Poison.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*
 * A poison value that can be used to fill a memory space with
 * an address that leads to a safe crash when dereferenced.
 */

#include "mozilla/Poison.h"

#include "mozilla/Assertions.h"
#ifdef _WIN32
# include <windows.h>
#elif !defined(__OS2__)
# include <unistd.h>
# include <sys/mman.h>
# ifndef MAP_ANON
#  ifdef MAP_ANONYMOUS
#   define MAP_ANON MAP_ANONYMOUS
#  else
#   error "Don't know how to get anonymous memory"
#  endif
# endif
#endif

extern "C" {
uintptr_t gMozillaPoisonValue;
uintptr_t gMozillaPoisonBase;
uintptr_t gMozillaPoisonSize;
}

// Freed memory is filled with a poison value, which we arrange to
// form a pointer either to an always-unmapped region of the address
// space, or to a page that has been reserved and rendered
// inaccessible via OS primitives.  See tests/TestPoisonArea.cpp for
// extensive discussion of the requirements for this page.  The code
// from here to 'class FreeList' needs to be kept in sync with that
// file.

#ifdef _WIN32
static void*
ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
{
  return VirtualAlloc((void*)aRegion, aSize, MEM_RESERVE, PAGE_NOACCESS);
}

static void
ReleaseRegion(void* aRegion, uintptr_t aSize)
{
  VirtualFree(aRegion, aSize, MEM_RELEASE);
}

static bool
ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
{
  SYSTEM_INFO sinfo;
  GetSystemInfo(&sinfo);
  if (aRegion >= (uintptr_t)sinfo.lpMaximumApplicationAddress &&
      aRegion + aSize >= (uintptr_t)sinfo.lpMaximumApplicationAddress) {
    return true;
  } else {
    return false;
  }
}

static uintptr_t
GetDesiredRegionSize()
{
  SYSTEM_INFO sinfo;
  GetSystemInfo(&sinfo);
  return sinfo.dwAllocationGranularity;
}

#define RESERVE_FAILED 0

#elif defined(__OS2__)
static void*
ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
{
  // OS/2 doesn't support allocation at an arbitrary address,
  // so return an address that is known to be invalid.
  return (void*)0xFFFD0000;
}

static void
ReleaseRegion(void* aRegion, uintptr_t aSize)
{
  return;
}

static bool
ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
{
  // There's no reliable way to probe an address in the system
  // arena other than by touching it and seeing if a trap occurs.
  return false;
}

static uintptr_t
GetDesiredRegionSize()
{
  // Page size is fixed at 4k.
  return 0x1000;
}

#define RESERVE_FAILED 0

#else // Unix

#include "mozilla/TaggedAnonymousMemory.h"

static void*
ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
{
  return MozTaggedAnonymousMmap(reinterpret_cast<void*>(aRegion), aSize,
                                PROT_NONE, MAP_PRIVATE|MAP_ANON, -1, 0,
                                "poison");
}

static void
ReleaseRegion(void* aRegion, uintptr_t aSize)
{
  munmap(aRegion, aSize);
}

static bool
ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
{
  if (madvise(reinterpret_cast<void*>(aRegion), aSize, MADV_NORMAL)) {
    return true;
  } else {
    return false;
  }
}

static uintptr_t
GetDesiredRegionSize()
{
  return sysconf(_SC_PAGESIZE);
}

#define RESERVE_FAILED MAP_FAILED

#endif // system dependencies

static_assert(sizeof(uintptr_t) == 4 || sizeof(uintptr_t) == 8, "");
static_assert(sizeof(uintptr_t) == sizeof(void*), "");

static uintptr_t
ReservePoisonArea(uintptr_t rgnsize)
{
  if (sizeof(uintptr_t) == 8) {
    // Use the hardware-inaccessible region.
    // We have to avoid 64-bit constants and shifts by 32 bits, since this
    // code is compiled in 32-bit mode, although it is never executed there.
    return
      (((uintptr_t(0x7FFFFFFFu) << 31) << 1 | uintptr_t(0xF0DEAFFFu))
       & ~(rgnsize-1));
  }

  // First see if we can allocate the preferred poison address from the OS.
  uintptr_t candidate = (0xF0DEAFFF & ~(rgnsize-1));
  void* result = ReserveRegion(candidate, rgnsize);
  if (result == (void*)candidate) {
    // success - inaccessible page allocated
    return candidate;
  }

  // That didn't work, so see if the preferred address is within a range
  // of permanently inacessible memory.
  if (ProbeRegion(candidate, rgnsize)) {
    // success - selected page cannot be usable memory
    if (result != RESERVE_FAILED) {
      ReleaseRegion(result, rgnsize);
    }
    return candidate;
  }

  // The preferred address is already in use.  Did the OS give us a
  // consolation prize?
  if (result != RESERVE_FAILED) {
    return uintptr_t(result);
  }

  // It didn't, so try to allocate again, without any constraint on
  // the address.
  result = ReserveRegion(0, rgnsize);
  if (result != RESERVE_FAILED) {
    return uintptr_t(result);
  }

  MOZ_CRASH("no usable poison region identified");
}

void
mozPoisonValueInit()
{
  gMozillaPoisonSize = GetDesiredRegionSize();
  gMozillaPoisonBase = ReservePoisonArea(gMozillaPoisonSize);

  if (gMozillaPoisonSize == 0) { // can't happen
    return;
  }
  gMozillaPoisonValue = gMozillaPoisonBase + gMozillaPoisonSize / 2 - 1;
}