linux-kernel - Re: /dev/urandom uses uninit bytes, leaks user data

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Date:	Thu, 20 Dec 2007 15:17:02 -0500
From:	Phillip Susi <psusi@....rr.com>
To:	Andrew Lutomirski <luto@...ealbox.com>
CC:	Theodore Tso <tytso@....edu>, David Newall <david@...idnewall.com>,
	John Reiser <jreiser@...wagon.com>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org,
	security@...nel.org
Subject: Re: /dev/urandom uses uninit bytes, leaks user data

Andrew Lutomirski wrote:
> I understand that there's no way that /dev/random can provide good
> output if there's insufficient entropy.  But it still shouldn't leak
> arbitrary bits of user data that were never meant to be put into the
> pool at all.

It doesn't leak it though, it consumes it, and it then vanishes into the 
entropy pool, never to be seen again.

> Step 1: Boot a system without a usable entropy source.
> Step 2: add some (predictable) "entropy" from userspace which isn't a
> multiple of 4, so up to three extra bytes get added.
> Step 3: Read a few bytes of /dev/random and send them over the network.

Only root can do 1 and 2, at which point, it's already game over.

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