random: make add_timer_randomness() fill the nonblocking pool first

Change add_timer_randomness() so that it directs incoming entropy to
the nonblocking pool first if it hasn't been fully initialized yet.
This matches the strategy we use in add_interrupt_randomness(), which
allows us to push the randomness where we need it the most during when
the system is first booting up, so that get_random_bytes() and
/dev/urandom become safe to use as soon as possible.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

drivers/char/random.c

@@ -761,6 +761,7 @@ static struct timer_rand_state input_timer_state;
  */
 static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 {
+	struct entropy_store	*r;
 	struct {
 		long jiffies;
 		unsigned cycles;
@@ -773,7 +774,8 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	sample.jiffies = jiffies;
 	sample.cycles = random_get_entropy();
 	sample.num = num;
-	mix_pool_bytes(&input_pool, &sample, sizeof(sample), NULL);
+	r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
+	mix_pool_bytes(r, &sample, sizeof(sample), NULL);
 
 	/*
 	 * Calculate number of bits of randomness we probably added.
@@ -807,8 +809,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 		 * Round down by 1 bit on general principles,
 		 * and limit entropy entimate to 12 bits.
 		 */
-		credit_entropy_bits(&input_pool,
-				    min_t(int, fls(delta>>1), 11));
+		credit_entropy_bits(r, min_t(int, fls(delta>>1), 11));
 	}
 	preempt_enable();
 }