mfbt/Poison.cpp

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * A poison value that can be used to fill a memory space with
   9  * an address that leads to a safe crash when dereferenced.
  10  */
  11
  12 #include "mozilla/Poison.h"
  13
  14 #include "mozilla/Assertions.h"
  15 #ifdef _WIN32
  16 # include <windows.h>
  17 #elif !defined(__OS2__)
  18 # include <unistd.h>
  19 # include <sys/mman.h>
  20 # ifndef MAP_ANON
  21 #  ifdef MAP_ANONYMOUS
  22 #   define MAP_ANON MAP_ANONYMOUS
  23 #  else
  24 #   error "Don't know how to get anonymous memory"
  25 #  endif
  26 # endif
  27 #endif
  28
  29 extern "C" {
  30 uintptr_t gMozillaPoisonValue;
  31 uintptr_t gMozillaPoisonBase;
  32 uintptr_t gMozillaPoisonSize;
  33 }
  34
  35 // Freed memory is filled with a poison value, which we arrange to
  36 // form a pointer either to an always-unmapped region of the address
  37 // space, or to a page that has been reserved and rendered
  38 // inaccessible via OS primitives.  See tests/TestPoisonArea.cpp for
  39 // extensive discussion of the requirements for this page.  The code
  40 // from here to 'class FreeList' needs to be kept in sync with that
  41 // file.
  42
  43 #ifdef _WIN32
  44 static void*
  45 ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
  46 {
  47   return VirtualAlloc((void*)aRegion, aSize, MEM_RESERVE, PAGE_NOACCESS);
  48 }
  49
  50 static void
  51 ReleaseRegion(void* aRegion, uintptr_t aSize)
  52 {
  53   VirtualFree(aRegion, aSize, MEM_RELEASE);
  54 }
  55
  56 static bool
  57 ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
  58 {
  59   SYSTEM_INFO sinfo;
  60   GetSystemInfo(&sinfo);
  61   if (aRegion >= (uintptr_t)sinfo.lpMaximumApplicationAddress &&
  62       aRegion + aSize >= (uintptr_t)sinfo.lpMaximumApplicationAddress) {
  63     return true;
  64   } else {
  65     return false;
  66   }
  67 }
  68
  69 static uintptr_t
  70 GetDesiredRegionSize()
  71 {
  72   SYSTEM_INFO sinfo;
  73   GetSystemInfo(&sinfo);
  74   return sinfo.dwAllocationGranularity;
  75 }
  76
  77 #define RESERVE_FAILED 0
  78
  79 #elif defined(__OS2__)
  80 static void*
  81 ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
  82 {
  83   // OS/2 doesn't support allocation at an arbitrary address,
  84   // so return an address that is known to be invalid.
  85   return (void*)0xFFFD0000;
  86 }
  87
  88 static void
  89 ReleaseRegion(void* aRegion, uintptr_t aSize)
  90 {
  91   return;
  92 }
  93
  94 static bool
  95 ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
  96 {
  97   // There's no reliable way to probe an address in the system
  98   // arena other than by touching it and seeing if a trap occurs.
  99   return false;
 100 }
 101
 102 static uintptr_t
 103 GetDesiredRegionSize()
 104 {
 105   // Page size is fixed at 4k.
 106   return 0x1000;
 107 }
 108
 109 #define RESERVE_FAILED 0
 110
 111 #else // Unix
 112
 113 #include "mozilla/TaggedAnonymousMemory.h"
 114
 115 static void*
 116 ReserveRegion(uintptr_t aRegion, uintptr_t aSize)
 117 {
 118   return MozTaggedAnonymousMmap(reinterpret_cast<void*>(aRegion), aSize,
 119                                 PROT_NONE, MAP_PRIVATE|MAP_ANON, -1, 0,
 120                                 "poison");
 121 }
 122
 123 static void
 124 ReleaseRegion(void* aRegion, uintptr_t aSize)
 125 {
 126   munmap(aRegion, aSize);
 127 }
 128
 129 static bool
 130 ProbeRegion(uintptr_t aRegion, uintptr_t aSize)
 131 {
 132   if (madvise(reinterpret_cast<void*>(aRegion), aSize, MADV_NORMAL)) {
 133     return true;
 134   } else {
 135     return false;
 136   }
 137 }
 138
 139 static uintptr_t
 140 GetDesiredRegionSize()
 141 {
 142   return sysconf(_SC_PAGESIZE);
 143 }
 144
 145 #define RESERVE_FAILED MAP_FAILED
 146
 147 #endif // system dependencies
 148
 149 static_assert(sizeof(uintptr_t) == 4 || sizeof(uintptr_t) == 8, "");
 150 static_assert(sizeof(uintptr_t) == sizeof(void*), "");
 151
 152 static uintptr_t
 153 ReservePoisonArea(uintptr_t rgnsize)
 154 {
 155   if (sizeof(uintptr_t) == 8) {
 156     // Use the hardware-inaccessible region.
 157     // We have to avoid 64-bit constants and shifts by 32 bits, since this
 158     // code is compiled in 32-bit mode, although it is never executed there.
 159     return
 160       (((uintptr_t(0x7FFFFFFFu) << 31) << 1 | uintptr_t(0xF0DEAFFFu))
 161        & ~(rgnsize-1));
 162   }
 163
 164   // First see if we can allocate the preferred poison address from the OS.
 165   uintptr_t candidate = (0xF0DEAFFF & ~(rgnsize-1));
 166   void* result = ReserveRegion(candidate, rgnsize);
 167   if (result == (void*)candidate) {
 168     // success - inaccessible page allocated
 169     return candidate;
 170   }
 171
 172   // That didn't work, so see if the preferred address is within a range
 173   // of permanently inacessible memory.
 174   if (ProbeRegion(candidate, rgnsize)) {
 175     // success - selected page cannot be usable memory
 176     if (result != RESERVE_FAILED) {
 177       ReleaseRegion(result, rgnsize);
 178     }
 179     return candidate;
 180   }
 181
 182   // The preferred address is already in use.  Did the OS give us a
 183   // consolation prize?
 184   if (result != RESERVE_FAILED) {
 185     return uintptr_t(result);
 186   }
 187
 188   // It didn't, so try to allocate again, without any constraint on
 189   // the address.
 190   result = ReserveRegion(0, rgnsize);
 191   if (result != RESERVE_FAILED) {
 192     return uintptr_t(result);
 193   }
 194
 195   // no usable poison region identified
 196   MOZ_CRASH();
 197   return 0;
 198 }
 199
 200 void
 201 mozPoisonValueInit()
 202 {
 203   gMozillaPoisonSize = GetDesiredRegionSize();
 204   gMozillaPoisonBase = ReservePoisonArea(gMozillaPoisonSize);
 205
 206   if (gMozillaPoisonSize == 0) { // can't happen
 207     return;
 208   }
 209   gMozillaPoisonValue = gMozillaPoisonBase + gMozillaPoisonSize / 2 - 1;
 210 }