Linux sandbox: add a new low-level broker process mechanism.

sandbox/linux/services/broker_process.cc

+// Copyright (c) 2012 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 "sandbox/linux/services/broker_process.h"
+
+#include <fcntl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+#include "base/pickle.h"
+#include "base/posix/eintr_wrapper.h"
+#include "base/posix/unix_domain_socket.h"
+
+namespace {
+
+static const int kCommandOpen = 'O';
+static const size_t kMaxMessageLength = 4096;
+
+// Some flags will need special treatment on the client side and are not
+// supported for now.
+int ForCurrentProcessFlagsMask() {
+  return O_CLOEXEC | O_NONBLOCK;
+}
+
+// Check whether |requested_filename| is in |allowed_file_names|.
+// See GetFileNameIfAllowedAccess() for an explaination of |file_to_open|.
+// async signal safe if |file_to_open| is NULL.
+// TODO(jln): assert signal safety.
+bool GetFileNameInWhitelist(const std::vector<std::string>& allowed_file_names,
+                            const std::string& requested_filename,
+                            const char** file_to_open) {
+  if (file_to_open && *file_to_open) {
+    // Make sure that callers never pass a non-empty string. In case callers
+    // wrongly forget to check the return value and look at the string
+    // instead, this could catch bugs.
+    RAW_LOG(FATAL, "*file_to_open should be NULL");
+    return false;
+  }
+  std::vector<std::string>::const_iterator it;
+  it = std::find(allowed_file_names.begin(), allowed_file_names.end(),
+                 requested_filename);
+  if (it < allowed_file_names.end()) {  // requested_filename was found?
+    if (file_to_open)
+      *file_to_open = it->c_str();
+    return true;
+  }
+  return false;
+}
+
+// We maintain a list of flags that have been reviewed for "sanity" and that
+// we're ok to allow in the broker.
+// I.e. here is where we wouldn't add O_RESET_FILE_SYSTEM.
+bool IsAllowedOpenFlags(int flags) {
+  // First, check the access mode
+  const int access_mode = flags & O_ACCMODE;
+  if (access_mode != O_RDONLY && access_mode != O_WRONLY &&
+      access_mode != O_RDWR) {
+    return false;
+  }
+
+  // Some flags affect the behavior of the current process. We don't support
+  // them and don't allow them for now.
+  if (flags & ForCurrentProcessFlagsMask()) {
+    return false;
+  }
+
+  // Now check that all the flags are known to us.
+  const int creation_and_status_flags = flags & ~O_ACCMODE;
+
+  const int known_flags =
+    O_APPEND | O_ASYNC | O_CLOEXEC | O_CREAT | O_DIRECT |
+    O_DIRECTORY | O_EXCL | O_LARGEFILE | O_NOATIME | O_NOCTTY |
+    O_NOFOLLOW | O_NONBLOCK | O_NDELAY | O_SYNC | O_TRUNC;
+
+  const int unknown_flags = ~known_flags;
+  const bool has_unknown_flags = creation_and_status_flags & unknown_flags;
+  return !has_unknown_flags;
+}
+
+}  // namespace
+
+namespace sandbox {
+
+BrokerProcess::BrokerProcess(const std::vector<std::string>& allowed_r_files,
+                             const std::vector<std::string>& allowed_w_files,
+                             bool fast_check_in_client,
+                             bool quiet_failures_for_tests)
+    : initialized_(false),
+      is_child_(false),
+      fast_check_in_client_(fast_check_in_client),
+      quiet_failures_for_tests_(quiet_failures_for_tests),
+      broker_pid_(-1),
+      allowed_r_files_(allowed_r_files),
+      allowed_w_files_(allowed_w_files),
+      ipc_socketpair_(-1) {
+}
+
+BrokerProcess::~BrokerProcess() {
+  if (initialized_ && ipc_socketpair_ != -1) {
+    void (HANDLE_EINTR(close(ipc_socketpair_)));
+  }
+}
+
+bool BrokerProcess::Init(void* sandbox_callback) {
+  CHECK(!initialized_);
+  CHECK_EQ(sandbox_callback, (void*) NULL) <<
+      "sandbox_callback is not implemented";
+  int socket_pair[2];
+  // Use SOCK_SEQPACKET, because we need to preserve message boundaries
+  // but we also want to be notified (recvmsg should return and not block)
+  // when the connection has been broken (one of the processes died).
