[PATCH v4] random: reseed more often immediately after booting

["Jason A. Donenfeld" <Jason@zx2c4.com>]

In order to chip away at the "premature first" problem, we augment our
existing entropy accounting with more frequent reseedings at boot.

The idea is that at boot, we're getting entropy from various places, and
we're not very sure which of early boot entropy is good and which isn't.
Even when we're crediting the entropy, we're still not totally certain
that it's any good. Since boot is the one time (aside from a compromise)
that we have zero entropy, it's important that we shepherd entropy into
the crng fairly often.

At the same time, we don't want a "premature next" problem, whereby an
attacker can brute force individual bits of added entropy. In lieu of
going full-on Fortuna (for now), we can pick a simpler strategy of just
reseeding more often during the first 5 minutes after boot. This is
still bounded by the 256-bit entropy credit requirement, so we'll skip a
reseeding if we haven't reached that, but in case entropy /is/ coming
in, this ensures that it makes its way into the crng rather rapidly
during these early stages.

Ordinarily we reseed if the previous reseeding is 300 seconds old. This
commit changes things so that for the first 600 seconds of boot time, we
reseed if the previous reseeding is uptime / 2 seconds old. That means
that we'll reseed at the very least double the uptime of the previous
reseeding.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
v4 uses Eric's formulation relative to the last reseed time, rather than
my prior one relative to boot time alone.

 drivers/char/random.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 19a602c69f2f..defdba110d1d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -327,6 +327,28 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
 	memzero_explicit(first_block, sizeof(first_block));
 }
 
+/*
+ * Return whether the crng seed is considered to be sufficiently
+ * old that a reseeding might be attempted. This happens if the last
+ * reseeding was CRNG_RESEED_INTERVAL ago, or during early boot, at
+ * an interval proportional to the uptime.
+ */
+static bool crng_has_old_seed(void)
+{
+	static bool early_boot = true;
+	unsigned long interval = CRNG_RESEED_INTERVAL;
+
+	if (unlikely(READ_ONCE(early_boot))) {
+		time64_t uptime = ktime_get_seconds();
+		if (uptime >= CRNG_RESEED_INTERVAL / HZ * 2)
+			WRITE_ONCE(early_boot, false);
+		else
+			interval = max_t(unsigned int, 5 * HZ,
+					 (unsigned int)uptime / 2 * HZ);
+	}
+	return time_after(jiffies, READ_ONCE(base_crng.birth) + interval);
+}
+
 /*
  * This function returns a ChaCha state that you may use for generating
  * random data. It also returns up to 32 bytes on its own of random data
@@ -360,10 +382,10 @@ static void crng_make_state(u32 chacha_state[CHACHA_STATE_WORDS],
 	}
 
 	/*
-	 * If the base_crng is more than 5 minutes old, we reseed, which
-	 * in turn bumps the generation counter that we check below.
+	 * If the base_crng is old enough, we try to reseed, which in turn
+	 * bumps the generation counter that we check below.
 	 */
-	if (unlikely(time_after(jiffies, READ_ONCE(base_crng.birth) + CRNG_RESEED_INTERVAL)))
+	if (unlikely(crng_has_old_seed()))
 		crng_reseed(false);
 
 	local_lock_irqsave(&crngs.lock, flags);
-- 
2.35.1