random: avoid initializing twice in credit race

Since all changes of crng_init now go through credit_init_bits(), we can
fix a long standing race in which two concurrent callers of
credit_init_bits() have the new bit count >= some threshold, but are
doing so with crng_init as a lower threshold, checked outside of a lock,
resulting in crng_reseed() or similar being called twice.

In order to fix this, we can use the original cmpxchg value of the bit
count, and only change crng_init when the bit count transitions from
below a threshold to meeting the threshold.
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -822,7 +822,7 @@ static void extract_entropy(void *buf, size_t nbytes)
 
 static void credit_init_bits(size_t nbits)
 {
-	unsigned int init_bits, orig, add;
+	unsigned int new, orig, add;
 	unsigned long flags;
 
 	if (crng_ready() || !nbits)
@@ -832,12 +832,12 @@ static void credit_init_bits(size_t nbits)
 
 	do {
 		orig = READ_ONCE(input_pool.init_bits);
-		init_bits = min_t(unsigned int, POOL_BITS, orig + add);
-	} while (cmpxchg(&input_pool.init_bits, orig, init_bits) != orig);
+		new = min_t(unsigned int, POOL_BITS, orig + add);
+	} while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
 
-	if (!crng_ready() && init_bits >= POOL_READY_BITS)
+	if (orig < POOL_READY_BITS && new >= POOL_READY_BITS)
 		crng_reseed();
-	else if (unlikely(crng_init == CRNG_EMPTY && init_bits >= POOL_EARLY_BITS)) {
+	else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
 		spin_lock_irqsave(&base_crng.lock, flags);
 		if (crng_init == CRNG_EMPTY) {
 			extract_entropy(base_crng.key, sizeof(base_crng.key));