serenity/Kernel/Random.cpp

/*
 * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
 * Copyright (c) 2020, Peter Elliott <pelliott@ualberta.ca>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <AK/Singleton.h>
#include <Kernel/Arch/i386/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) {
        klog() << "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.
        klog() << "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();
            this->resource().add_random_event(hpet_time, i % 32);
        }
    } else {
        // Fallback to RTC
        klog() << "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()
{
    ASSERT(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(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!
    ASSERT(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);
    ASSERT(result);
}

}