Add ChildPortHandshake and its test

util/mach/child_port_handshake.h

Index: util/mach/child_port_handshake.h
diff --git a/util/mach/child_port_handshake.h b/util/mach/child_port_handshake.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_MACH_CHILD_PORT_HANDSHAKE_H_
+#define CRASHPAD_UTIL_MACH_CHILD_PORT_HANDSHAKE_H_
+
+#include <mach/mach.h>
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/files/scoped_file.h"
+#include "util/mach/child_port_server.h"
+
+namespace crashpad {
+
+namespace test {
+namespace {
+class ChildPortHandshakeTest;
+}  // namespace
+}  // namespace test
+
+//! \brief Implements a handshake protocol that allows a parent process to
+//!     obtain a Mach port right from a child process.
+//!
+//! Ordinarily, there is no way for parent and child processes to exchange port
+//! rights, outside of the rights that children inherit from their parents.
+//! These include task-special ports and exception ports, but all of these have
+//! system-defined uses, and cannot reliably be replaced: in a multi-threaded
+//! parent, it is impossible to temporarily change one an inheritable port while
+//! maintaining a guarantee that another thread will not attempt to use it, and
+//! in children, it difficult to guarantee that nothing will attempt to use an
+//! inheritable port before it can be replaced with the correct one. This latter
+//! concern is becoming increasingly more pronounced as system libraries perform
+//! more operations that rely on an inheritable port in module initializers.
+//!
+//! The protocol implemented by this class involves a server that runs in the
+//! parent process. The server is published with the bootstrap server, which the
+//! child has access to because the bootstrap port is one of the inherited
+//! task-special ports. The parent and child also share a pipe, which the parent
+//! can write to and the child can read from. After launching a child process,
+//! the parent will write a random token to this pipe, along with the name under
+//! which its server has been registered with the bootstrap server. The child
+//! can then obtain a send right to this server with `bootstrap_look_up()`, and
+//! send a check-in message containing the token value and the port right of its
+//! choice by calling `child_port_check_in()`.
+//!
+//! The inclusion of the token authenticates the child to its parent. This is
+//! necessary because the service is published with the bootstrap server, which
+//! opens up access to it to more than the child process. Because the token is
+//! passed to the child by a shared pipe, it constitutes a shared secret not
+//! known by other processes that may have incidental access to the server. The
+//! ChildPortHandshake server considers its randomly-generated token valid until
+//! a client checks in with it. This mechanism is used instead of examining the
+//! request message’s audit trailer to verify the sender’s process ID because in
+//! some process architectures, it may be impossible to verify the child’s
+//! process ID. This may happen when the child disassociates from the parent
+//! with a double fork(), and the actual client is the parent’s grandchild. In
+//! this case, the child would not check in, but the grandchild, in possession
+//! of the token, would check in.
+//!
+//! The shared pipe serves another purpose: the server monitors it for an
+//! end-of-file (no readers) condition. Once detected, it will stop its blocking
+//! wait for a client to check in. This mechanism was chosen over monitoring a
+//! child process directly for exit to account for the possibility that the
+//! child might disassociate with a double fork().
+//!
+//! This class can be used to allow a child process to provide its parent with
+//! a send right to its task port, in cases where it is desirable for the parent
+//! to have such access. It can also be used to allow a child process to
+//! establish its own server and provide its parent with a send right to that
+//! server, for cases where a service is provided and it is undesirable or
+//! impossible to provide it via the bootstrap or launchd interfaces.
+class ChildPortHandshake : public ChildPortServer::Interface {
+ public:
+  //! \brief Initializes the server.
+  //!
+  //! This creates the pipe so that the “read” side can be obtained by calling
+  //! ReadPipeFD().
+  ChildPortHandshake();
+
+  ~ChildPortHandshake();
+
+  //! \brief Obtains the “read” side of the pipe, to be used by the client.
+  //!
+  //! Callers must obtain this file descriptor and arrange for the caller to
+  //! have access to it before calling RunServer().
+  //!
+  //! \return The file descriptor that the client should read from.
+  int ReadPipeFD() const;
+
+  //! \brief Runs the server.
+  //!
+  //! This method performs these tasks:
+  //!  - Closes the “read” side of the pipe in-process, so that the client
+  //!    process holds the only file descriptor that can read from the pipe.
+  //!  - Creates a random token and sends it via the pipe.
+  //!  - Checks its service in with the bootstrap server, and sends the name
+  //!    of its bootstrap service mapping via the pipe.
+  //!  - Simultaneously receives messages on its Mach server and monitors the
+  //!    pipe for end-of-file. This is a blocking operation.
+  //!  - When a Mach message is received, calls HandleChildPortCheckIn() to
+  //!    interpret and validate it, and if the message is valid, returns the
+  //!    port right extracted from the message. If the message is not valid,
+  //!    this method will continue waiting for a valid message. Valid messages
+  //!    are properly formatted and have the correct token. If a valid message
+  //!    carries a send or send-once right, it will be returned. If a valid
+  //!    message contains a receive right, it will be destroyed and
+  //!    `MACH_PORT_NULL` will be returned. If a message is not valid, this
+  //!    method will continue waiting for pipe EOF or a valid message.
