random: cap the rate which the /dev/urandom pool gets reseeded

In order to avoid draining the input pool of its entropy at too high
of a rate, enforce a minimum time interval between reseedings of the
urandom pool.  This is set to 60 seconds by default.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

drivers/char/random.c

@@ -306,6 +306,13 @@ static int random_read_wakeup_thresh = 64;
  */
 static int random_write_wakeup_thresh = 128;
 
+/*
+ * The minimum number of seconds between urandom pool resending.  We
+ * do this to limit the amount of entropy that can be drained from the
+ * input pool even if there are heavy demands on /dev/urandom.
+ */
+static int random_min_urandom_seed = 60;
+
 /*
  * When the input pool goes over trickle_thresh, start dropping most
  * samples to avoid wasting CPU time and reduce lock contention.
@@ -438,6 +445,7 @@ struct entropy_store {
 	struct entropy_store *pull;
 
 	/* read-write data: */
+	unsigned long last_pulled;
 	spinlock_t lock;
 	unsigned short add_ptr;
 	unsigned short input_rotate;
@@ -887,6 +895,14 @@ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes)
 {
 	__u32	tmp[OUTPUT_POOL_WORDS];
 
+	if (r->limit == 0 && random_min_urandom_seed) {
+		unsigned long now = jiffies;
+
+		if (time_before(now,
+				r->last_pulled + random_min_urandom_seed * HZ))
+			return;
+		r->last_pulled = now;
+	}
 	if (r->pull &&
 	    r->entropy_count < (nbytes << (ENTROPY_SHIFT + 3)) &&
 	    r->entropy_count < r->poolinfo->poolfracbits) {
@@ -1190,6 +1206,7 @@ static void init_std_data(struct entropy_store *r)
 	r->entropy_count = 0;
 	r->entropy_total = 0;
 	r->last_data_init = 0;
+	r->last_pulled = jiffies;
 	mix_pool_bytes(r, &now, sizeof(now), NULL);
 	for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
 		if (!arch_get_random_long(&rv))
@@ -1540,6 +1557,13 @@ struct ctl_table random_table[] = {
 		.extra1		= &min_write_thresh,
 		.extra2		= &max_write_thresh,
 	},
+	{
+		.procname	= "urandom_min_reseed_secs",
+		.data		= &random_min_urandom_seed,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 	{
 		.procname	= "boot_id",
 		.data		= &sysctl_bootid,