base/allocator/partition_allocator/address_space_randomization.h

// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_ALLOCATOR_PARTITION_ALLOCATOR_ADDRESS_SPACE_RANDOMIZATION_H_
#define BASE_ALLOCATOR_PARTITION_ALLOCATOR_ADDRESS_SPACE_RANDOMIZATION_H_

#include "base/allocator/partition_allocator/page_allocator.h"
#include "base/base_export.h"
#include "build/build_config.h"

namespace base {

// Sets the seed for the random number generator used by GetRandomPageBase in
// order to generate a predictable sequence of addresses. May be called multiple
// times.
BASE_EXPORT void SetRandomPageBaseSeed(int64_t seed);

// Calculates a random preferred mapping address. In calculating an address, we
// balance good ASLR against not fragmenting the address space too badly.
BASE_EXPORT void* GetRandomPageBase();

namespace internal {

constexpr uintptr_t AslrAddress(uintptr_t mask) {
  return mask & kPageAllocationGranularityBaseMask;
}
constexpr uintptr_t AslrMask(uintptr_t bits) {
  return AslrAddress((1ULL << bits) - 1ULL);
}

// Turn off formatting, because the thicket of nested ifdefs below is
// incomprehensible without indentation. It is also incomprehensible with
// indentation, but the only other option is a combinatorial explosion of
// *_{win,linux,mac,foo}_{32,64}.h files.
//
// clang-format off

#if defined(ARCH_CPU_64_BITS)

  #if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)

    // We shouldn't allocate system pages at all for sanitizer builds. However,
    // we do, and if random hint addresses interfere with address ranges
    // hard-coded in those tools, bad things happen. This address range is
    // copied from TSAN source but works with all tools. See
    // https://crbug.com/539863.
    constexpr uintptr_t kASLRMask = AslrAddress(0x007fffffffffULL);
    constexpr uintptr_t kASLROffset = AslrAddress(0x7e8000000000ULL);

  #elif defined(OS_WIN)

    // Windows 8.10 and newer support the full 48 bit address range. Older
    // versions of Windows only support 44 bits. Since kASLROffset is non-zero
    // and may cause a carry, use 47 and 43 bit masks. See
    // http://www.alex-ionescu.com/?p=246
    constexpr uintptr_t kASLRMask = AslrMask(47);
    constexpr uintptr_t kASLRMaskBefore8_10 = AslrMask(43);
    // Try not to map pages into the range where Windows loads DLLs by default.
    constexpr uintptr_t kASLROffset = 0x80000000ULL;

  #elif defined(OS_MACOSX)

    // macOS as of 10.12.5 does not clean up entries in page map levels 3/4
    // [PDP/PML4] created from mmap or mach_vm_allocate, even after the region
    // is destroyed. Using a virtual address space that is too large causes a
    // leak of about 1 wired [can never be paged out] page per call to mmap. The
    // page is only reclaimed when the process is killed. Confine the hint to a
    // 39-bit section of the virtual address space.
    //
    // This implementation adapted from
    // https://chromium-review.googlesource.com/c/v8/v8/+/557958. The difference
    // is that here we clamp to 39 bits, not 32.
    //
    // TODO(crbug.com/738925): Remove this limitation if/when the macOS behavior
    // changes.
    constexpr uintptr_t kASLRMask = AslrMask(38);
    constexpr uintptr_t kASLROffset = AslrAddress(0x1000000000ULL);

  #elif defined(OS_POSIX) || defined(OS_FUCHSIA)

    #if defined(ARCH_CPU_X86_64)

      // Linux (and macOS) support the full 47-bit user space of x64 processors.
      // Use only 46 to allow the kernel a chance to fulfill the request.
      constexpr uintptr_t kASLRMask = AslrMask(46);
      constexpr uintptr_t kASLROffset = AslrAddress(0);

    #elif defined(ARCH_CPU_ARM64)

      #if defined(OS_ANDROID)

      // Restrict the address range on Android to avoid a large performance
      // regression in single-process WebViews. See https://crbug.com/837640.
      constexpr uintptr_t kASLRMask = AslrMask(30);
      constexpr uintptr_t kASLROffset = AslrAddress(0x20000000ULL);

      #else

      // ARM64 on Linux has 39-bit user space. Use 38 bits since kASLROffset
      // could cause a carry.
      constexpr uintptr_t kASLRMask = AslrMask(38);
      constexpr uintptr_t kASLROffset = AslrAddress(0x1000000000ULL);

      #endif

    #elif defined(ARCH_CPU_PPC64)

