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

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Date:   Sun, 28 Jan 2018 21:47:28 -0000
From:   "David Woodhouse" <dwmw2@...radead.org>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
Cc:     "David Woodhouse" <dwmw2@...radead.org>,
        "Andy Lutomirski" <luto@...capital.net>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
        "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.

Right. Andy's concern was about VMX non-root (i.e. guest) ring 0
attempting to write IBRS but it's trapped and actually happens in the
host.

As long as it *remains* set when the host re-enters the vCPU that should
be fine.


-- 
dwmw2