init: run vendor commands in a separate SELinux context
One of the major aspects of treble is the compartmentalization of system
and vendor components, however init leaves a huge gap here, as vendor
init scripts run in the same context as system init scripts and thus can
access and modify the same properties, files, etc as the system can.
This change is meant to close that gap. It forks a separate 'subcontext'
init that runs in a different SELinux context with permissions that match
what vendors should have access to. Commands get sent over a socket to
this 'subcontext' init that then runs them in this SELinux context and
returns the result.
Note that not all commands run in the subcontext; some commands such as
those dealing with services only make sense in the context of the main
init process.
Bug: 62875318
Test: init unit tests, boot bullhead, boot sailfish
Change-Id: Idf4a4ebf98842d27b8627f901f961ab9eb412aee
diff --git a/init/subcontext.cpp b/init/subcontext.cpp
new file mode 100644
index 0000000..1306c7d
--- /dev/null
+++ b/init/subcontext.cpp
@@ -0,0 +1,282 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#include "subcontext.h"
+
+#include <fcntl.h>
+#include <poll.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/strings.h>
+#include <selinux/android.h>
+
+#include "action.h"
+#include "system/core/init/subcontext.pb.h"
+#include "util.h"
+
+using android::base::GetExecutablePath;
+using android::base::Join;
+using android::base::Socketpair;
+using android::base::Split;
+using android::base::StartsWith;
+using android::base::unique_fd;
+
+namespace android {
+namespace init {
+
+const std::string kInitContext = "u:object_r:init:s0";
+const std::string kVendorContext = "u:object_r:vendor_init:s0";
+
+namespace {
+
+constexpr size_t kBufferSize = 4096;
+
+Result<std::string> ReadMessage(int socket) {
+ char buffer[kBufferSize] = {};
+ auto result = TEMP_FAILURE_RETRY(recv(socket, buffer, sizeof(buffer), 0));
+ if (result <= 0) {
+ return ErrnoError();
+ }
+ return std::string(buffer, result);
+}
+
+template <typename T>
+Result<Success> SendMessage(int socket, const T& message) {
+ std::string message_string;
+ if (!message.SerializeToString(&message_string)) {
+ return Error() << "Unable to serialize message";
+ }
+
+ if (message_string.size() > kBufferSize) {
+ return Error() << "Serialized message too long to send";
+ }
+
+ if (auto result =
+ TEMP_FAILURE_RETRY(send(socket, message_string.c_str(), message_string.size(), 0));
+ result != static_cast<long>(message_string.size())) {
+ return ErrnoError() << "send() failed to send message contents";
+ }
+ return Success();
+}
+
+class SubcontextProcess {
+ public:
+ SubcontextProcess(const KeywordFunctionMap* function_map, std::string context, int init_fd)
+ : function_map_(function_map), context_(std::move(context)), init_fd_(init_fd){};
+ void MainLoop();
+
+ private:
+ void RunCommand(const SubcontextCommand::ExecuteCommand& execute_command,
+ SubcontextReply::ResultMessage* result_message) const;
+
+ const KeywordFunctionMap* function_map_;
+ const std::string context_;
+ const int init_fd_;
+};
+
+void SubcontextProcess::RunCommand(const SubcontextCommand::ExecuteCommand& execute_command,
+ SubcontextReply::ResultMessage* result_message) const {
+ // Need to use ArraySplice instead of this code.
+ auto args = std::vector<std::string>();
+ for (const auto& string : execute_command.args()) {
+ args.emplace_back(string);
+ }
+
+ auto map_result = function_map_->FindFunction(args);
+ Result<Success> result;
+ if (!map_result) {
+ result = Error() << "Cannot find command: " << map_result.error();
+ } else {
+ result = RunBuiltinFunction(map_result->second, args, context_);
+ }
+
+ if (result) {
+ result_message->set_success(true);
+ } else {
+ result_message->set_success(false);
+ result_message->set_error_string(result.error_string());
+ result_message->set_error_errno(result.error_errno());
+ }
+}
+
+void SubcontextProcess::MainLoop() {
+ pollfd ufd[1];
+ ufd[0].events = POLLIN;
+ ufd[0].fd = init_fd_;
+
+ while (true) {
+ ufd[0].revents = 0;
+ int nr = TEMP_FAILURE_RETRY(poll(ufd, arraysize(ufd), -1));
+ if (nr == 0) continue;
+ if (nr < 0) {
+ PLOG(FATAL) << "poll() of subcontext socket failed, continuing";
+ }
+
+ auto init_message = ReadMessage(init_fd_);
+ if (!init_message) {
