random: tie batched entropy generation to base_crng generation

Now that we have an explicit base_crng generation counter, we don't need
a separate one for batched entropy. Rather, we can just move the
generation forward every time we change crng_init state or update the
base_crng key.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -430,8 +430,6 @@ static DEFINE_PER_CPU(struct crng, crngs) = {
 
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 
-static void invalidate_batched_entropy(void);
-
 /*
  * crng_fast_load() can be called by code in the interrupt service
  * path.  So we can't afford to dilly-dally. Returns the number of
@@ -454,7 +452,7 @@ static size_t crng_fast_load(const void *cp, size_t len)
 		src++; crng_init_cnt++; len--; ret++;
 	}
 	if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
-		invalidate_batched_entropy();
+		++base_crng.generation;
 		crng_init = 1;
 	}
 	spin_unlock_irqrestore(&base_crng.lock, flags);
@@ -531,7 +529,6 @@ static void crng_reseed(void)
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
 	if (crng_init < 2) {
-		invalidate_batched_entropy();
 		crng_init = 2;
 		finalize_init = true;
 	}
@@ -1256,8 +1253,9 @@ int __init rand_initialize(void)
 	mix_pool_bytes(utsname(), sizeof(*(utsname())));
 
 	extract_entropy(base_crng.key, sizeof(base_crng.key));
+	++base_crng.generation;
+
 	if (arch_init && trust_cpu && crng_init < 2) {
-		invalidate_batched_entropy();
 		crng_init = 2;
 		pr_notice("crng init done (trusting CPU's manufacturer)\n");
 	}
@@ -1607,8 +1605,6 @@ static int __init random_sysctls_init(void)
 device_initcall(random_sysctls_init);
 #endif	/* CONFIG_SYSCTL */
 
-static atomic_t batch_generation = ATOMIC_INIT(0);
-
 struct batched_entropy {
 	union {
 		/*
@@ -1622,8 +1618,8 @@ struct batched_entropy {
 		u32 entropy_u32[CHACHA_BLOCK_SIZE * 3 / (2 * sizeof(u32))];
 	};
 	local_lock_t lock;
+	unsigned long generation;
 	unsigned int position;
-	int generation;
 };
 
 /*
@@ -1643,14 +1639,14 @@ u64 get_random_u64(void)
 	unsigned long flags;
 	struct batched_entropy *batch;
 	static void *previous;
-	int next_gen;
+	unsigned long next_gen;
 
 	warn_unseeded_randomness(&previous);
 
 	local_lock_irqsave(&batched_entropy_u64.lock, flags);
 	batch = raw_cpu_ptr(&batched_entropy_u64);
 
-	next_gen = atomic_read(&batch_generation);
+	next_gen = READ_ONCE(base_crng.generation);
 	if (batch->position >= ARRAY_SIZE(batch->entropy_u64) ||
 	    next_gen != batch->generation) {
 		_get_random_bytes(batch->entropy_u64, sizeof(batch->entropy_u64));
@@ -1677,14 +1673,14 @@ u32 get_random_u32(void)
 	unsigned long flags;
 	struct batched_entropy *batch;
 	static void *previous;
-	int next_gen;
+	unsigned long next_gen;
 
 	warn_unseeded_randomness(&previous);
 
 	local_lock_irqsave(&batched_entropy_u32.lock, flags);
 	batch = raw_cpu_ptr(&batched_entropy_u32);
 
-	next_gen = atomic_read(&batch_generation);
+	next_gen = READ_ONCE(base_crng.generation);
 	if (batch->position >= ARRAY_SIZE(batch->entropy_u32) ||
 	    next_gen != batch->generation) {
 		_get_random_bytes(batch->entropy_u32, sizeof(batch->entropy_u32));
@@ -1700,15 +1696,6 @@ u32 get_random_u32(void)
 }
 EXPORT_SYMBOL(get_random_u32);
 
-/* It's important to invalidate all potential batched entropy that might
- * be stored before the crng is initialized, which we can do lazily by
- * bumping the generation counter.
- */
-static void invalidate_batched_entropy(void)
-{
-	atomic_inc(&batch_generation);
-}
-
 /**
  * randomize_page - Generate a random, page aligned address
  * @start:	The smallest acceptable address the caller will take.