In order to chip away at the "premature first" problem, we augment our
existing entropy accounting with increased 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 shephard 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. For this we start at
5 seconds after boot, and double that interval until it's more than 5
minutes. After that, we then move to our normal schedule of reseeding
not more than once per 5 minutes.

Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Changes v1->v2:
- Simplified arithmetic, prevented overflow.

 drivers/char/random.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0ceda9a12bfe..8c08186205f4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -333,6 +333,27 @@ 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 is the case 5,
+ * 10, 20, 40, 80, and 160 seconds after boot, and after if the
+ * last reseeding was CRNG_RESEED_INTERVAL ago.
+ */
+static bool crng_has_old_seed(void)
+{
+	static unsigned int next_init_secs = 5;
+
+	if (unlikely(next_init_secs < CRNG_RESEED_INTERVAL / HZ)) {
+		unsigned int uptime = min_t(u64, INT_MAX, ktime_get_seconds());
+		if (uptime >= READ_ONCE(next_init_secs)) {
+			WRITE_ONCE(next_init_secs, 5U << fls(uptime / 5));
+			return true;
+		}
+		return false;
+	}
+	return time_after(jiffies, READ_ONCE(base_crng.birth) + CRNG_RESEED_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
@@ -366,10 +387,10 @@ 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

Hi Jason,

On Wed, Mar 09, 2022 at 12:18:50PM -0700, Jason A. Donenfeld wrote:
> In order to chip away at the "premature first" problem, we augment our
> existing entropy accounting with increased reseedings at boot.

I'm very glad to see this; this is something that I've been concerned about.
I think this is basically the right solution until something more sophisticated
can be implemented (as you said).

A few comments below.

> 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 shephard 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. For this we start at
> 5 seconds after boot, and double that interval until it's more than 5
> minutes. After that, we then move to our normal schedule of reseeding
> not more than once per 5 minutes.

Break up the above into multiple paragraphs?

> +/*
> + * Return whether the crng seed is considered to be sufficiently
> + * old that a reseeding might be attempted. This is the case 5,
> + * 10, 20, 40, 80, and 160 seconds after boot, and after if the
> + * last reseeding was CRNG_RESEED_INTERVAL ago.
> + */
> +static bool crng_has_old_seed(void)
> +{
> +	static unsigned int next_init_secs = 5;
> +
> +	if (unlikely(next_init_secs < CRNG_RESEED_INTERVAL / HZ)) {

The read of 'next_init_secs' needs READ_ONCE(), since it can be written to
concurrently.

> +		unsigned int uptime = min_t(u64, INT_MAX, ktime_get_seconds());
> +		if (uptime >= READ_ONCE(next_init_secs)) {
> +			WRITE_ONCE(next_init_secs, 5U << fls(uptime / 5));
> +			return true;
> +		}
> +		return false;

The '5U << fls(uptime / 5)' expression is a little hard to understand, but it
appears to work as intended.

However, one thing that seems a bit odd is that this method can result in two
reseeds with very little time in between.  For example, if no one is using the
RNG from second 40-78, but then it is used in seconds 79-80, then it will be
reseeded at both seconds 79 and 80 if there is entropy available.

Perhaps the condition should still be:

	time_after(jiffies, READ_ONCE(base_crng.birth) + interval);

... as it is in the non-early case, but where 'interval' is a function of
'uptime' rather than always CRNG_RESEED_INTERVAL?  Maybe something like:

	interval = CRNG_RESEED_INTERVAL;
	if (uptime < 2 * CRNG_RESEED_INTERVAL / HZ)
		interval = max(5, uptime / 2) * HZ;

- Eric

Hey Eric,

On Wed, Mar 9, 2022 at 9:57 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > not more than once per 5 minutes.
>
> Break up the above into multiple paragraphs?

Will do.

>
> > +/*
> > + * Return whether the crng seed is considered to be sufficiently
> > + * old that a reseeding might be attempted. This is the case 5,
> > + * 10, 20, 40, 80, and 160 seconds after boot, and after if the
> > + * last reseeding was CRNG_RESEED_INTERVAL ago.
> > + */
> > +static bool crng_has_old_seed(void)
> > +{
> > +     static unsigned int next_init_secs = 5;
> > +
> > +     if (unlikely(next_init_secs < CRNG_RESEED_INTERVAL / HZ)) {
>
> The read of 'next_init_secs' needs READ_ONCE(), since it can be written to
> concurrently.

Thanks, will fix.

