[mojo] Use a pipe path to initialise Mojo in elevated utility processes.

mojo/edk/embedder/named_platform_channel_pair_win.cc

+// Copyright 2016 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 "mojo/edk/embedder/named_platform_channel_pair.h"
+
+#include <windows.h>
+
+#include <string>
+#include <utility>
+
+#include "base/command_line.h"
+#include "base/logging.h"
+#include "base/rand_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/stringprintf.h"
+#include "base/win/windows_version.h"
+#include "mojo/edk/embedder/platform_handle.h"
+
+namespace mojo {
+namespace edk {
+
+namespace {
+
+const char kMojoNamedPlatformChannelPipeSwitch[] =
+    "mojo-named-platform-channel-pipe";

[ncarter (slow), 2016/05/11 00:14:23]

It seems unusual to have switches defined outside of a switches namespace.

[Anand Mistry (off Chromium), 2016/05/11 04:15:20]

I'm following the pattern in PlatformChannelPair. PlatformChannelPair needs a
bit of a refactor (it has some dead code and obsolete comments) and I'd prefer
to keep these two looking similar until I get around to it.

+
+std::wstring GeneratePipeName() {
+  return base::StringPrintf(L"\\\\.\\pipe\\mojo.%u.%u.%I64u",
+                            GetCurrentProcessId(), GetCurrentThreadId(),
+                            base::RandUint64());
+
+}
+
+}  // namespace
+
+NamedPlatformChannelPair::NamedPlatformChannelPair() {
+  pipe_name_ = GeneratePipeName();
+
+  const DWORD kOpenMode =
+      PIPE_ACCESS_DUPLEX | FILE_FLAG_OVERLAPPED | FILE_FLAG_FIRST_PIPE_INSTANCE;
+  const DWORD kPipeMode = PIPE_TYPE_BYTE | PIPE_READMODE_BYTE;

[forshaw, 2016/05/11 00:01:40]

I'd recommend specifying PIPE_REJECT_REMOTE_CLIENTS here just in case. Even
though the name should be pretty hard to guess or exploit, no reason to expose
it remotely.

[Anand Mistry (off Chromium), 2016/05/11 04:15:20]

Done.

+  PlatformHandle handle(
+      CreateNamedPipeW(pipe_name_.c_str(), kOpenMode, kPipeMode,
+                       1,           // Max instances.
+                       4096,        // Out buffer size.
+                       4096,        // In buffer size.
+                       5000,        // Timeout in milliseconds.
+                       nullptr));   // Default security descriptor.

[forshaw, 2016/05/11 00:01:40]

Is there any advantage to another user on the system (say on a terminal server)
being able to open the server pipe with only read access? The default DACL gives
access to Everyone and the Anonymous user for Read. If there's some benefit to
having this limited access then we might want to ensure the DACL is explicitly
locked down. Perhaps as this is only expected to be opened by an elevated user
could specify only Administrators group.

[Anand Mistry (off Chromium), 2016/05/11 04:15:20]

I've tried to add an acl here to restrict access, but can't get it to work.
Frankly, this is far beyond my level of windows knowledge.

[forshaw, 2016/05/11 12:03:15]

Well it's probably not needed, but a simple example would be this:

#include <sddl.h>

PSECURITY_DESCRIPTOR security_desc = nullptr;
ULONG security_desc_len = 0;
if
(!::ConvertStringSecurityDescriptorToSecurityDescriptor(L"D:(A;;GA;;;SY)(A;;GA;;;BA)(A;;GA;;;OW)",

    SDDL_REVISION_1, &security_desc, &security_desc_len)) {
  return false;
}
std::unique_ptr<void, decltype(::LocalFree)*> p(security_desc, ::LocalFree);
SECURITY_ATTRIBUTES sa = {};
sa.nLength = sizeof(sa);
sa.lpSecurityDescriptor = security_desc;

Just pass the sa variable as the last parameter to CreateNamedPipeW. That will
setup the pipe so only Local System, Local Administrators and the Owner of the
pipe can access it. This removes the implicit Everyone and Anonymous group
access control entries.

[Anand Mistry (off Chromium), 2016/05/12 01:17:07]

Thanks. Added and it appears to work on my windows box. Lets see if it passes
the trybots.

+  handle.needs_connection = true;
+  server_handle_.reset(handle);
+  PCHECK(server_handle_.is_valid());
+}
+
+NamedPlatformChannelPair::~NamedPlatformChannelPair() {}
+
+ScopedPlatformHandle NamedPlatformChannelPair::PassServerHandle() {
+  return std::move(server_handle_);
+}
+
+// static
+ScopedPlatformHandle
+NamedPlatformChannelPair::PassClientHandleFromParentProcess(
+    const base::CommandLine& command_line) {
+  std::wstring client_handle_string =
+      command_line.GetSwitchValueNative(kMojoNamedPlatformChannelPipeSwitch);
+
+  if (client_handle_string.empty())
+    return ScopedPlatformHandle();
+
+  // Note: This may block.
+  BOOL ok = WaitNamedPipeW(client_handle_string.c_str(),
+                           NMPWAIT_USE_DEFAULT_WAIT);
+  if (!ok)
+    return ScopedPlatformHandle();
+
+  // In order to support passing the pipe name on the command line, the pipe
+  // handle is lazily created from the pipe name when requested.
+  const DWORD kDesiredAccess = GENERIC_READ | GENERIC_WRITE;
+  // The SECURITY_ANONYMOUS flag means that the server side cannot impersonate
+  // the client.
+  const DWORD kFlags =
+      SECURITY_SQOS_PRESENT | SECURITY_ANONYMOUS | FILE_FLAG_OVERLAPPED;
+  ScopedPlatformHandle handle(
+      PlatformHandle(CreateFileW(client_handle_string.c_str(), kDesiredAccess,
+                                 0,  // No sharing.
+                                 nullptr, OPEN_EXISTING, kFlags,
+                                 nullptr)));  // No template file.
+  PCHECK(handle.is_valid());
+  return handle;
+}
+
+void NamedPlatformChannelPair::PrepareToPassClientHandleToChildProcess(
+    base::CommandLine* command_line) const {
+  DCHECK(command_line);
+
+  // Log a warning if the command line already has the switch, but "clobber" it
+  // anyway, since it's reasonably likely that all the switches were just copied
+  // from the parent.
+  LOG_IF(WARNING,
+         command_line->HasSwitch(kMojoNamedPlatformChannelPipeSwitch))
+      << "Child command line already has switch --"
+      << kMojoNamedPlatformChannelPipeSwitch << "="
+      << command_line->GetSwitchValueNative(
+          kMojoNamedPlatformChannelPipeSwitch);
+  // (Any existing switch won't actually be removed from the command line, but
+  // the last one appended takes precedence.)
+  command_line->AppendSwitchNative(
+      kMojoNamedPlatformChannelPipeSwitch, pipe_name_);
+}
+
+}  // namespace edk
+}  // namespace mojo