From b7b67d1391a831eb9de133e85a2230e2e81a2ce4 Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Sun, 1 May 2022 13:51:34 +0200 Subject: [PATCH] random: mix in timestamps and reseed on system restore 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 Signed-off-by: Jason A. Donenfeld --- drivers/char/random.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/drivers/char/random.c b/drivers/char/random.c index bd6fdb624861..2d10942ba534 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -53,6 +53,7 @@ #include #include #include +#include #include #include #include @@ -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;