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- // Copyright 2020 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.
- #include "base/memory/shared_memory_security_policy.h"
- #include <algorithm>
- #include <atomic>
- #include <limits>
- #include "base/bits.h"
- #include "base/memory/page_size.h"
- #include "base/numerics/checked_math.h"
- #include "build/build_config.h"
- #include "third_party/abseil-cpp/absl/types/optional.h"
- namespace base {
- namespace {
- // Note: pointers are 32 bits on all architectures in NaCl. See
- // https://bugs.chromium.org/p/nativeclient/issues/detail?id=1162
- #if defined(ARCH_CPU_32_BITS) || BUILDFLAG(IS_NACL)
- // No effective limit on 32-bit, since there simply isn't enough address space
- // for ASLR to be particularly effective.
- constexpr size_t kTotalMappedSizeLimit = std::numeric_limits<size_t>::max();
- #elif defined(ARCH_CPU_64_BITS)
- // 32 GB of mappings ought to be enough for anybody.
- constexpr size_t kTotalMappedSizeLimit = 32ULL * 1024 * 1024 * 1024;
- #endif
- static std::atomic_size_t total_mapped_size_;
- absl::optional<size_t> AlignWithPageSize(size_t size) {
- #if BUILDFLAG(IS_WIN)
- // TODO(crbug.com/210609): Matches alignment requirements defined in
- // platform_shared_memory_region_win.cc:PlatformSharedMemoryRegion::Create.
- // Remove this when NaCl is gone.
- static const size_t kSectionSize = 65536;
- const size_t page_size = std::max(kSectionSize, GetPageSize());
- #else
- const size_t page_size = GetPageSize();
- #endif // BUILDFLAG(IS_WIN)
- size_t rounded_size = bits::AlignUp(size, page_size);
- // Fail on overflow.
- if (rounded_size < size)
- return absl::nullopt;
- return rounded_size;
- }
- } // namespace
- // static
- bool SharedMemorySecurityPolicy::AcquireReservationForMapping(size_t size) {
- size_t previous_mapped_size =
- total_mapped_size_.load(std::memory_order_relaxed);
- size_t total_mapped_size;
- absl::optional<size_t> page_aligned_size = AlignWithPageSize(size);
- if (!page_aligned_size)
- return false;
- // Relaxed memory ordering is all that's needed since all atomicity is all
- // that's required. If the value is stale, compare_exchange_weak() will fail
- // and the loop will retry the operation with an updated total mapped size.
- do {
- if (!CheckAdd(previous_mapped_size, *page_aligned_size)
- .AssignIfValid(&total_mapped_size)) {
- return false;
- }
- if (total_mapped_size >= kTotalMappedSizeLimit)
- return false;
- } while (!total_mapped_size_.compare_exchange_weak(
- previous_mapped_size, total_mapped_size, std::memory_order_relaxed,
- std::memory_order_relaxed));
- return true;
- }
- // static
- void SharedMemorySecurityPolicy::ReleaseReservationForMapping(size_t size) {
- // Note #1: relaxed memory ordering is sufficient since atomicity is all
- // that's required.
- // Note #2: |size| should never overflow when aligned to page size, since
- // this should only be called if AcquireReservationForMapping() returned true.
- absl::optional<size_t> page_aligned_size = AlignWithPageSize(size);
- total_mapped_size_.fetch_sub(*page_aligned_size, std::memory_order_relaxed);
- }
- } // namespace base
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