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kernel: Fixes for z_early_rand_get

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The early random get function was making many wrong assumptions
about random subsys and entropy drivers. First, it was assuming
that entropy_get_entropy() would be ISR safe, that is not right,
the driver has an ISR safe callback and if it is not implemented
or not working it is not ok using the other callback.
Second, the fallback to the random subsys is even more problematic
since they can use kernel services to protect internal states and be
thread-safe.

Another incorrect thing in this function was the guard around it.
It was needed by features like stack randomization and stack canaries,
and not when those conditions were match. Just remove it and in case
it is not needed the linker will take care of it.

The drawback of this change is that in the absence of an entropy
generator with support to be called from ISR the randomness is very
weak.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
This commit is contained in:
Flavio Ceolin 2023-10-09 20:52:07 -07:00 • committed by Johan Hedberg
commit 6c0fad2888
1 changed files with 23 additions and 31 deletions
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54
kernel/init.c
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@ -508,46 +508,38 @@ static FUNC_NORETURN void switch_to_main_thread(char *stack_ptr)
}
#endif /* CONFIG_MULTITHREADING */
#if defined(CONFIG_ENTROPY_HAS_DRIVER) || defined(CONFIG_TEST_RANDOM_GENERATOR)
__boot_func
void __weak z_early_rand_get(uint8_t *buf, size_t length)
{
#ifdef CONFIG_ENTROPY_HAS_DRIVER
const struct device *const entropy = DEVICE_DT_GET_OR_NULL(DT_CHOSEN(zephyr_entropy));
static uint64_t state = 123456789UL;
int rc;
if (!device_is_ready(entropy)) {
goto sys_rand_fallback;
#ifdef CONFIG_ENTROPY_HAS_DRIVER
const struct device *const entropy = DEVICE_DT_GET_OR_NULL(DT_CHOSEN(zephyr_entropy));
if ((entropy != NULL) && device_is_ready(entropy)) {
/* Try to see if driver provides an ISR-specific API */
rc = entropy_get_entropy_isr(entropy, buf, length, ENTROPY_BUSYWAIT);
if (rc > 0) {
length -= rc;
buf += rc;
}
}
/* Try to see if driver provides an ISR-specific API */
rc = entropy_get_entropy_isr(entropy, buf, length, ENTROPY_BUSYWAIT);
if (rc == -ENOTSUP) {
/* Driver does not provide an ISR-specific API, assume it can
* be called from ISR context
*/
rc = entropy_get_entropy(entropy, buf, length);
}
if (rc >= 0) {
return;
}
/* Fall through to fallback */
sys_rand_fallback:
#endif
/* FIXME: this assumes sys_rand32_get() won't use any synchronization
* primitive, like semaphores or mutexes. It's too early in the boot
* process to use any of them. Ideally, only the path where entropy
* devices are available should be built, this is only a fallback for
* those devices without a HWRNG entropy driver.
*/
sys_rand_get(buf, length);
while (length > 0) {
uint32_t val;
state = state + k_cycle_get_32();
state = state * 2862933555777941757ULL + 3037000493ULL;
val = (uint32_t)(state >> 32);
rc = MIN(length, sizeof(val));
z_early_memcpy((void *)buf, &val, rc);
length -= rc;
buf += rc;
}
}
/* defined(CONFIG_ENTROPY_HAS_DRIVER) || defined(CONFIG_TEST_RANDOM_GENERATOR) */
#endif
/**
*