random: schedule jitter credit for next jiffy, not in two jiffies

[ 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>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 8dfb28d5ae3fab9b57202e0cb5b7a2965cfa80a2..5defbc479a5c7b5311f39fb538f139893d48e247 100644 (file)
--- 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();