[patch 02/12] random: fix bound check ordering (CVE-2007-3105)

[Greg KH <gregkh@suse.de>]

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-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Matt Mackall <mpm@selenic.com>

If root raised the default wakeup threshold over the size of the
output pool, the pool transfer function could overflow the stack with
RNG bytes, causing a DoS or potential privilege escalation.

(Bug reported by the PaX Team <pageexec@freemail.hu>)

Cc: Theodore Tso <tytso@mit.edu>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 drivers/char/random.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -693,9 +693,14 @@ static void xfer_secondary_pool(struct e
 
 	if (r->pull && r->entropy_count < nbytes * 8 &&
 	    r->entropy_count < r->poolinfo->POOLBITS) {
-		int bytes = max_t(int, random_read_wakeup_thresh / 8,
-				min_t(int, nbytes, sizeof(tmp)));
+		/* If we're limited, always leave two wakeup worth's BITS */
 		int rsvd = r->limit ? 0 : random_read_wakeup_thresh/4;
+		int bytes = nbytes;
+
+		/* pull at least as many as BYTES as wakeup BITS */
+		bytes = max_t(int, bytes, random_read_wakeup_thresh / 8);
+		/* but never more than the buffer size */
+		bytes = min_t(int, bytes, sizeof(tmp));
 
 		DEBUG_ENT("going to reseed %s with %d bits "
 			  "(%d of %d requested)\n",

--