random: fix bound check ordering (CVE-2007-3105)

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: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7f5271272f91..397c714cf2ba 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -693,9 +693,14 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
 
         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",