+  //!  - When notified of pipe EOF, returns `MACH_PORT_NULL`.
+  //!  - Regardless of return value, destroys the server’s receive right and
+  //!    closes the pipe.
+  //!
+  //! \return On success, the send or send-once right to the port provided by
+  //!     the client. The caller takes ownership of this right. On failure,
+  //!     `MACH_PORT_NULL`, indicating that the client did not check in properly
+  //!     before terminating, where termination is detected by noticing that the
+  //!     read side of the shared pipe has closed. On failure, a message
+  //!     indiciating the nature of the failure will be logged.
+  mach_port_t RunServer();
+
+  // ChildPortServer::Interface:
+  kern_return_t HandleChildPortCheckIn(child_port_server_t server,
+                                       child_port_token_t token,
+                                       mach_port_t port,
+                                       mach_msg_type_name_t right_type,
+                                       bool* destroy_complex_request) override;
+
+  //! \brief Runs the client.
+  //!
+  //! This function performs these tasks:
+  //!  - Reads the token from the pipe.
+  //!  - Reads the bootstrap service name from the pipe.
+  //!  - Obtains a send right to the server by calling `bootstrap_look_up()`.
+  //!  - Sends a check-in message to the server by calling
+  //!    `child_port_check_in()`, providing the token and the user-supplied port
+  //!    right.
+  //!  - Deallocates the send right to the server, and closes the pipe.
+  //!
+  //! There is no return value because `child_port_check_in()` is a MIG
+  //! `simpleroutine`, and the server does not send a reply. This allows
+  //! check-in to occur without blocking to wait for a reply.
+  //!
+  //! \param[in] pipe_read The “read” side of the pipe shared with the server
+  //!     process.
+  //! \param[in] port The port that will be passed to the server by
+  //!     `child_port_check_in()`.
+  //! \param[in] right_type The right type to furnish the parent with. If \a
+  //!     port is a send right, this can be `MACH_MSG_TYPE_COPY_SEND` or
+  //!     `MACH_MSG_TYPE_MOVE_SEND`. If \a port is a send-once right, this can
+  //!      be `MACH_MSG_TYPE_MOVE_SEND_ONCE`. If \a port is a receive right,
+  //!      this can be `MACH_MSG_TYPE_MAKE_SEND`. `MACH_MSG_TYPE_MOVE_RECEIVE`
+  //!      is supported by the client interface but will be silently rejected by
+  //!      server run by RunServer(), which expects to receive only send or
+  //!      send-once rights.
+  static void RunClient(int pipe_read,
+                        mach_port_t port,
+                        mach_msg_type_name_t right_type);
+
+ private:
+  //! \brief Runs the read-from-pipe portion of the client’s side of the
+  //!     handshake. This is an implementation detail of RunClient and is only
+  //!     exposed for testing purposes.
+  //!
+  //! \param[in] pipe_read The “read” side of the pipe shared with the server
+  //!     process.
+  //! \param[out] token The token value read from \a pipe_read.
+  //! \param[out] service_name The service name as registered with the bootstrap
+  //!     server, read from \a pipe_read.
+  static void RunClientInternal_ReadPipe(int pipe_read,
+                                         child_port_token_t* token,
+                                         std::string* service_name);
+
+  //! \brief Runs the check-in portion of the client’s side of the handshake.
+  //!     This is an implementation detail of RunClient and is only exposed for
+  //!     testing purposes.
+  //!
+  //! \param[in] service_name The service name as registered with the bootstrap
+  //!     server, to be looked up with `bootstrap_look_up()`.
+  //! \param[in] token The token value to provide during check-in.
+  //! \param[in] port The port that will be passed to the server by
+  //!     `child_port_check_in()`.
+  //! \param[in] right_type The right type to furnish the parent with.
+  static void RunClientInternal_SendCheckIn(const std::string& service_name,
+                                            child_port_token_t token,
+                                            mach_port_t port,
+                                            mach_msg_type_name_t right_type);
+
+  // Communicates the token from RunServer(), where it’s generated, to
+  // HandleChildPortCheckIn(), where it’s validated.
+  child_port_token_t token_;
+
+  base::ScopedFD pipe_read_;
+  base::ScopedFD pipe_write_;
+
+  // Communicates the port received from the client from
+  // HandleChildPortCheckIn(), where it’s received, to RunServer(), where it’s
+  // returned. This is strongly-owned, but ownership is transferred to
+  // RunServer()’s caller.
+  mach_port_t child_port_;
+
+  // Communicates that a check-in with a valid token was received by
+  // HandleChildPortCheckIn(), and that the value of child_port_ should be
+  // returned to RunServer()’s caller.
+  bool checked_in_;
+
+  friend class test::ChildPortHandshakeTest;
+
+  DISALLOW_COPY_AND_ASSIGN(ChildPortHandshake);
+};
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_MACH_CHILD_PORT_HANDSHAKE_H_