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diff --git a/ipc/gtest/TestSharedMemory.cpp b/ipc/gtest/TestSharedMemory.cpp
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+/* -*- 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/. */
+
+#include "gtest/gtest.h"
+
+#include "base/shared_memory.h"
+
+#include "mozilla/RefPtr.h"
+#include "mozilla/ipc/SharedMemory.h"
+#include "mozilla/ipc/SharedMemoryBasic.h"
+
+#ifdef XP_LINUX
+#  include <errno.h>
+#  include <linux/magic.h>
+#  include <stdio.h>
+#  include <string.h>
+#  include <sys/statfs.h>
+#  include <sys/utsname.h>
+#endif
+
+#ifdef XP_WIN
+#  include <windows.h>
+#endif
+
+namespace mozilla {
+
+// Try to map a frozen shm for writing.  Threat model: the process is
+// compromised and then receives a frozen handle.
+TEST(IPCSharedMemory, FreezeAndMapRW)
+{
+  base::SharedMemory shm;
+
+  // Create and initialize
+  ASSERT_TRUE(shm.CreateFreezeable(1));
+  ASSERT_TRUE(shm.Map(1));
+  auto mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_TRUE(mem);
+  *mem = 'A';
+
+  // Freeze
+  ASSERT_TRUE(shm.Freeze());
+  ASSERT_FALSE(shm.memory());
+
+  // Re-create as writeable
+  auto handle = shm.TakeHandle();
+  ASSERT_TRUE(shm.IsHandleValid(handle));
+  ASSERT_FALSE(shm.IsValid());
+  ASSERT_TRUE(shm.SetHandle(std::move(handle), /* read-only */ false));
+  ASSERT_TRUE(shm.IsValid());
+
+  // This should fail
+  EXPECT_FALSE(shm.Map(1));
+}
+
+// Try to restore write permissions to a frozen mapping.  Threat
+// model: the process has mapped frozen shm normally and then is
+// compromised, or as for FreezeAndMapRW (see also the
+// proof-of-concept at https://crbug.com/project-zero/1671 ).
+TEST(IPCSharedMemory, FreezeAndReprotect)
+{
+  base::SharedMemory shm;
+
+  // Create and initialize
+  ASSERT_TRUE(shm.CreateFreezeable(1));
+  ASSERT_TRUE(shm.Map(1));
+  auto mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_TRUE(mem);
+  *mem = 'A';
+
+  // Freeze
+  ASSERT_TRUE(shm.Freeze());
+  ASSERT_FALSE(shm.memory());
+
+  // Re-map
+  ASSERT_TRUE(shm.Map(1));
+  mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_EQ(*mem, 'A');
+
+  // Try to alter protection; should fail
+  EXPECT_FALSE(ipc::SharedMemory::SystemProtectFallible(
+      mem, 1, ipc::SharedMemory::RightsReadWrite));
+}
+
+#ifndef XP_WIN
+// This essentially tests whether FreezeAndReprotect would have failed
+// without the freeze.  It doesn't work on Windows: VirtualProtect
+// can't exceed the permissions set in MapViewOfFile regardless of the
+// security status of the original handle.
+TEST(IPCSharedMemory, Reprotect)
+{
+  base::SharedMemory shm;
+
+  // Create and initialize
+  ASSERT_TRUE(shm.CreateFreezeable(1));
+  ASSERT_TRUE(shm.Map(1));
+  auto mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_TRUE(mem);
+  *mem = 'A';
+
+  // Re-create as read-only
+  auto handle = shm.TakeHandle();
+  ASSERT_TRUE(shm.IsHandleValid(handle));
+  ASSERT_FALSE(shm.IsValid());
+  ASSERT_TRUE(shm.SetHandle(std::move(handle), /* read-only */ true));
+  ASSERT_TRUE(shm.IsValid());
+
+  // Re-map
+  ASSERT_TRUE(shm.Map(1));
+  mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_EQ(*mem, 'A');
+
+  // Try to alter protection; should succeed, because not frozen
+  EXPECT_TRUE(ipc::SharedMemory::SystemProtectFallible(
+      mem, 1, ipc::SharedMemory::RightsReadWrite));
+}
+#endif
+
+#ifdef XP_WIN
+// Try to regain write permissions on a read-only handle using
+// DuplicateHandle; this will succeed if the object has no DACL.
