text: /*
   * 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;
  }
  
  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!)";
          time_t current_time = 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 *= 0x574a;
              current_time += 0x40b2;
          }
      }
  }
  
  void KernelRng::wait_for_entropy()
  {
      ScopedSpinLock lock(get_lock());
      if (!resource().is_ready()) {
          dbgln("Entropy starvation...");
          m_seed_queue.wait_on({}, "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);
  }
  
  }
  