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

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Date:	Mon, 17 Dec 2007 12:59:04 -0800
From:	"David Schwartz" <davids@...master.com>
To:	"Theodore Tso" <tytso@....edu>, <jreiser@...wagon.com>
Cc:	"Matt Mackall" <mpm@...enic.com>, <linux-kernel@...r.kernel.org>,
	<security@...nel.org>
Subject: RE: /dev/urandom uses uninit bytes, leaks user data


> The bottom line:  At a cost of at most three unpredictable branches
> (whether to clear the bytes in the last word with indices congruent
> to 1, 2, or 3 modulo 4), then the code can reduce the risk from something
> small but positive, to zero.  This is very inexpensive insurance.

> John Reiser, jreiser@...Wagon.com

Even if you're right, the change isn't free. You've simply presented
evidence of one non-zero benefit of it. You've given no ability to assess
the size of this benefit and no way to figure if it exceeds the cost. There
is also a non-zero *security* cost to this change.

DS


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