      #if defined(OS_AIX)

        // AIX has 64 bits of virtual addressing, but we limit the address range
        // to (a) minimize segment lookaside buffer (SLB) misses; and (b) use
        // extra address space to isolate the mmap regions.
        constexpr uintptr_t kASLRMask = AslrMask(30);
        constexpr uintptr_t kASLROffset = AslrAddress(0x400000000000ULL);

      #elif defined(ARCH_CPU_BIG_ENDIAN)

        // Big-endian Linux PPC has 44 bits of virtual addressing. Use 42.
        constexpr uintptr_t kASLRMask = AslrMask(42);
        constexpr uintptr_t kASLROffset = AslrAddress(0);

      #else  // !defined(OS_AIX) && !defined(ARCH_CPU_BIG_ENDIAN)

        // Little-endian Linux PPC has 48 bits of virtual addressing. Use 46.
        constexpr uintptr_t kASLRMask = AslrMask(46);
        constexpr uintptr_t kASLROffset = AslrAddress(0);

      #endif  // !defined(OS_AIX) && !defined(ARCH_CPU_BIG_ENDIAN)

    #elif defined(ARCH_CPU_S390X)

      // Linux on Z uses bits 22 - 32 for Region Indexing, which translates to
      // 42 bits of virtual addressing. Truncate to 40 bits to allow kernel a
      // chance to fulfill the request.
      constexpr uintptr_t kASLRMask = AslrMask(40);
      constexpr uintptr_t kASLROffset = AslrAddress(0);

    #elif defined(ARCH_CPU_S390)

      // 31 bits of virtual addressing. Truncate to 29 bits to allow the kernel
      // a chance to fulfill the request.
      constexpr uintptr_t kASLRMask = AslrMask(29);
      constexpr uintptr_t kASLROffset = AslrAddress(0);

    #else  // !defined(ARCH_CPU_X86_64) && !defined(ARCH_CPU_PPC64) &&
           // !defined(ARCH_CPU_S390X) && !defined(ARCH_CPU_S390)

      // For all other POSIX variants, use 30 bits.
      constexpr uintptr_t kASLRMask = AslrMask(30);

      #if defined(OS_SOLARIS)

        // For our Solaris/illumos mmap hint, we pick a random address in the
        // bottom half of the top half of the address space (that is, the third
        // quarter). Because we do not MAP_FIXED, this will be treated only as a
        // hint -- the system will not fail to mmap because something else
        // happens to already be mapped at our random address. We deliberately
        // set the hint high enough to get well above the system's break (that
        // is, the heap); Solaris and illumos will try the hint and if that
        // fails allocate as if there were no hint at all. The high hint
        // prevents the break from getting hemmed in at low values, ceding half
        // of the address space to the system heap.
        constexpr uintptr_t kASLROffset = AslrAddress(0x80000000ULL);

      #elif defined(OS_AIX)

        // The range 0x30000000 - 0xD0000000 is available on AIX; choose the
        // upper range.
        constexpr uintptr_t kASLROffset = AslrAddress(0x90000000ULL);

      #else  // !defined(OS_SOLARIS) && !defined(OS_AIX)

        // The range 0x20000000 - 0x60000000 is relatively unpopulated across a
        // variety of ASLR modes (PAE kernel, NX compat mode, etc) and on macOS
        // 10.6 and 10.7.
        constexpr uintptr_t kASLROffset = AslrAddress(0x20000000ULL);

      #endif  // !defined(OS_SOLARIS) && !defined(OS_AIX)

    #endif  // !defined(ARCH_CPU_X86_64) && !defined(ARCH_CPU_PPC64) &&
            // !defined(ARCH_CPU_S390X) && !defined(ARCH_CPU_S390)

  #endif  // defined(OS_POSIX)

#elif defined(ARCH_CPU_32_BITS)

  // This is a good range on 32-bit Windows and Android (the only platforms on
  // which we support 32-bitness). Allocates in the 0.5 - 1.5 GiB region. There
  // is no issue with carries here.
  constexpr uintptr_t kASLRMask = AslrMask(30);
  constexpr uintptr_t kASLROffset = AslrAddress(0x20000000ULL);

#else

  #error Please tell us about your exotic hardware! Sounds interesting.

#endif  // defined(ARCH_CPU_32_BITS)

// clang-format on

}  // namespace internal

}  // namespace base

#endif  // BASE_ALLOCATOR_PARTITION_ALLOCATOR_ADDRESS_SPACE_RANDOMIZATION_H_