random: add back async readiness notifier

This is required by vsprint, because it can't do things synchronously
from hardirq context, and it will be useful for an EFI notifier as well.
I didn't initially want to do this, but with two potential consumers
now, it seems worth it.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -84,6 +84,7 @@ static DEFINE_STATIC_KEY_FALSE(crng_is_ready);
 /* Various types of waiters for crng_init->CRNG_READY transition. */
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct fasync_struct *fasync;
+static ATOMIC_NOTIFIER_HEAD(random_ready_notifier);
 
 /* Control how we warn userspace. */
 static struct ratelimit_state urandom_warning =
@@ -140,6 +141,26 @@ int wait_for_random_bytes(void)
 }
 EXPORT_SYMBOL(wait_for_random_bytes);
 
+/*
+ * Add a callback function that will be invoked when the crng is initialised,
+ * or immediately if it already has been. Only use this is you are absolutely
+ * sure it is required. Most users should instead be able to test
+ * `rng_is_initialized()` on demand, or make use of `get_random_bytes_wait()`.
+ */
+int __cold execute_with_initialized_rng(struct notifier_block *nb)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&random_ready_notifier.lock, flags);
+	if (crng_ready())
+		nb->notifier_call(nb, 0, NULL);
+	else
+		ret = raw_notifier_chain_register((struct raw_notifier_head *)&random_ready_notifier.head, nb);
+	spin_unlock_irqrestore(&random_ready_notifier.lock, flags);
+	return ret;
+}
+
 #define warn_unseeded_randomness() \
 	if (IS_ENABLED(CONFIG_WARN_ALL_UNSEEDED_RANDOM) && !crng_ready()) \
 		printk_deferred(KERN_NOTICE "random: %s called from %pS with crng_init=%d\n", \
@@ -697,6 +718,7 @@ static void __cold _credit_init_bits(size_t bits)
 		crng_reseed(NULL); /* Sets crng_init to CRNG_READY under base_crng.lock. */
 		if (static_key_initialized)
 			execute_in_process_context(crng_set_ready, &set_ready);
+		atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 		pr_notice("crng init done\n");

include/linux/random.h

@@ -120,6 +120,7 @@ void __init random_init_early(const char *command_line);
 void __init random_init(void);
 bool rng_is_initialized(void);
 int wait_for_random_bytes(void);
+int execute_with_initialized_rng(struct notifier_block *nb);
 
 /* Calls wait_for_random_bytes() and then calls get_random_bytes(buf, nbytes).
  * Returns the result of the call to wait_for_random_bytes. */