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/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2020, Peter Elliott <pelliott@ualberta.ca>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <AK/Singleton.h>
#include <Kernel/Arch/x86/CPU.h>
#include <Kernel/Devices/RandomDevice.h>
#include <Kernel/Random.h>
#include <Kernel/Time/HPET.h>
#include <Kernel/Time/RTC.h>
#include <Kernel/Time/TimeManagement.h>
namespace Kernel {
static AK::Singleton<KernelRng> s_the;
KernelRng& KernelRng::the()
{
return *s_the;
}
UNMAP_AFTER_INIT KernelRng::KernelRng()
{
bool supports_rdseed = Processor::current().has_feature(CPUFeature::RDSEED);
bool supports_rdrand = Processor::current().has_feature(CPUFeature::RDRAND);
if (supports_rdseed || supports_rdrand) {
dmesgln("KernelRng: Using RDSEED or RDRAND as entropy source");
for (size_t i = 0; i < resource().pool_count * resource().reseed_threshold; ++i) {
u32 value = 0;
if (supports_rdseed) {
asm volatile(
"1:\n"
"rdseed %0\n"
"jnc 1b\n"
: "=r"(value));
} else {
asm volatile(
"1:\n"
"rdrand %0\n"
"jnc 1b\n"
: "=r"(value));
}
this->resource().add_random_event(value, i % 32);
}
} else if (TimeManagement::the().can_query_precise_time()) {
// Add HPET as entropy source if we don't have anything better.
dmesgln("KernelRng: Using HPET as entropy source");
for (size_t i = 0; i < resource().pool_count * resource().reseed_threshold; ++i) {
u64 hpet_time = HPET::the().read_main_counter_unsafe();
this->resource().add_random_event(hpet_time, i % 32);
}
} else {
// Fallback to RTC
dmesgln("KernelRng: Using RTC as entropy source (bad!)");
auto current_time = static_cast<u64>(RTC::now());
for (size_t i = 0; i < resource().pool_count * resource().reseed_threshold; ++i) {
this->resource().add_random_event(current_time, i % 32);
current_time *= 0x574au;
current_time += 0x40b2u;
}
}
}
void KernelRng::wait_for_entropy()
{
ScopedSpinLock lock(get_lock());
if (!resource().is_ready()) {
dbgln("Entropy starvation...");
m_seed_queue.wait_forever("KernelRng");
}
}
void KernelRng::wake_if_ready()
{
VERIFY(get_lock().is_locked());
if (resource().is_ready()) {
m_seed_queue.wake_all();
}
}
size_t EntropySource::next_source { static_cast<size_t>(EntropySource::Static::MaxHardcodedSourceIndex) };
static void do_get_fast_random_bytes(u8* buffer, size_t buffer_size)
{
static Atomic<u32, AK::MemoryOrder::memory_order_relaxed> next = 1;
union {
u8 bytes[4];
u32 value;
} u;
size_t offset = 4;
for (size_t i = 0; i < buffer_size; ++i) {
if (offset >= 4) {
auto current_next = next.load();
for (;;) {
auto new_next = current_next * 1103515245 + 12345;
if (next.compare_exchange_strong(current_next, new_next)) {
u.value = new_next;
break;
}
}
offset = 0;
}
buffer[i] = u.bytes[offset++];
}
}
bool get_good_random_bytes(u8* buffer, size_t buffer_size, bool allow_wait, bool fallback_to_fast)
{
bool result = false;
auto& kernel_rng = KernelRng::the();
// FIXME: What if interrupts are disabled because we're in an interrupt?
bool can_wait = are_interrupts_enabled();
if (!can_wait && allow_wait) {
// If we can't wait but the caller would be ok with it, then we
// need to definitely fallback to *something*, even if it's less
// secure...
fallback_to_fast = true;
}
if (can_wait && allow_wait) {
for (;;) {
{
Locker locker(KernelRng::the().lock());
if (kernel_rng.resource().get_random_bytes(buffer, buffer_size)) {
result = true;
break;
}
}
kernel_rng.wait_for_entropy();
}
} else {
// We can't wait/block here, or we are not allowed to block/wait
if (kernel_rng.resource().get_random_bytes(buffer, buffer_size)) {
result = true;
} else if (fallback_to_fast) {
// If interrupts are disabled
do_get_fast_random_bytes(buffer, buffer_size);
result = true;
}
}
// NOTE: The only case where this function should ever return false and
// not actually return random data is if fallback_to_fast == false and
// allow_wait == false and interrupts are enabled!
VERIFY(result || !fallback_to_fast);
return result;
}
void get_fast_random_bytes(u8* buffer, size_t buffer_size)
{
// Try to get good randomness, but don't block if we can't right now
// and allow falling back to fast randomness
auto result = get_good_random_bytes(buffer, buffer_size, false, true);
VERIFY(result);
}
}
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