| Message ID | 20220313014110.54451-1-Jason@zx2c4.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Delegated to: | Herbert Xu |
| Headers | show |
| Series | [v4] random: reseed more often immediately after booting | expand |
On Sat, Mar 12, 2022 at 06:41:10PM -0700, Jason A. Donenfeld wrote: > 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); > Looks good, Reviewed-by: Eric Biggers <ebiggers@google.com> - Eric
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);
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(-)