+// See also https://crbug.com/338538
+TEST(IPCSharedMemory, WinUnfreeze)
+{
+  base::SharedMemory shm;
+
+  // Create and initialize
+  ASSERT_TRUE(shm.CreateFreezeable(1));
+  ASSERT_TRUE(shm.Map(1));
+  auto mem = reinterpret_cast<char*>(shm.memory());
+  ASSERT_TRUE(mem);
+  *mem = 'A';
+
+  // Freeze
+  ASSERT_TRUE(shm.Freeze());
+  ASSERT_FALSE(shm.memory());
+
+  // Extract handle.
+  auto handle = shm.TakeHandle();
+  ASSERT_TRUE(shm.IsHandleValid(handle));
+  ASSERT_FALSE(shm.IsValid());
+
+  // Unfreeze.
+  HANDLE newHandle = INVALID_HANDLE_VALUE;
+  bool unfroze = ::DuplicateHandle(
+      GetCurrentProcess(), handle.release(), GetCurrentProcess(), &newHandle,
+      FILE_MAP_ALL_ACCESS, false, DUPLICATE_CLOSE_SOURCE);
+  ASSERT_FALSE(unfroze);
+}
+#endif
+
+// Test that a read-only copy sees changes made to the writeable
+// mapping in the case that the page wasn't accessed before the copy.
+TEST(IPCSharedMemory, ROCopyAndWrite)
+{
+  base::SharedMemory shmRW, shmRO;
+
+  // Create and initialize
+  ASSERT_TRUE(shmRW.CreateFreezeable(1));
+  ASSERT_TRUE(shmRW.Map(1));
+  auto memRW = reinterpret_cast<char*>(shmRW.memory());
+  ASSERT_TRUE(memRW);
+
+  // Create read-only copy
+  ASSERT_TRUE(shmRW.ReadOnlyCopy(&shmRO));
+  EXPECT_FALSE(shmRW.IsValid());
+  ASSERT_EQ(shmRW.memory(), memRW);
+  ASSERT_EQ(shmRO.max_size(), size_t(1));
+
+  // Map read-only
+  ASSERT_TRUE(shmRO.IsValid());
+  ASSERT_TRUE(shmRO.Map(1));
+  auto memRO = reinterpret_cast<const char*>(shmRO.memory());
+  ASSERT_TRUE(memRO);
+  ASSERT_NE(memRW, memRO);
+
+  // Check
+  *memRW = 'A';
+  EXPECT_EQ(*memRO, 'A');
+}
+
+// Test that a read-only copy sees changes made to the writeable
+// mapping in the case that the page was accessed before the copy
+// (and, before that, sees the state as of when the copy was made).
+TEST(IPCSharedMemory, ROCopyAndRewrite)
+{
+  base::SharedMemory shmRW, shmRO;
+
+  // Create and initialize
+  ASSERT_TRUE(shmRW.CreateFreezeable(1));
+  ASSERT_TRUE(shmRW.Map(1));
+  auto memRW = reinterpret_cast<char*>(shmRW.memory());
+  ASSERT_TRUE(memRW);
+  *memRW = 'A';
+
+  // Create read-only copy
+  ASSERT_TRUE(shmRW.ReadOnlyCopy(&shmRO));
+  EXPECT_FALSE(shmRW.IsValid());
+  ASSERT_EQ(shmRW.memory(), memRW);
+  ASSERT_EQ(shmRO.max_size(), size_t(1));
+
+  // Map read-only
+  ASSERT_TRUE(shmRO.IsValid());
+  ASSERT_TRUE(shmRO.Map(1));
+  auto memRO = reinterpret_cast<const char*>(shmRO.memory());
+  ASSERT_TRUE(memRO);
+  ASSERT_NE(memRW, memRO);
+
+  // Check
+  ASSERT_EQ(*memRW, 'A');
+  EXPECT_EQ(*memRO, 'A');
+  *memRW = 'X';
+  EXPECT_EQ(*memRO, 'X');
+}
+
+// See FreezeAndMapRW.
