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hwrng: core - start hwrng kthread also for untrusted sources

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Start the hwrng kthread even if the hwrng source has a quality setting
of zero. Then, every crng reseed interval, one batch of data from this
zero-quality hwrng source will be mixed into the CRNG pool.

This patch is based on the assumption that data from a hwrng source
will not actively harm the CRNG state. Instead, many hwrng sources
(such as TPM devices), even though they are assigend a quality level of
zero, actually provide some entropy, which is good enough to mix into
the CRNG pool every once in a while.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Dominik Brodowski 2022-09-22 15:59:31 +02:00 committed by Herbert Xu
parent 4edff849f7
commit b006c439d5
1 changed files with 10 additions and 26 deletions
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36
drivers/char/hw_random/core.c
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@ -52,7 +52,7 @@ MODULE_PARM_DESC(default_quality,
static void drop_current_rng(void);
static int hwrng_init(struct hwrng *rng);
static void hwrng_manage_rngd(struct hwrng *rng);
static int hwrng_fillfn(void *unused);
static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
int wait);
@ -96,6 +96,15 @@ static int set_current_rng(struct hwrng *rng)
drop_current_rng();
current_rng = rng;
/* if necessary, start hwrng thread */
if (!hwrng_fill) {
hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
if (IS_ERR(hwrng_fill)) {
pr_err("hwrng_fill thread creation failed\n");
hwrng_fill = NULL;
}
}
return 0;
}
@ -167,8 +176,6 @@ skip_init:
rng->quality = 1024;
current_quality = rng->quality; /* obsolete */
hwrng_manage_rngd(rng);
return 0;
}
@ -454,10 +461,6 @@ static ssize_t rng_quality_store(struct device *dev,
/* the best available RNG may have changed */
ret = enable_best_rng();
/* start/stop rngd if necessary */
if (current_rng)
hwrng_manage_rngd(current_rng);
out:
mutex_unlock(&rng_mutex);
return ret ? ret : len;
@ -513,9 +516,6 @@ static int hwrng_fillfn(void *unused)
put_rng(rng);
if (!quality)
break;
if (rc <= 0)
continue;
@ -534,22 +534,6 @@ static int hwrng_fillfn(void *unused)
return 0;
}
static void hwrng_manage_rngd(struct hwrng *rng)
{
if (WARN_ON(!mutex_is_locked(&rng_mutex)))
return;
if (rng->quality == 0 && hwrng_fill)
kthread_stop(hwrng_fill);
if (rng->quality > 0 && !hwrng_fill) {
hwrng_fill = kthread_run(hwrng_fillfn, NULL, "hwrng");
if (IS_ERR(hwrng_fill)) {
pr_err("hwrng_fill thread creation failed\n");
hwrng_fill = NULL;
}
}
}
int hwrng_register(struct hwrng *rng)
{
int err = -EINVAL;