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: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 24 insertions, 0 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index b8809d4ae186..a68b4a093272 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -307,6 +307,13 @@ 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))
@@ -1541,6 +1558,13 @@ struct ctl_table random_table[] = {
                 .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,
                 .maxlen         = 16,