From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755231AbYEQG21 (ORCPT ); Sat, 17 May 2008 02:28:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751763AbYEQG2R (ORCPT ); Sat, 17 May 2008 02:28:17 -0400 Received: from ozlabs.org ([203.10.76.45]:37317 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751767AbYEQG2Q (ORCPT ); Sat, 17 May 2008 02:28:16 -0400 From: Rusty Russell To: "H. Peter Anvin" Subject: Re: [PATCH 2/2] lguest: virtio-rng support Date: Sat, 17 May 2008 16:28:03 +1000 User-Agent: KMail/1.9.9 Cc: Johannes Berg , virtualization@lists.linux-foundation.org, Jeff Garzik , Herbert Xu , Christian Borntraeger , LKML , Theodore Tso , Matt Mackall References: <482C8595.5030509@garzik.org> <200805171446.39814.rusty@rustcorp.com.au> <482E6417.8040602@zytor.com> In-Reply-To: <482E6417.8040602@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200805171628.03801.rusty@rustcorp.com.au> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Saturday 17 May 2008 14:50:31 H. Peter Anvin wrote: > Rusty Russell wrote: > > On Friday 16 May 2008 20:49:41 Johannes Berg wrote: > >>> + > >>> +/* Our random number generator device reads from /dev/urandom into the > >>> Guest's + * input buffers. The usual case is that the Guest doesn't > >>> want random numbers + * and so has no buffers although /dev/urandom is > >>> still readable, whereas + * console is the reverse. > >> > >> Is it really a good idea to use the hosts /dev/urandom to fill the > >> guests /dev/random? > > > > Technically it's up to rngd in the guest to decide whether to feed > > entropy or not (ie. /dev/urandom or /dev/random). > > Uhm, no. It's not. Unless the host provides actual entropy > information, you have a security hole. Huh? We just can't assume it adds entropy. AFAICT rngd -H0 is what we want here. > > If we use /dev/random in the host, we risk a DoS. But since /dev/random > > is 0666 on my system, perhaps noone actually cares? > > There is no point in feeding the host /dev/urandom to the guest (except > for seeding, which can be handled through other means); it will do its > own mixing anyway. Seeding is good, but unlikely to be done properly for first boot of some standard virtualized container. In practice, feeding /dev/urandom from the host will make /dev/urandom harder to predict in the guest. > The reason to provide anything at all from the host > is to give it "golden" entropy bits. But you did not address the DoS question: can we ignore it? Or are we trading off a DoS in the host against a potential security weakness in the guest? If so, how do we resolve it? Thanks, Rusty.