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 <[e-mail]>)

Cc: Theodore Tso <[e-mail]>
Cc: Willy Tarreau <[e-mail]>
Signed-off-by: Matt Mackall <[e-mail]>
Signed-off-by: Chris Wright <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/random.c

···
 
 	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",