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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 <tuple>
#include "base/files/scoped_file.h"
#include "util/mach/child_port_types.h"
namespace crashpad {
namespace test {
namespace {
class ChildPortHandshakeTest;
} // namespace
} // namespace test
//! \brief Implements a handshake protocol that allows processes to exchange
//! port rights.
//!
//! 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 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 inherited port in module initializers.
//!
//! The protocol implemented by this class involves a server that runs in one
//! process. The server is published with the bootstrap server, which the other
//! process has access to because the bootstrap port is one of the inherited
//! task-special ports. The two processes also share a pipe, which the server
//! can write to and the client can read from. The server will write a random
//! token to this pipe, along with the name under which its service has been
//! registered with the bootstrap server. The client can then obtain a send
//! right to this service 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 client to the server. This is
//! necessary because the service is published with the bootstrap server, which
//! opens up access to it to more than the intended client. Because the token is
//! passed to the client 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 client’s
//! process ID.
//!
//! 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 also chosen for its
//! ability to function properly in diverse process architectures.
//!
//! 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 parent process to
//! transfer a receive right to a child process that implements the server for
//! that right, or for 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.
//!
//! Example parent process, running a client that sends a receive right to its
//! child:
//! \code
//! ChildPortHandshake child_port_handshake;
//! base::ScopedFD server_write_fd = child_port_handshake.ServerWriteFD();
//! std::string server_write_fd_string =
//! base::StringPrintf("%d", server_write_fd.get());
//!
//! pid_t pid = fork();
//! if (pid == 0) {
//! // Child
//!
//! // Close all file descriptors above STDERR_FILENO except for
//! // server_write_fd. Let the child know what file descriptor to use for
//! // server_write_fd by passing it as argv[1]. Example code for the child
//! // process is below.
//! CloseMultipleNowOrOnExec(STDERR_FILENO + 1, server_write_fd.get());
//! execlp("./child", "child", server_write_fd_string.c_str(), nullptr);
//! }
//!
//! // Parent
//!
//! // Close the child’s end of the pipe.
//! server_write_fd.reset();
//!
//! // Make a new Mach receive right.
//! base::mac::ScopedMachReceiveRight
//! receive_right(NewMachPort(MACH_PORT_RIGHT_RECEIVE));
//!
//! // Make a send right corresponding to the receive right.
//! mach_port_t send_right;
//! mach_msg_type_name_t send_right_type;
//! mach_port_extract_right(mach_task_self(),
//! receive_right.get(),
//! MACH_MSG_TYPE_MAKE_SEND,
//! &send_right,
//! &send_right_type);
//! base::mac::ScopedMachSendRight send_right_owner(send_right);
//!
//! // Send the receive right to the child process, retaining the send right
//! // for use in the parent process.
//! if (child_port_handshake.RunClient(receive_right.get(),
//! MACH_MSG_TYPE_MOVE_RECEIVE)) {
//! std::ignore = receive_right.release();
//! }
//! \endcode
//!
//! Example child process, running a server that receives a receive right from
//! its parent:
//! \code
//! int main(int argc, char* argv[]) {
//! // The parent passed server_write_fd in argv[1].
//! base::ScopedFD server_write_fd(atoi(argv[1]));
//!
//! // Obtain a receive right from the parent process.
//! base::mac::ScopedMachReceiveRight receive_right(
//! ChildPortHandshake::RunServerForFD(
//! std::move(server_write_fd),
//! ChildPortHandshake::PortRightType::kReceiveRight));
//! }
//! \endcode
class ChildPortHandshake {
public:
//! \brief Controls whether a receive or send right is expected to be
//! obtained from the client by the server’s call to RunServer().
enum class PortRightType {
//! \brief The server expects to receive a receive right.
kReceiveRight = 0,
//! \brief The server expects to receive a send or send-once right.
kSendRight,
};
ChildPortHandshake();
ChildPortHandshake(const ChildPortHandshake&) = delete;
ChildPortHandshake& operator=(const ChildPortHandshake&) = delete;
~ChildPortHandshake();
//! \brief Obtains the “read” side of the pipe, to be used by the client.
//!
//! This file descriptor must be passed to RunClientForFD().
//!
