On 06/03/15 12:23, George Dunlap wrote:
> On 06/03/2015 04:58 PM, Andrew Cooper wrote:
>> On 03/06/15 16:26, George Dunlap wrote:
>>> On 05/22/2015 04:50 PM, Don Slutz wrote:
>>>> Summary is that VMware treats "in (%dx),%eax" (or "out %eax,(%dx)")
>>>> to port 0x5658 specially. Note: since many operations return data
>>>> in EAX, "in (%dx),%eax" is the one to use. The other lengths like
>>>> "in (%dx),%al" will still do things, only AL part of EAX will be
>>>> changed. For "out %eax,(%dx)" of all lengths, EAX will remain
>>>> unchanged.
>>>>
>>>> This instruction is allowed to be used from ring 3. To
>>>> support this the vmexit for GP needs to be enabled. I have not
>>>> fully tested that nested HVM is doing the right thing for this.
>>>>
>>>> Enable no-fault of pio in x86_emulate for VMware port
>>>>
>>>> Also adjust the emulation registers after doing a VMware
>>>> backdoor operation.
>>>>
>>>> Add new routine hvm_emulate_one_gp() to be used by the #GP fault
>>>> handler.
>>>>
>>>> Some of the best info is at:
>>>>
>>>> https://sites.google.com/site/chitchatvmback/backdoor
>>>>
>>>> Signed-off-by: Don Slutz <dsl...@verizon.com>
>>> So let me get this straight.
>>>
>>> VMWare allows ring3 to access the magic port regardless of whether the
>>> guest OS has enabled access to that IO port or not.
>>>
>>> In order to emulate this, we need to:
>>> * Trap to Xen on #GPs rather than just letting the hardware handle it
>>> * Emulate all instructions which cause a #GP, just to see if they might
>>> be an IO instruction accessing the magic port.
>>> * If it is an IO instruction, and it's accessing the magic port, then we
>>> skip the ioport access checks (which will cause the instruction to
>>> execute as though it had been given access).
>>> * Under all other circumstances (we hope) the emulator in Xen will do
>>> exactly what the hardware just did, and deliver a #GP to the guest.
>>>
>>> In an attempt to make this more safe, emulation ops that write (such as
>>> write and cmpxchg) are replaced with stubs which always return an error.
>>>
>>> Is that about right?
>>>
>>> That sounds completely insane. It opens up an almost infinite surface
>>> of attack onto the Xen emulator.
>>>
>>> I understand that having the "VMWare compatible" is a nice tick-box to
>>> have, but seriously, I cannot imagine that having unprivileged
>>> user-space tools know the real clock frequency without having to involve
>>> the OS is anywhere close to worth the risk involved.
>>
>> The attack surface sadly is not enlarged in the slightest by this change.
>>
>> We already trap and emulate all #UD exceptions in an attempt to support
>> migration of VMs between Intel and AMD hardware. See XSA-105. (There
>> is a good argument to be made for not trapping #UD, but that doesn't
>> completely close the hole)
>
> So at the moment, an attacker on Intel can force the emulation of any
> AMD-only instruction (and vice versa), is that right?
>
> This would allow an attacker to force the emulation of every #GP
> condition of every instruction we emulate.
>
> Those two sets may be within an order of magnitude of each other, but
> they will only overlap a little bit. So my guess is that enabling this
> would double the surface of attack (give or take).
>
> I'd be a lot happier with this patch if we could make it so that on a
> #GP the only instruction that could get emulated would be an IO instruction.
>
You mean like I said in:
Message-ID: <54c67d830200007800059...@mail.emea.novell.com>
X-Mailer: Novell GroupWise Internet Agent 14.0.1
Date: Mon, 26 Jan 2015 16:46:43 +0000
From: Jan Beulich <jbeul...@suse.com>
To: Don Slutz <dsl...@verizon.com>
CC: Suravee Suthikulpanit <suravee.suthikulpa...@amd.com>, Andrew Cooper
<andrew.coop...@citrix.com>, Ian Campbell <ian.campb...@citrix.com>,
"George
Dunlap" <george.dun...@eu.citrix.com>, Ian Jackson
<ian.jack...@eu.citrix.com>, Stefano Stabellini
<stefano.stabell...@eu.citrix.com>, Eddie Dong <eddie.d...@intel.com>,
"Jun
Nakajima" <jun.nakaj...@intel.com>, Kevin Tian <kevin.t...@intel.com>,
<xen-devel@lists.xen.org>, Boris Ostrovsky <boris.ostrov...@oracle.com>,
Konrad Rzeszutek Wilk <konrad.w...@oracle.com>, Keir Fraser
<k...@xen.org>,
Tim Deegan <t...@xen.org>
Subject: Re: [PATCH for-4.5 v8 4/7] xen: Add vmware_port support
References: <1412285417-19180-1-git-send-email-dsl...@verizon.com>
<1412285417-19180-5-git-send-email-dsl...@verizon.com>
<542dca92.1030...@terremark.com> <542dd44f.6030...@terremark.com>
<54b8f1740200007800055...@mail.emea.novell.com>
<54bfe768.3090...@terremark.com>
<54c0c39f0200007800057...@mail.emea.novell.com> <54c6643...@terremark.com>
In-Reply-To: <54c6643...@terremark.com>
-Don Slutz
> -George
>
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