[PATCH 5.19 533/717] random: schedule jitter credit for next jiffy, not in two jiffies

Greg Kroah-Hartman
2022-10-22T08:47:29Z

From: Jason A. Donenfeld <Jason@zx2c4.com>

[ Upstream commit 122733471384be8c23f019fbbd46bdf7be561dcd ]

Counterintuitively, mod_timer(..., jiffies + 1) will cause the timer to
fire not in the next jiffy, but in two jiffies. The way to cause
the timer to fire in the next jiffy is with mod_timer(..., jiffies).
Doing so then lets us bump the upper bound back up again.

Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
Fixes: 829d680e82a9 ("random: cap jitter samples per bit to factor of HZ")
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/char/random.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 8dfb28d5ae3f..5defbc479a5c 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1178,7 +1178,7 @@ static void __cold entropy_timer(struct timer_list *timer)
*/
static void __cold try_to_generate_entropy(void)
{
- enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
+ enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
struct entropy_timer_state stack;
unsigned int i, num_different = 0;
unsigned long last = random_get_entropy();
@@ -1197,7 +1197,7 @@ static void __cold try_to_generate_entropy(void)
timer_setup_on_stack(&stack.timer, entropy_timer, 0);
while (!crng_ready() && !signal_pending(current)) {
if (!timer_pending(&stack.timer))
- mod_timer(&stack.timer, jiffies + 1);
+ mod_timer(&stack.timer, jiffies);
mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
schedule();
stack.entropy = random_get_entropy();
--
2.35.1