+  if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, socket_pair)) {
+    LOG(ERROR) << "Failed to create socketpair";
+    return false;
+  }
+
+  int child_pid = fork();
+  if (child_pid == -1) {
+    (void) HANDLE_EINTR(close(socket_pair[0]));
+    (void) HANDLE_EINTR(close(socket_pair[1]));
+    return false;
+  }
+  if (child_pid) {
+    // We are the parent and we have just forked our broker process.
+    (void) HANDLE_EINTR(close(socket_pair[0]));
+    // We should only be able to write to the IPC channel. We'll always send
+    // a new file descriptor to receive the reply on.
+    shutdown(socket_pair[1], SHUT_RD);
+    ipc_socketpair_ = socket_pair[1];
+    is_child_ = false;
+    broker_pid_ = child_pid;
+    initialized_ = true;
+    return true;
+  } else {
+    // We are the broker.
+    (void) HANDLE_EINTR(close(socket_pair[1]));
+    // We should only be able to read from this IPC channel. We will send our
+    // replies on a new file descriptor attached to the requests.
+    shutdown(socket_pair[0], SHUT_WR);
+    ipc_socketpair_ = socket_pair[0];
+    is_child_ = true;
+    // TODO(jln): activate a sandbox here.
+    initialized_ = true;
+    for (;;) {
+      HandleRequest();
+    }
+    _exit(1);
+  }
+  NOTREACHED();
+}
+
+// This function needs to be async signal safe.
+int BrokerProcess::Open(const char* pathname, int flags) const {
+  RAW_CHECK(initialized_);  // async signal safe CHECK().
+  if (!pathname)
+    return -EFAULT;
+  // There is no point in forwarding a request that we know will be denied.
+  // Of course, the real security check needs to be on the other side of the
+  // IPC.
+  if (fast_check_in_client_) {
+    if (!GetFileNameIfAllowedAccess(pathname, flags, NULL))
+      return -EPERM;
+  }
+
+  Pickle write_pickle;
+  write_pickle.WriteInt(kCommandOpen);
+  write_pickle.WriteString(pathname);
+  write_pickle.WriteInt(flags);
+  RAW_CHECK(write_pickle.size() <= kMaxMessageLength);
+
+  int returned_fd = -1;
+  uint8_t reply_buf[kMaxMessageLength];
+  // Send a request (in write_pickle) as well that will include a new
+  // temporary socketpair (created internally by SendRecvMsg()).
+  // Then read the reply on this new socketpair in reply_buf and put an
+  // eventual attached file descriptor in |returned_fd|.
+  // TODO(jln): this API needs some rewriting and documentation.
+  ssize_t msg_len = UnixDomainSocket::SendRecvMsg(ipc_socketpair_,
+                                                  reply_buf,
+                                                  sizeof(reply_buf),
+                                                  &returned_fd,
+                                                  write_pickle);
+  if (msg_len <= 0) {
+    if (!quiet_failures_for_tests_)
+      RAW_LOG(ERROR, "Could not make request to broker process");
+    return -ENOMEM;
+  }
+
+  Pickle read_pickle(reinterpret_cast<char*>(reply_buf), msg_len);
+  PickleIterator iter(read_pickle);
+  int return_value = -1;
+  // Now deserialize the return value and eventually return the file
+  // descriptor.
+  if (read_pickle.ReadInt(&iter, &return_value)) {
+    if (return_value < 0) {
+      RAW_CHECK(returned_fd == -1);
+      return return_value;
+    } else {
+      // We have a real file descriptor to return.
+      RAW_CHECK(returned_fd >= 0);
+      return returned_fd;
+    }
+  } else {
+    RAW_LOG(ERROR, "Could not read pickle");
+    return -1;
+  }
+}
+
+// Handle a request on the IPC channel ipc_socketpair_.
+// A request should have a file descriptor attached on which we will reply and
+// that we will then close.
+// A request should start with an int that will be used as the command type.
+bool BrokerProcess::HandleRequest() const {
+
+  std::vector<int> fds;
+  char buf[kMaxMessageLength];
+  errno = 0;
+  const ssize_t msg_len = UnixDomainSocket::RecvMsg(ipc_socketpair_, buf,
+                                                    sizeof(buf), &fds);
+
+  if (msg_len == 0 || (msg_len == -1 && errno == ECONNRESET)) {
+    // EOF from our parent, or our parent died, we should die.
