BACKPORT: random32: add periodic reseeding

Clean cherry pick of commmit 6d31920246a9fc80be4f16acd27c0bbe8d7b8494.

The current Tausworthe PRNG is never reseeded with truly random data after
the first attempt in late_initcall. As this PRNG is used for some critical
random data as e.g. UDP port randomization we should try better and reseed
the PRNG once in a while with truly random data from get_random_bytes().

When we reseed with prandom_seed we now make also sure to throw the first
output away. This suffices the reseeding procedure.

The delay calculation is based on a proposal from Eric Dumazet.

Joint work with Daniel Borkmann.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Bug: http://b/29621447
Change-Id: I990d00f4a29a56a22357cec1c17477c4721054ae
(cherry picked from commit 6d31920246a9fc80be4f16acd27c0bbe8d7b8494)

lib/random32.c

@@ -142,6 +142,7 @@ void prandom_seed(u32 entropy)
 	for_each_possible_cpu (i) {
 		struct rnd_state *state = &per_cpu(net_rand_state, i);
 		state->s1 = __seed(state->s1 ^ entropy, 2);
+		prandom_u32_state(state);
 	}
 }
 EXPORT_SYMBOL(prandom_seed);
@@ -174,6 +175,27 @@ static int __init prandom_init(void)
 }
 core_initcall(prandom_init);
 
+static void __prandom_timer(unsigned long dontcare);
+static DEFINE_TIMER(seed_timer, __prandom_timer, 0, 0);
+
+static void __prandom_timer(unsigned long dontcare)
+{
+	u32 entropy;
+
+	get_random_bytes(&entropy, sizeof(entropy));
+	prandom_seed(entropy);
+	/* reseed every ~60 seconds, in [40 .. 80) interval with slack */
+	seed_timer.expires = jiffies + (40 * HZ + (prandom_u32() % (40 * HZ)));
+	add_timer(&seed_timer);
+}
+
+static void prandom_start_seed_timer(void)
+{
+	set_timer_slack(&seed_timer, HZ);
+	seed_timer.expires = jiffies + 40 * HZ;
+	add_timer(&seed_timer);
+}
+
 /*
  *	Generate better values after random number generator
  *	is fully initialized.
@@ -194,6 +216,7 @@ static int __init prandom_reseed(void)
 		/* mix it in */
 		prandom_u32_state(state);
 	}
+	prandom_start_seed_timer();
 	return 0;
 }
 late_initcall(prandom_reseed);