linux-kernel - [PATCH 4.19 091/234] random: tie batched entropy generation to base

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Message-Id: <20220623164345.633845719@linuxfoundation.org>
Date:   Thu, 23 Jun 2022 18:42:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Theodore Tso <tytso@....edu>,
        Eric Biggers <ebiggers@...gle.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH 4.19 091/234] random: tie batched entropy generation to base_crng generation

From: "Jason A. Donenfeld" <Jason@...c4.com>

commit 0791e8b655cc373718f0f58800fdc625a3447ac5 upstream.

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: Eric Biggers <ebiggers@...gle.com>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/char/random.c |   29 ++++++++---------------------
 1 file changed, 8 insertions(+), 21 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -428,8 +428,6 @@ 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
@@ -452,7 +450,7 @@ static size_t crng_fast_load(const void
 		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);
@@ -529,7 +527,6 @@ 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;
 	}
@@ -1254,8 +1251,9 @@ 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");
 	}
@@ -1593,8 +1591,6 @@ struct ctl_table random_table[] = {
 };
 #endif	/* CONFIG_SYSCTL */
 
-static atomic_t batch_generation = ATOMIC_INIT(0);
-
 struct batched_entropy {
 	union {
 		/*
@@ -1607,8 +1603,8 @@ struct batched_entropy {
 		u64 entropy_u64[CHACHA20_BLOCK_SIZE * 3 / (2 * sizeof(u64))];
 		u32 entropy_u32[CHACHA20_BLOCK_SIZE * 3 / (2 * sizeof(u32))];
 	};
+	unsigned long generation;
 	unsigned int position;
-	int generation;
 };
 
 /*
@@ -1627,14 +1623,14 @@ 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_irq_save(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));
@@ -1660,14 +1656,14 @@ 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_irq_save(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));
@@ -1683,15 +1679,6 @@ 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.