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 region, uintptr_t size)
  46 {
  47   return VirtualAlloc((void *)region, size, MEM_RESERVE, PAGE_NOACCESS);
  48 }
  49
  50 static void
  51 ReleaseRegion(void *region, uintptr_t size)
  52 {
  53   VirtualFree(region, size, MEM_RELEASE);
  54 }
  55
  56 static bool
  57 ProbeRegion(uintptr_t region, uintptr_t size)
  58 {
  59   SYSTEM_INFO sinfo;
  60   GetSystemInfo(&sinfo);
  61   if (region >= (uintptr_t)sinfo.lpMaximumApplicationAddress &&
  62       region + size >= (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 region, uintptr_t size)
  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 *region, uintptr_t size)
  90 {
  91   return;
  92 }
  93
  94 static bool
  95 ProbeRegion(uintptr_t region, uintptr_t size)
  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 static void *
 114 ReserveRegion(uintptr_t region, uintptr_t size)
 115 {
 116   return mmap(reinterpret_cast<void*>(region), size, PROT_NONE, MAP_PRIVATE|MAP_ANON, -1, 0);
 117 }
 118
 119 static void
 120 ReleaseRegion(void *region, uintptr_t size)
 121 {
 122   munmap(region, size);
 123 }
 124
 125 static bool
 126 ProbeRegion(uintptr_t region, uintptr_t size)
 127 {
 128   if (madvise(reinterpret_cast<void*>(region), size, MADV_NORMAL)) {
 129     return true;
 130   } else {
 131     return false;
 132   }
 133 }
 134
 135 static uintptr_t
 136 GetDesiredRegionSize()
 137 {
 138   return sysconf(_SC_PAGESIZE);
 139 }
 140
 141 #define RESERVE_FAILED MAP_FAILED
 142
 143 #endif // system dependencies
 144
 145 static_assert(sizeof(uintptr_t) == 4 || sizeof(uintptr_t) == 8, "");
 146 static_assert(sizeof(uintptr_t) == sizeof(void *), "");
 147
 148 static uintptr_t
 149 ReservePoisonArea(uintptr_t rgnsize)
 150 {
 151   if (sizeof(uintptr_t) == 8) {
 152     // Use the hardware-inaccessible region.
 153     // We have to avoid 64-bit constants and shifts by 32 bits, since this
 154     // code is compiled in 32-bit mode, although it is never executed there.
 155     return
 156       (((uintptr_t(0x7FFFFFFFu) << 31) << 1 | uintptr_t(0xF0DEAFFFu))
 157        & ~(rgnsize-1));
 158
 159   } else {
 160     // First see if we can allocate the preferred poison address from the OS.
 161     uintptr_t candidate = (0xF0DEAFFF & ~(rgnsize-1));
 162     void *result = ReserveRegion(candidate, rgnsize);
 163     if (result == (void *)candidate) {
 164       // success - inaccessible page allocated
 165       return candidate;
 166     }
 167
 168     // That didn't work, so see if the preferred address is within a range
 169     // of permanently inacessible memory.
 170     if (ProbeRegion(candidate, rgnsize)) {
 171       // success - selected page cannot be usable memory
 172       if (result != RESERVE_FAILED)
 173         ReleaseRegion(result, rgnsize);
 174       return candidate;
 175     }
 176
 177     // The preferred address is already in use.  Did the OS give us a
 178     // consolation prize?
 179     if (result != RESERVE_FAILED) {
 180       return uintptr_t(result);
 181     }
 182
 183     // It didn't, so try to allocate again, without any constraint on
 184     // the address.
 185     result = ReserveRegion(0, rgnsize);
 186     if (result != RESERVE_FAILED) {
 187       return uintptr_t(result);
 188     }
 189
 190     // no usable poison region identified
 191     MOZ_CRASH();
 192     return 0;
 193   }
 194 }
 195
 196 void
 197 mozPoisonValueInit()
 198 {
 199   gMozillaPoisonSize = GetDesiredRegionSize();
 200   gMozillaPoisonBase = ReservePoisonArea(gMozillaPoisonSize);
 201
 202   if (gMozillaPoisonSize == 0) // can't happen
 203     return;
 204
 205   gMozillaPoisonValue = gMozillaPoisonBase + gMozillaPoisonSize/2 - 1;
 206 }