> However, one thing that seems a bit odd is that this method can result in two
> reseeds with very little time in between.  For example, if no one is using the
> RNG from second 40-78, but then it is used in seconds 79-80, then it will be
> reseeded at both seconds 79 and 80 if there is entropy available.

I've been sort of going back and forth on this. I think the idea is
that there are two relative time measurements. The ordinary one we use
is time since last reseeding. But at boot, we're trying to account for
the fact that entropy is coming in relative to the init process of the
system, which means we need it to be relative to boot time rather than
relative to the last reseeding. As you point out, this is a little
wonky with how things are now, because we only ever reseed on usage.
To get closer to what I have in mind, we could reseed in a timer, so
that it hits it exactly on the 5,10,20,40,etc dot. But that seems a
bit cumbersome and maybe unnecessary. The effect of the behavior of
this v1 you pointed out is:

- We might reseed at 79, and then fail to reseed at 80. Consequence:
we lose 1 second of entropy that could have made it for that try.
- We might reseed at 79, and then also reseed at 80 too. Consequence:
that's a fairly quick refresh, but on the other hand, we're still
requiring 256 bit credits, so maybe not so bad, and if we've got so
much entropy coming in during that small period of time, maybe it
really isn't a concern.

So I'm not sure either of these cases really matter that much.

> Perhaps the condition should still be:
>
>         time_after(jiffies, READ_ONCE(base_crng.birth) + interval);
>
> ... as it is in the non-early case, but where 'interval' is a function of
> 'uptime' rather than always CRNG_RESEED_INTERVAL?  Maybe something like:
>
>         interval = CRNG_RESEED_INTERVAL;
>         if (uptime < 2 * CRNG_RESEED_INTERVAL / HZ)
>                 interval = max(5, uptime / 2) * HZ;

I'd experimented with things like that, for example making it exponential:

  static bool early_boot = true;
  unsigned long interval = CRNG_RESEED_INTERVAL;

  if (unlikely(READ_ONCE(early_boot))) {
    unsigned int uptime = min_t(u64, INT_MAX, ktime_get_seconds());
    if (uptime >= CRNG_RESEED_INTERVAL / HZ)
      WRITE_ONCE(early_boot, false);
    else
      interval = (5U << fls(uptime / 5)) * HZ;
  }
  return time_after(jiffies, READ_ONCE(base_crng.birth) + interval);

But the whole thing of combining relative-to-last-reseed with
relative-to-boottime seems really strange. I'm not quite sure what
that's supposed to represent, whereas what I have in v1 is
"exponentially increasing intervals from boottime" which is fairly
easy to understand.

Jason

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 shephard 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.

For this we start at 5 seconds after boot, and double that interval
until it's more than 5 minutes. After that, we then move to our normal
schedule of reseeding not more than once per 5 minutes.

One interesting caveat is that reseeding is presently on-demand, so
we're not reseeding at precisely the 5, 10, 20, 40, 80, 160 marks, but
rather whenever the crng is actually used and we've matched or exceeded
one of those marks. This means that we might wind up trying to reseed at
both second 79 and second 80, or at second 41 and second 80, or anywhere
in between. In that sense, this isn't quite imposing a minimum frequency
on reseeds during early boot, but simply bounding a maximum frequency.
Since we're still requiring 256 bits of entropy credits, this shouldn't
be a super large issue in terms of "premature next", and in the event
that we reseed a bit late and then miss the next reseeding due to not
enough entropy credits, we still have acquired more entropy by virtue of
having reseeded later.

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>
---
Changes v1->v2:
- Note "on-demand" funkiness in commit message.
- Always use READ_ONCE() with next_init_secs.

 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 d94d5ba414ee..27d91cfc3cd9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -333,6 +333,28 @@ 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 is the case 5,
+ * 10, 20, 40, 80, and 160 seconds after boot, and after if the
+ * last reseeding was CRNG_RESEED_INTERVAL ago.
+ */
+static bool crng_has_old_seed(void)
+{
+	static unsigned int next_init_secs = 5;
+	unsigned int this_init_secs = READ_ONCE(next_init_secs);
+
+	if (unlikely(this_init_secs < CRNG_RESEED_INTERVAL / HZ)) {
+		unsigned int uptime = min_t(u64, INT_MAX, ktime_get_seconds());
+		if (uptime >= this_init_secs) {
+			WRITE_ONCE(next_init_secs, 5U << fls(uptime / 5));
+			return true;
+		}
+		return false;
+	}
+	return time_after(jiffies, READ_ONCE(base_crng.birth) + CRNG_RESEED_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
@@ -366,10 +388,10 @@ 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