Since the RNG loses freshness 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@xxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
drivers/char/random.c | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0935a140795e..48eac27214ea 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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>
@@ -966,6 +967,37 @@ 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 (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().
@@ -1009,6 +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;
--
2.35.1
