linux-kernel - [PATCH v2] random: tie batched entropy generation to base

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Message-Id: <20220210131304.97224-1-Jason@zx2c4.com>
Date:   Thu, 10 Feb 2022 14:13:04 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: [PATCH v2] 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@....edu>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
v2 always increments the generation after extraction, as suggested by
Dominik.

 drivers/char/random.c | 29 ++++++++---------------------
 1 file changed, 8 insertions(+), 21 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 5beb421ec12b..57d36f13e3a6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -431,8 +431,6 @@ 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
@@ -455,7 +453,7 @@ 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);
@@ -530,7 +528,6 @@ static void crng_reseed(void)
 	WRITE_ONCE(base_crng.generation, next_gen);
 	base_crng.birth = jiffies;
 	if (crng_init < 2) {
-		invalidate_batched_entropy();
 		crng_init = 2;
 		finalize_init = true;
 	}
@@ -1277,8 +1274,9 @@ 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");
 	}
@@ -1628,8 +1626,6 @@ 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 {
 		/* We make this 1.5x a ChaCha block, so that we get the
@@ -1642,8 +1638,8 @@ 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;
 };
 
 /*
@@ -1662,14 +1658,14 @@ 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) == 0 ||
 	    next_gen != batch->generation) {
 		_get_random_bytes(batch->entropy_u64, sizeof(batch->entropy_u64));
@@ -1695,14 +1691,14 @@ 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) == 0 ||
 	    next_gen != batch->generation) {
 		_get_random_bytes(batch->entropy_u32, sizeof(batch->entropy_u32));
@@ -1718,15 +1714,6 @@ 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.
-- 
2.35.0