linux-kernel - [PATCH v2] random: unify cycles

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Date:   Thu, 24 Feb 2022 22:30:30 +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: unify cycles_t and jiffies usage and types

random_get_entropy() returns a cycles_t, not an unsigned long, which is
sometimes 64 bits on various 32-bit platforms, including x86.
Conversely, jiffies is always unsigned long. This commit fixes things to
use cycles_t for fields that use random_get_entropy() and unsigned long
for fields that use jiffies. It's also good to mix in a cycles_t and a
jiffies in the same way for both add_device_randomness and
add_timer_randomness, rather than using xor in one case. Finally, we
unify the order of these volatile reads, always reading the more precise
cycles counter, and then jiffies, so that the cycle counter is as close
to the event as possible.

Cc: Theodore Ts'o <tytso@....edu>
Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
Changes v1->v2:
- [Dominik] Use unsigned long, not long, for jiffies, except when it
  represents a delta.
- Also unify how jiffies and cycles combine, where they're used, and the
  order in which they're read.
- Stylistic fixes.

 drivers/char/random.c | 46 +++++++++++++++++++++----------------------
 1 file changed, 22 insertions(+), 24 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 19bf44b9ba0f..12a1b2cfd702 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1000,12 +1000,6 @@ int __init rand_initialize(void)
 	return 0;
 }
 
-/* There is one of these per entropy source */
-struct timer_rand_state {
-	cycles_t last_time;
-	long last_delta, last_delta2;
-};
-
 /*
  * Add device- or boot-specific data to the input pool to help
  * initialize it.
@@ -1016,19 +1010,26 @@ struct timer_rand_state {
  */
 void add_device_randomness(const void *buf, size_t size)
 {
-	unsigned long time = random_get_entropy() ^ jiffies;
-	unsigned long flags;
+	cycles_t cycles = random_get_entropy();
+	unsigned long now = jiffies, flags;
 
 	if (crng_init == 0 && size)
 		crng_pre_init_inject(buf, size, false);
 
 	spin_lock_irqsave(&input_pool.lock, flags);
+	_mix_pool_bytes(&cycles, sizeof(cycles));
+	_mix_pool_bytes(&now, sizeof(now));
 	_mix_pool_bytes(buf, size);
-	_mix_pool_bytes(&time, sizeof(time));
 	spin_unlock_irqrestore(&input_pool.lock, flags);
 }
 EXPORT_SYMBOL(add_device_randomness);
 
+/* There is one of these per entropy source */
+struct timer_rand_state {
+	unsigned long last_time;
+	long last_delta, last_delta2;
+};
+
 /*
  * This function adds entropy to the entropy "pool" by using timing
  * delays.  It uses the timer_rand_state structure to make an estimate
@@ -1037,29 +1038,26 @@ EXPORT_SYMBOL(add_device_randomness);
  * The number "num" is also added to the pool - it should somehow describe
  * the type of event which just happened.  This is currently 0-255 for
  * keyboard scan codes, and 256 upwards for interrupts.
- *
  */
 static void add_timer_randomness(struct timer_rand_state *state, unsigned int num)
 {
-	struct {
-		long jiffies;
-		unsigned int cycles;
-		unsigned int num;
-	} sample;
+	cycles_t cycles = random_get_entropy();
+	unsigned long now = jiffies, flags;
 	long delta, delta2, delta3;
 
-	sample.jiffies = jiffies;
-	sample.cycles = random_get_entropy();
-	sample.num = num;
-	mix_pool_bytes(&sample, sizeof(sample));
+	spin_lock_irqsave(&input_pool.lock, flags);
+	_mix_pool_bytes(&cycles, sizeof(cycles));
+	_mix_pool_bytes(&now, sizeof(now));
+	_mix_pool_bytes(&num, sizeof(num));
+	spin_unlock_irqrestore(&input_pool.lock, flags);
 
 	/*
 	 * Calculate number of bits of randomness we probably added.
 	 * We take into account the first, second and third-order deltas
 	 * in order to make our estimate.
 	 */
-	delta = sample.jiffies - READ_ONCE(state->last_time);
-	WRITE_ONCE(state->last_time, sample.jiffies);
+	delta = now - READ_ONCE(state->last_time);
+	WRITE_ONCE(state->last_time, now);
 
 	delta2 = delta - READ_ONCE(state->last_delta);
 	WRITE_ONCE(state->last_delta, delta);
@@ -1291,10 +1289,10 @@ static void mix_interrupt_randomness(struct work_struct *work)
 void add_interrupt_randomness(int irq)
 {
 	enum { MIX_INFLIGHT = 1U << 31 };
+	cycles_t cycles = random_get_entropy();
+	unsigned long now = jiffies;
 	struct fast_pool *fast_pool = this_cpu_ptr(&irq_randomness);
 	struct pt_regs *regs = get_irq_regs();
-	unsigned long now = jiffies;
-	cycles_t cycles = random_get_entropy();
 	unsigned int new_count;
 
 	if (cycles == 0)
@@ -1356,7 +1354,7 @@ static void entropy_timer(struct timer_list *t)
 static void try_to_generate_entropy(void)
 {
 	struct {
-		unsigned long now;
+		cycles_t now;
 		struct timer_list timer;
 	} stack;
 
-- 
2.35.1