From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:34208) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SUieD-0001g5-RR for qemu-devel@nongnu.org; Wed, 16 May 2012 14:08:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SUie7-0008Uo-Ea for qemu-devel@nongnu.org; Wed, 16 May 2012 14:08:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58889) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SUie7-0008UG-71 for qemu-devel@nongnu.org; Wed, 16 May 2012 14:08:23 -0400 Date: Wed, 16 May 2012 22:51:43 +0530 From: Amit Shah Message-ID: <20120516172143.GA16342@amit.redhat.com> References: <4FB3AA86.3080507@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4FB3AA86.3080507@codemonkey.ws> Subject: Re: [Qemu-devel] [PATCH 1/1] virtio-rng: device to send host entropy to guest List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: qemu list On (Wed) 16 May 2012 [08:24:22], Anthony Liguori wrote: > On 05/16/2012 06:30 AM, Amit Shah wrote: > >The Linux kernel already has a virtio-rng driver, this is the device > >implementation. > > > >When Linux needs more entropy, it puts a buffer in the vq. We then put > >entropy into that buffer, and push it back to the guest. > > > >Feeding randomness from host's /dev/urandom into the guest is > >sufficient, so this is a simple implementation that opens /dev/urandom > >and reads from it whenever required. > > > >Invocation is simple: > > > > qemu ... -device virtio-rng-pci > > > >In the guest, we see > > > > $ cat /sys/devices/virtual/misc/hw_random/rng_available > > virtio > > > > $ cat /sys/devices/virtual/misc/hw_random/rng_current > > virtio > > > >There are ways to extend the device to be more generic and collect > >entropy from other sources, but this is simple enough and works for now. > > > >Signed-off-by: Amit Shah > > It's not this simple unfortunately. > > If you did this with libvirt, one guest could exhaust the available > entropy for the remaining guests. This could be used as a mechanism > for one guest to attack another (reducing the available entropy for > key generation). > > You need to rate limit the amount of entropy that a guest can obtain > to allow management tools to mitigate this attack. Hm, rate-limiting is a good point. However, we're using /dev/urandom here, which is nonblocking, and will keep on providing data as long as we keep reading. Amit