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;
Subject: Re: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit
 mode
To:     Wanpeng Li <kernellwp@gmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>
CC:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
References: <1521707651-9375-1-git-send-email-wanpengli@tencent.com>
 <aadc47c3-9d5f-b4d5-92d3-cb888a29563c@redhat.com>
 <49454fe4-16e2-4d8b-7ad5-9e488afc786e@citrix.com>
 <d1126eb9-7203-7044-f72b-174cac6607ab@redhat.com>
 <e9db6804-4881-be93-f570-e9631dbccac8@citrix.com>
 <9bd82cb0-d88f-4891-a111-3704802e1d4e@redhat.com>
 <CANRm+CwmJQCFZ7oe1_YatqoOXzzEU+SP9Z5qR6zU_ybd_DoGQQ@mail.gmail.com>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Message-ID: <94bbfac5-2022-ab92-0b9a-1c3cd2275054@citrix.com>
Date:   Thu, 22 Mar 2018 13:53:25 +0000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.6.0
MIME-Version: 1.0
In-Reply-To: <CANRm+CwmJQCFZ7oe1_YatqoOXzzEU+SP9Z5qR6zU_ybd_DoGQQ@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Content-Language: en-GB
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 22/03/18 13:39, Wanpeng Li wrote:
> 2018-03-22 20:38 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> On 22/03/2018 12:04, Andrew Cooper wrote:
>>> We've got a Force Emulation Prefix (ud2a; .ascii "xen") for doing
>>> magic.  Originally, this was used for PV guests to explicitly request an
>>> emulated CPUID, but I extended it to HVM guests for "emulate the next
>>> instruction", after we had some guest user => guest kernel privilege
>>> escalations because of incorrect emulation.
>> Wanpeng, why don't you add it behind a new kvm module parameter? :)
> Great point! I will have a try. Thanks Paolo and Andrew. :)

Using the force emulation prefix requires intercepting #UD, which is in
general a BadThing(tm) for security.  Therefore, we have a build time
configuration option to compile in support, and require that test
systems explicitly opt into using it via a command line parameter.

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/hvm/hvm.c;h=db52312882205e65b32e587106ca795f4bfab2eb;hb=refs/heads/staging#l3741
is the general #UD intercept handler if you want a reference.  (You can
ignore the cross-vendor part, which is leftovers from
http://developer.amd.com/wordpress/media/2012/10/CrossVendorMigration.pdf )

~Andrew