+TEST(IPCSharedMemory, ROCopyAndMapRW)
+{
+  base::SharedMemory shmRW, shmRO;
+
+  // Create and initialize
+  ASSERT_TRUE(shmRW.CreateFreezeable(1));
+  ASSERT_TRUE(shmRW.Map(1));
+  auto memRW = reinterpret_cast<char*>(shmRW.memory());
+  ASSERT_TRUE(memRW);
+  *memRW = 'A';
+
+  // Create read-only copy
+  ASSERT_TRUE(shmRW.ReadOnlyCopy(&shmRO));
+  ASSERT_TRUE(shmRO.IsValid());
+
+  // Re-create as writeable
+  auto handle = shmRO.TakeHandle();
+  ASSERT_TRUE(shmRO.IsHandleValid(handle));
+  ASSERT_FALSE(shmRO.IsValid());
+  ASSERT_TRUE(shmRO.SetHandle(std::move(handle), /* read-only */ false));
+  ASSERT_TRUE(shmRO.IsValid());
+
+  // This should fail
+  EXPECT_FALSE(shmRO.Map(1));
+}
+
+// See FreezeAndReprotect
+TEST(IPCSharedMemory, ROCopyAndReprotect)
+{
+  base::SharedMemory shmRW, shmRO;
+
+  // Create and initialize
+  ASSERT_TRUE(shmRW.CreateFreezeable(1));
+  ASSERT_TRUE(shmRW.Map(1));
+  auto memRW = reinterpret_cast<char*>(shmRW.memory());
+  ASSERT_TRUE(memRW);
+  *memRW = 'A';
+
+  // Create read-only copy
+  ASSERT_TRUE(shmRW.ReadOnlyCopy(&shmRO));
+  ASSERT_TRUE(shmRO.IsValid());
+
+  // Re-map
+  ASSERT_TRUE(shmRO.Map(1));
+  auto memRO = reinterpret_cast<char*>(shmRO.memory());
+  ASSERT_EQ(*memRO, 'A');
+
+  // Try to alter protection; should fail
+  EXPECT_FALSE(ipc::SharedMemory::SystemProtectFallible(
+      memRO, 1, ipc::SharedMemory::RightsReadWrite));
+}
+
+TEST(IPCSharedMemory, IsZero)
+{
+  base::SharedMemory shm;
+
+  static constexpr size_t kSize = 65536;
+  ASSERT_TRUE(shm.Create(kSize));
+  ASSERT_TRUE(shm.Map(kSize));
+
+  auto* mem = reinterpret_cast<char*>(shm.memory());
+  for (size_t i = 0; i < kSize; ++i) {
+    ASSERT_EQ(mem[i], 0) << "offset " << i;
+  }
+}
+
+#ifndef FUZZING
+TEST(IPCSharedMemory, BasicIsZero)
+{
+  auto shm = MakeRefPtr<ipc::SharedMemoryBasic>();
+
+  static constexpr size_t kSize = 65536;
+  ASSERT_TRUE(shm->Create(kSize));
+  ASSERT_TRUE(shm->Map(kSize));
+
+  auto* mem = reinterpret_cast<char*>(shm->memory());
+  for (size_t i = 0; i < kSize; ++i) {
+    ASSERT_EQ(mem[i], 0) << "offset " << i;
+  }
+}
+#endif
+
+#if defined(XP_LINUX) && !defined(ANDROID)
+// Test that memfd_create is used where expected.
+//
+// More precisely: if memfd_create support is expected, verify that
+// shared memory isn't subject to a filesystem size limit.
+TEST(IPCSharedMemory, IsMemfd)
+{
+  static constexpr int kMajor = 3;
+  static constexpr int kMinor = 17;
+
+  struct utsname uts;
+  ASSERT_EQ(uname(&uts), 0) << strerror(errno);
+  ASSERT_STREQ(uts.sysname, "Linux");
+  int major, minor;
+  ASSERT_EQ(sscanf(uts.release, "%d.%d", &major, &minor), 2);
+  bool expectMemfd = major > kMajor || (major == kMajor && minor >= kMinor);
+
+  base::SharedMemory shm;
+  ASSERT_TRUE(shm.Create(1));
+  UniqueFileHandle fd = shm.TakeHandle();
+  ASSERT_TRUE(fd);
+
+  struct statfs fs;
+  ASSERT_EQ(fstatfs(fd.get(), &fs), 0) << strerror(errno);
+  EXPECT_EQ(fs.f_type, TMPFS_MAGIC);
+  static constexpr decltype(fs.f_blocks) kNoLimit = 0;
+  if (expectMemfd) {
+    EXPECT_EQ(fs.f_blocks, kNoLimit);
+  } else {
+    // On older kernels, we expect the memfd / no-limit test to fail.
+    // (In theory it could succeed if backported memfd support exists;
+    // if that ever happens, this check can be removed.)
+    EXPECT_NE(fs.f_blocks, kNoLimit);
+  }
+}
+#endif
+
+}  // namespace mozilla