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Date: Fri, 14 Dec 2007 11:34:09 -0800
From: John Reiser <jreiser@...Wagon.com>
To: Matt Mackall <mpm@...enic.com>
CC: linux-kernel@...r.kernel.org, security@...nel.org
Subject: /dev/urandom uses uninit bytes, leaks user data
xfer_secondary_pool() in drivers/char/random.c tells add_entropy_words()
to use uninitialized tmp[] whenever bytes is not a multiple of 4.
Besides being unfriendly to automated dynamic checkers, this is a
potential leak of user data into the output stream. When called from
extract_entropy_user, then uninit tmp[] can capture leftover data
from a previous copy_from_user().
Signed off by: jreiser@...Wagon.com
--- ./drivers/char/random.c.orig 2007-12-14 11:06:03.000000000 -0800
+++ ./drivers/char/random.c 2007-12-14 11:06:57.000000000 -0800
@@ -708,7 +708,19 @@
bytes=extract_entropy(r->pull, tmp, bytes,
random_read_wakeup_thresh / 8, rsvd);
- add_entropy_words(r, tmp, (bytes + 3) / 4);
+ /*
+ * 2007-12-13 (valgrind/memcheck) Do not use undefined bytes.
+ * Avoid info leak when called from extract_entropy_user:
+ * uninit tmp[] can have data from previous copy_from_user().
+ * Instead: fill last word using first bytes.
+ */
+ {
+ __u8 *src = (__u8 *)&tmp[0];
+ __u8 *dst = bytes + src;
+ for (; 0!=(3 & bytes); ++bytes)
+ *dst++ = *src++;
+ }
+ add_entropy_words(r, tmp, bytes>>2);
credit_entropy_store(r, bytes*8);
}
--
John Reiser, jreiser@...Wagon.com
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