From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Standardizing an MSR or other hypercall to get an RNG seed?
Date: Fri, 19 Sep 2014 16:29:53 -0700
Message-ID: <541CBC71.9040709@zytor.com>
References: <0180a8dfcad746a895755c4374853c16@BY2PR03MB585.namprd03.prod.outlook.com>
	<541B5553.7020203@zytor.com>
	<CALCETrUq71jpO1de4pwbVo+hnE-D2FkuqBDFbdyLiDQzNdENEQ@mail.gmail.com>
	<CAL54oT1Q8kABge=t4s5REVWWakboON-6vfszMRkVz=ks_3vRoA@mail.gmail.com>
	<CALCETrUXz1B=x6rjkQri_qU5Tv-XM5du=rUr-=E-EfNSvOnThg@mail.gmail.com>
	<CALCETrWZ1cF23aT82yGfTKS48d2G+_Od7hEkAzDWzDhqpHNVqA@mail.gmail.com>
	<CAL54oT0sibBiLSHf+eajdRgxqs1jgSzdTTCWBUH9M25ZL10EzA@mail.gmail.com>
	<541C5C8A.6030304@zytor.com> <20140919220537.GR26995@thunk.org>
	<CALCETrXct3eqJH7wATfR=H3NrAHXMkydRKsctVhpzZ4jsv87bw@mail.gmail.com>
	<20140919225727.GT26995@thunk.org>
	<CALCETrVbhvB3ExxxNQRB21NYbpvoXgKpX6XxzMKsd=b47rKCeA@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: Mathew John <mathewj@microsoft.com>, kvm list <kvm@vger.kernel.org>,
	Gleb Natapov <gleb@kernel.org>, Niels Ferguson <niels@microsoft.com>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	David Hepkin <davidhep@microsoft.com>, Jake Oshins <jakeo@microsoft.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	John Starks <John.Starks@microsoft.com>
To: Andy Lutomirski <luto@amacapital.net>, "Theodore Ts'o" <tytso@mit.edu>
Return-path: <virtualization-bounces@lists.linux-foundation.org>
In-Reply-To: <CALCETrVbhvB3ExxxNQRB21NYbpvoXgKpX6XxzMKsd=b47rKCeA@mail.gmail.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
List-Id: kvm.vger.kernel.org

On 09/19/2014 04:12 PM, Andy Lutomirski wrote:
> 
> To force deterministic execution.
> 
> I incorrectly thought that the kernel could switch RDRAND on and off.
> It turns out that a hypervisor can do this, but not the kernel.  Also,
> determinism is lost anyway because of TSX, which *also* can't be
> turned on and off.
> 

Actually, a much bigger reason is because it lets rogue guest *user
space*, even will a well-behaved guest OS, do something potentially
harmful to the host.

	-hpa