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 <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bd6fdb6..2d10942 100644 (file)
--- 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>
@@ -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;