the guest.
> 
> I believe it does. Guest kernel is protected from any guest userspace
> predictions learned before IBRS was last set to 1 in *any* mode,
> including host.

the specification requires you to write a 1 on each transition to higher privilege.


> 
> > For that matter, what are the semantics of VMRESUME doing a write to
> > IBRS as part of its MSR switch?  Is it treated as IBRS=1 from guest
> > context?

the guest ring 3 wouldn't have had time to do anything evil in the mean time so the vmresume write is valid. (anything else would be unworkable)