//! \return The file descriptor that the client should read from.
base::ScopedFD ClientReadFD();
//! \brief Obtains the “write” side of the pipe, to be used by the server.
//!
//! This file descriptor must be passed to RunServerForFD().
//!
//! \return The file descriptor that the server should write to.
base::ScopedFD ServerWriteFD();
//! \brief Runs the server.
//!
//! This method 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.
//! It then calls RunServerForFD() using the “write” side of the pipe. If
//! ClientReadFD() has already been called in the server process, the caller
//! must ensure that the file descriptor returned by ClientReadFD() is closed
//! prior to calling this method.
mach_port_t RunServer(PortRightType port_right_type);
//! \brief Runs the client.
//!
//! This method closes the “write” side of the pipe in-process, so that the
//! server process holds the only file descriptor that can write to the pipe.
//! It then calls RunClientForFD() using the “read” side of the pipe. If
//! ServerWriteFD() has already been called in the client process, the caller
//! must ensure that the file descriptor returned by ServerWriteFD() is closed
//! prior to calling this method.
//!
//! \return `true` on success, `false` on failure with a message logged.
bool RunClient(mach_port_t port, mach_msg_type_name_t right_type);
//! \brief Runs the server.
//!
//! If a ChildPortHandshake object is available, don’t call this static
//! function. Instead, call RunServer(), which wraps this function. When using
//! this function, the caller is responsible for ensuring that the client
//! “read” side of the pipe is closed in the server process prior to calling
//! this function.
//!
//! This function performs these tasks:
//! - 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. The right carried in
//! a valid message 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.
//!
//! \param[in] server_write_fd The write side of the pipe shared with the
//! client process. This function takes ownership of this file descriptor,
//! and will close it prior to returning.
//! \param[in] port_right_type The port right type expected to be received
//! from the client. If the port right received from the client does not
//! match the expected type, the received port right will be destroyed,
//! and `MACH_PORT_NULL` will be returned.
//!
//! \return On success, the port right 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 detecting that the read
//! side of the shared pipe has closed. On failure, a message indicating
//! the nature of the failure will be logged.
static mach_port_t RunServerForFD(base::ScopedFD server_write_fd,
PortRightType port_right_type);
//! \brief Runs the client.
//!
//! If a ChildPortHandshake object is available, don’t call this static
//! function. Instead, call RunClient(), which wraps this function. When using
//! this function, the caller is responsible for ensuring that the server
//! “write” side of the pipe is closed in the client process prior to calling
//! this function.
//!
//! 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] client_read_fd The “read” side of the pipe shared with the
//! server process. This function takes ownership of this file descriptor,
//! and will close it prior to returning.
//! \param[in] port The port right that will be passed to the server by
//! `child_port_check_in()`.
//! \param[in] right_type The right type to furnish the server 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_MAKE_SEND_ONCE`, or
//! `MACH_MSG_TYPE_MOVE_RECEIVE`.
//!
//! \return `true` on success, `false` on failure with a message logged. On
//! failure, the port right corresponding to a \a right_type of
//! `MACH_MSG_TYPE_MOVE_*` is not consumed, and the caller must dispose of
//! the right if necessary.
static bool RunClientForFD(base::ScopedFD client_read_fd,
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.
//!
//! When using this function and RunClientInternal_SendCheckIn(), the caller
//! is responsible for closing \a pipe_read at an appropriate time, normally
//! after calling RunClientInternal_SendCheckIn().
//!
//! \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.
//!
//! \return `true` on success, `false` on failure with a message logged.
static bool 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.
//!
//! When using this RunClientInternal_ReadPipe() and this function, the caller
//! is responsible for closing the “read” side of the pipe at an appropriate
//! time, normally after calling this function.
//!
//! \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 server with.
//!
//! \return `true` on success, `false` on failure with a message logged.
static bool RunClientInternal_SendCheckIn(const std::string& service_name,
child_port_token_t token,
mach_port_t port,
mach_msg_type_name_t right_type);
base::ScopedFD client_read_fd_;
base::ScopedFD server_write_fd_;
friend class test::ChildPortHandshakeTest;
};
} // namespace crashpad
#endif // CRASHPAD_UTIL_MACH_CHILD_PORT_HANDSHAKE_H_