random: fix locking for crng_init in crng_reseed()

crng_init is protected by primary_crng->lock. Therefore, we need
to hold this lock when increasing crng_init to 2. As we shouldn't
hold this lock for too long, only hold it for those parts which
require protection.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -502,6 +502,7 @@ static void crng_reseed(void)
 	int entropy_count;
 	unsigned long next_gen;
 	u8 key[CHACHA_KEY_SIZE];
+	bool finalize_init = false;
 
 	/*
 	 * First we make sure we have POOL_MIN_BITS of entropy in the pool,
@@ -529,12 +530,14 @@ static void crng_reseed(void)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
-	spin_unlock_irqrestore(&base_crng.lock, flags);
-	memzero_explicit(key, sizeof(key));
-
 	if (crng_init < 2) {
 		invalidate_batched_entropy();
 		crng_init = 2;
+		finalize_init = true;
+	}
+	spin_unlock_irqrestore(&base_crng.lock, flags);
+	memzero_explicit(key, sizeof(key));
+	if (finalize_init) {
 		process_random_ready_list();
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);