Provide a memory region that never swaps to disk.

SecureMemoryRegion.cpp

// -*- coding:utf-8; mode:c++; mode:auto-fill; fill-column:80; -*-

/// @file      SecureMemoryRegion.cpp
/// @brief     Provide region of memory that never swaps to disk.
/// @author    J. Arrieta <[email protected]>
/// @date      October 15, 2017
/// @copyright (c) 2017 Nabla Zero Labs
///
///     $ clang++ -o SecureMemoryRegion SecureMemoryRegion.cpp -std=c++1z \
///     -Wall -Wextra -Ofast -march=native
///
///     $ ./SecureMemoryRegion
///     SecureMemoryRegion(64)
///     Data: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
///
///     $ ./SecureMemoryRegion 32
///     SecureMemoryRegion(32)
///     Data: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
///

// C++ Standard Library
#include <cerrno>
#include <string>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <stdexcept>

// POSIX Libraries
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>

std::string
ErrnoWithPrefix(const std::string& prefix)
{ return prefix + ":" + std::strerror(errno); }

[[noreturn]]
void
Fail(const std::string& msg)
{ throw std::runtime_error(msg); }

class SecureMemoryRegion {
 public:

  SecureMemoryRegion(std::size_t m_size);

  ~SecureMemoryRegion() noexcept(true);

  const char*
  data() const
  { return m_data; }

  char*
  data()
  { return m_data; }

  std::size_t
  size() const
  { return m_size; }

 private:

  void
  zero_out()
  {
    // In production, I would zero out using '\0' instead of 'X'
    std::fill(m_data, std::next(m_data, m_size), 'X');
  }

  char* m_data { nullptr };
  std::size_t m_size { 0 };
};

SecureMemoryRegion::SecureMemoryRegion(std::size_t size) : m_size(size)
{
  const auto protection = PROT_READ | PROT_WRITE;
  const auto mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS;
  m_data = reinterpret_cast<char*>(::mmap(nullptr, size, protection, mmap_flags, -1, 0));

  if (m_data == MAP_FAILED) {
    Fail(ErrnoWithPrefix("Failed to mmap"));
  }

  if (::mlock(m_data, m_size) != 0) {
    Fail(ErrnoWithPrefix("Failed to lock memory"));
  }

#if defined(MADV_DONTDUMP)
  ::madvise(m_data, m_size, MADV_DONTDUMP);
#endif

  zero_out();
}

SecureMemoryRegion::~SecureMemoryRegion() noexcept(true)
{
  if (m_data != nullptr) {
    zero_out();
#if (defined(MADV_DONTDUMP) && defined(MADV_DODUMP))
    ::madvise(m_data, m_size, MADV_DODUMP);
#endif
    ::munlock(m_data, m_size);
    ::munmap(m_data, m_size);
    m_data = nullptr;
    m_size = 0;
  }
}

std::ostream&
operator<<(std::ostream& os, const SecureMemoryRegion& r)
{
  os << "SecureMemoryRegion(" << r.size() << ")\nData: ";
  std::copy(r.data(), std::next(r.data(), r.size()),
            std::ostreambuf_iterator<char>(os));
  return os;
}


int
main(int argc, char* argv[])
{
  std::size_t size {1 << 6};

  if (argc == 2) {
    size = std::stoi(argv[1]);
  }

  SecureMemoryRegion smr(size);
  std::cout << smr << std::endl;
}