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SubjectRe: [PATCH] random: mix in timestamps and reseed on system restore
Am Sun, May 01, 2022 at 02:38:49PM +0200 schrieb Jason A. Donenfeld:
> 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@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> 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;
> +}

Should this also wake up any thread waiting in add_hwgenerator_randomness()
/ "use" the input already in store there?

Thanks,
Dominik

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