+ LOG(FATAL) << "Could not read message from init: " << init_message.error();
+ }
+
+ auto subcontext_command = SubcontextCommand();
+ if (!subcontext_command.ParseFromString(*init_message)) {
+ LOG(FATAL) << "Unable to parse message from init";
+ }
+
+ auto reply = SubcontextReply();
+ switch (subcontext_command.command_case()) {
+ case SubcontextCommand::kExecuteCommand: {
+ RunCommand(subcontext_command.execute_command(), reply.mutable_result());
+ break;
+ }
+ default:
+ LOG(FATAL) << "Unknown message type from init: "
+ << subcontext_command.command_case();
+ }
+
+ if (auto result = SendMessage(init_fd_, reply); !result) {
+ LOG(FATAL) << "Failed to send message to init: " << result.error();
+ }
+ }
+}
+
+} // namespace
+
+int SubcontextMain(int argc, char** argv, const KeywordFunctionMap* function_map) {
+ if (argc < 4) LOG(FATAL) << "Fewer than 4 args specified to subcontext (" << argc << ")";
+
+ auto context = std::string(argv[2]);
+ auto init_fd = std::atoi(argv[3]);
+
+ auto subcontext_process = SubcontextProcess(function_map, context, init_fd);
+ subcontext_process.MainLoop();
+ return 0;
+}
+
+void Subcontext::Fork() {
+ unique_fd subcontext_socket;
+ if (!Socketpair(AF_UNIX, SOCK_SEQPACKET | SOCK_CLOEXEC, 0, &socket_, &subcontext_socket)) {
+ LOG(FATAL) << "Could not create socket pair to communicate to subcontext";
+ return;
+ }
+
+ auto result = fork();
+
+ if (result == -1) {
+ LOG(FATAL) << "Could not fork subcontext";
+ } else if (result == 0) {
+ socket_.reset();
+
+ // We explicitly do not use O_CLOEXEC here, such that we can reference this FD by number
+ // in the subcontext process after we exec.
+ int child_fd = dup(subcontext_socket);
+ if (child_fd < 0) {
+ PLOG(FATAL) << "Could not dup child_fd";
+ }
+
+ if (setexeccon(context_.c_str()) < 0) {
+ PLOG(FATAL) << "Could not set execcon for '" << context_ << "'";
+ }
+
+ auto init_path = GetExecutablePath();
+ auto child_fd_string = std::to_string(child_fd);
+ const char* args[] = {init_path.c_str(), "subcontext", context_.c_str(),
+ child_fd_string.c_str(), nullptr};
+ execv(init_path.data(), const_cast<char**>(args));
+
+ PLOG(FATAL) << "Could not execv subcontext init";
+ } else {
+ subcontext_socket.reset();
+ pid_ = result;
+ LOG(INFO) << "Forked subcontext for '" << context_ << "' with pid " << pid_;
+ }
+}
+
+void Subcontext::Restart() {
+ LOG(ERROR) << "Restarting subcontext '" << context_ << "'";
+ if (pid_) {
+ kill(pid_, SIGKILL);
+ }
+ pid_ = 0;
+ socket_.reset();
+ Fork();
+}
+
+Result<Success> Subcontext::Execute(const std::vector<std::string>& args) {
+ auto subcontext_command = SubcontextCommand();
+ std::copy(
+ args.begin(), args.end(),
+ RepeatedPtrFieldBackInserter(subcontext_command.mutable_execute_command()->mutable_args()));
+
+ if (auto result = SendMessage(socket_, subcontext_command); !result) {
+ Restart();
+ return ErrnoError() << "Failed to send message to subcontext";
+ }
+
+ auto subcontext_message = ReadMessage(socket_);
+ if (!subcontext_message) {
+ Restart();
+ return Error() << "Failed to receive result from subcontext: " << subcontext_message.error();
+ }
+
+ auto subcontext_reply = SubcontextReply();
+ if (!subcontext_reply.ParseFromString(*subcontext_message)) {
+ Restart();
+ return Error() << "Unable to parse message from subcontext";
+ }
+
+ switch (subcontext_reply.reply_case()) {
+ case SubcontextReply::kResult: {
+ auto result = subcontext_reply.result();
+ if (result.success()) {
+ return Success();
+ } else {
+ return ResultError(result.error_string(), result.error_errno());
+ }
+ }
+ default:
+ return Error() << "Unknown message type from subcontext: "
+ << subcontext_reply.reply_case();
+ }
+}
+
+static std::vector<Subcontext> subcontexts;
+
+std::vector<Subcontext>* InitializeSubcontexts() {
+ static const char* const paths_and_secontexts[][2] = {
+ // TODO: Enable this once the SEPolicy is in place.
+ // {"/vendor", kVendorContext.c_str()},
+ };
+ for (const auto& [path_prefix, secontext] : paths_and_secontexts) {
+ subcontexts.emplace_back(path_prefix, secontext);
+ }
+ return &subcontexts;
+}
+
+bool SubcontextChildReap(pid_t pid) {
+ for (auto& subcontext : subcontexts) {
+ if (subcontext.pid() == pid) {
+ subcontext.Restart();
+ return true;
+ }
+ }
+ return false;
+}
+
+} // namespace init
+} // namespace android