Re: [PATCH 2/2] lguest: virtio-rng support

From: H. Peter Anvin
Date: Sat May 17 2008 - 00:53:53 EST

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.

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?

/dev/random = give me actual entropy, if you have some.
/dev/urandom = give me what you have, regardless of quality.

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. The reason to provide anything at all from the host is to give it "golden" entropy bits.

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