Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Message-ID: <5facc2ddb1d59bcc17837977eb495b57.squirrel@twosheds.infradead.org>
In-Reply-To: <0575AF4FD06DD142AD198903C74E1CC87A602447@ORSMSX103.amr.corp.intel.com>
References: <1517167750-23485-1-git-send-email-karahmed@amazon.de>
    <4DCAF18F-C86A-4CBC-A9CC-CC01BF63313F@oracle.com>
    <86F5D9C0-7B11-448D-954F-D2EF6CBE80EC@amacapital.net>
    <1517172296.6624.84.camel@infradead.org>
    <0575AF4FD06DD142AD198903C74E1CC87A602447@ORSMSX103.amr.corp.intel.com>
Date:   Sun, 28 Jan 2018 21:47:28 -0000
Subject: RE: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From:   "David Woodhouse" <dwmw2@infradead.org>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>
Cc:     "David Woodhouse" <dwmw2@infradead.org>,
        "Andy Lutomirski" <luto@amacapital.net>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>,
        "KarimAllah Ahmed" <karahmed@amazon.de>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Mallick, Asit K" <asit.k.mallick@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Andi Kleen" <ak@linux.intel.com>,
        "Andrea Arcangeli" <aarcange@redhat.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Tim Chen" <tim.c.chen@linux.intel.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "Paolo Bonzini" <pbonzini@redhat.com>,
        "Greg KH" <gregkh@linuxfoundation.org>,
        "Andy Lutomirski" <luto@kernel.org>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "daniel.kiper@oracle.com" <daniel.kiper@oracle.com>
User-Agent: SquirrelMail/1.4.22-21.fc27
MIME-Version: 1.0
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: 8bit
Importance: Normal
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


>>
>> 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