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random: mix in timestamps and reseed on system restore

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Since the RNG loses freshness with system suspend/hibernation, when we
resume, immediately reseed using whatever data we can, which for this
particular case is the various timestamps regarding system suspend time,
in addition to more generally the RDSEED/RDRAND/RDTSC values that happen
whenever the crng reseeds.

On systems that suspend and resume automatically all the time -- such as
Android -- we skip the reseeding on suspend resumption, since that could
wind up being far too busy. This is the same trade-off made in
WireGuard.

In addition to reseeding upon resumption always mix into the pool these
various stamps on every power notification event.

Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
This commit is contained in:
Jason A. Donenfeld 2022-05-01 13:51:34 +02:00
parent 78c768e619
commit b7b67d1391
1 changed files with 30 additions and 0 deletions
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30
drivers/char/random.c
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@ -53,6 +53,7 @@
#include <linux/completion.h>
#include <linux/uuid.h>
#include <linux/uaccess.h>
#include <linux/suspend.h>
#include <crypto/chacha.h>
#include <crypto/blake2s.h>
#include <asm/processor.h>
@ -970,6 +971,33 @@ static int __init parse_trust_bootloader(char *arg)
early_param("random.trust_cpu", parse_trust_cpu);
early_param("random.trust_bootloader", parse_trust_bootloader);
static int random_pm_notification(struct notifier_block *nb, unsigned long action, void *data)
{
unsigned long flags, entropy = random_get_entropy();
/*
* Encode a representation of how long the system has been suspended,
* in a way that is distinct from prior system suspends.
*/
ktime_t stamps[] = { ktime_get(), ktime_get_boottime(), ktime_get_real() };
spin_lock_irqsave(&input_pool.lock, flags);
_mix_pool_bytes(&action, sizeof(action));
_mix_pool_bytes(stamps, sizeof(stamps));
_mix_pool_bytes(&entropy, sizeof(entropy));
spin_unlock_irqrestore(&input_pool.lock, flags);
if (crng_ready() && (action == PM_RESTORE_PREPARE ||
(action == PM_POST_SUSPEND &&
!IS_ENABLED(CONFIG_PM_AUTOSLEEP) && !IS_ENABLED(CONFIG_ANDROID)))) {
crng_reseed(true);
pr_notice("crng reseeded on system resumption\n");
}
return 0;
}
static struct notifier_block pm_notifier = { .notifier_call = random_pm_notification };
/*
* The first collection of entropy occurs at system boot while interrupts
* are still turned off. Here we push in RDSEED, a timestamp, and utsname().
@ -1013,6 +1041,8 @@ int __init rand_initialize(void)
unseeded_warning.interval = 0;
}
WARN_ON(register_pm_notifier(&pm_notifier));
WARN(!random_get_entropy(), "Missing cycle counter and fallback timer; RNG "
"entropy collection will consequently suffer.");
return 0;