From: "Jason A. Donenfeld" Jason@zx2c4.com
stable inclusion from stable-v5.10.119 commit 20da9c6079df82d3c6e53e5d196950e30ddf5252 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6BB
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=...
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commit 7a7ff644aeaf071d433caffb3b8ea57354b55bd3 upstream.
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 Reviewed-by: Eric Biggers ebiggers@google.com Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Zheng Zengkai zhengzengkai@huawei.com Acked-by: Xie XiuQi xiexiuqi@huawei.com --- 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 90a3a09999d5..4187cc8e4cb7 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -337,6 +337,28 @@ 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 @@ -370,10 +392,10 @@ 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();
local_lock_irqsave(&crngs.lock, flags);