+    _exit(0);
+  }
+
+  // The parent should send exactly one file descriptor, on which we
+  // will write the reply.
+  if (msg_len < 0 || fds.size() != 1 || fds.at(0) < 0) {
+    PLOG(ERROR) << "Error reading message from the client";
+    return false;
+  }
+
+  const int temporary_ipc = fds.at(0);
+
+  Pickle pickle(buf, msg_len);
+  PickleIterator iter(pickle);
+  int command_type;
+  if (pickle.ReadInt(&iter, &command_type)) {
+    bool r = false;
+    // Go through all the possible IPC messages.
+    switch (command_type) {
+      case kCommandOpen:
+        // We reply on the file descriptor sent to us via the IPC channel.
+        r = HandleOpenRequest(temporary_ipc, pickle, iter);
+        (void) HANDLE_EINTR(close(temporary_ipc));
+        return r;
+      default:
+        NOTREACHED();
+        return false;
+    }
+  }
+
+  LOG(ERROR) << "Error parsing IPC request";
+  return false;
+}
+
+// Handle an open request contained in |read_pickle| and send the reply
+// on |reply_ipc|.
+bool BrokerProcess::HandleOpenRequest(int reply_ipc,
+                                      const Pickle& read_pickle,
+                                      PickleIterator iter) const {
+  std::string requested_filename;
+  int flags = 0;
+  if (!read_pickle.ReadString(&iter, &requested_filename) ||
+      !read_pickle.ReadInt(&iter, &flags)) {
+    return -1;
+  }
+
+  Pickle write_pickle;
+  std::vector<int> opened_files;
+
+  const char* file_to_open = NULL;
+  const bool safe_to_open_file = GetFileNameIfAllowedAccess(
+      requested_filename.c_str(), flags, &file_to_open);
+
+  if (safe_to_open_file) {
+    CHECK(file_to_open);
+    // O_CLOEXEC doesn't hurt (even though we won't execve()), and this
+    // property won't be passed to the client.
+    // We may want to think about O_NONBLOCK as well.
+    int opened_fd = open(file_to_open, flags | O_CLOEXEC);
+    if (opened_fd < 0) {
+      write_pickle.WriteInt(-errno);
+    } else {
+      // Success.
+      opened_files.push_back(opened_fd);
+      write_pickle.WriteInt(0);
+    }
+  } else {
+    write_pickle.WriteInt(-EPERM);
+  }
+
+  CHECK_LE(write_pickle.size(), kMaxMessageLength);
+  ssize_t sent = UnixDomainSocket::SendMsg(reply_ipc, write_pickle.data(),
+                                           write_pickle.size(), opened_files);
+
+  // Close anything we have opened in this process.
+  for (std::vector<int>::iterator it = opened_files.begin();
+       it < opened_files.end(); ++it) {
+    (void) HANDLE_EINTR(close(*it));
+  }
+
+  if (sent <= 0) {
+    LOG(ERROR) << "Could not send IPC reply";
+    return false;
+  }
+  return true;
+}
+
+// For paranoia, if |file_to_open| is not NULL, we will return the matching
+// string from the white list.
+// Async signal safe only if |file_to_open| is NULL.
+// Even if an attacker managed to fool the string comparison mechanism, we
+// would not open an attacker-controlled file name.
+// Return true if access should be allowed, false otherwise.
+bool BrokerProcess::GetFileNameIfAllowedAccess(const char* requested_filename,
+    int requested_flags, const char** file_to_open) const {
+  if (!IsAllowedOpenFlags(requested_flags)) {
+    return false;
+  }
+  switch (requested_flags & O_ACCMODE) {
+    case O_RDONLY:
+      return GetFileNameInWhitelist(allowed_r_files_, requested_filename,
+                                    file_to_open);
+    case O_WRONLY:
+      return GetFileNameInWhitelist(allowed_w_files_, requested_filename,
+                                    file_to_open);
+    case O_RDWR:
+    {
+      bool allowed_for_read_and_write =
+          GetFileNameInWhitelist(allowed_r_files_, requested_filename, NULL) &&
+          GetFileNameInWhitelist(allowed_w_files_, requested_filename,
+                                 file_to_open);
+      return allowed_for_read_and_write;
+    }
+    default:
+      return false;
+  }
+}
+
+}  // namespace sandbox.