subcontext.cpp revision 193e43494f8afffaa1098690f1d249a3e61f2d43 
1/*
2 * Copyright (C) 2017 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#include "subcontext.h"
18
19#include <fcntl.h>
20#include <poll.h>
21#include <sys/socket.h>
22#include <unistd.h>
23
24#include <android-base/file.h>
25#include <android-base/logging.h>
26#include <android-base/strings.h>
27#include <selinux/android.h>
28
29#include "action.h"
30#include "selinux.h"
31#include "system/core/init/subcontext.pb.h"
32#include "util.h"
33
34using android::base::GetExecutablePath;
35using android::base::Join;
36using android::base::Socketpair;
37using android::base::Split;
38using android::base::StartsWith;
39using android::base::unique_fd;
40
41namespace android {
42namespace init {
43
44const std::string kInitContext = "u:r:init:s0";
45const std::string kVendorContext = "u:r:vendor_init:s0";
46
47namespace {
48
49constexpr size_t kBufferSize = 4096;
50
51Result<std::string> ReadMessage(int socket) {
52    char buffer[kBufferSize] = {};
53    auto result = TEMP_FAILURE_RETRY(recv(socket, buffer, sizeof(buffer), 0));
54    if (result <= 0) {
55        return ErrnoError();
56    }
57    return std::string(buffer, result);
58}
59
60template <typename T>
61Result<Success> SendMessage(int socket, const T& message) {
62    std::string message_string;
63    if (!message.SerializeToString(&message_string)) {
64        return Error() << "Unable to serialize message";
65    }
66
67    if (message_string.size() > kBufferSize) {
68        return Error() << "Serialized message too long to send";
69    }
70
71    if (auto result =
72            TEMP_FAILURE_RETRY(send(socket, message_string.c_str(), message_string.size(), 0));
73        result != static_cast<long>(message_string.size())) {
74        return ErrnoError() << "send() failed to send message contents";
75    }
76    return Success();
77}
78
79class SubcontextProcess {
80  public:
81    SubcontextProcess(const KeywordFunctionMap* function_map, std::string context, int init_fd)
82        : function_map_(function_map), context_(std::move(context)), init_fd_(init_fd){};
83    void MainLoop();
84
85  private:
86    void RunCommand(const SubcontextCommand::ExecuteCommand& execute_command,
87                    SubcontextReply::ResultMessage* result_message) const;
88
89    const KeywordFunctionMap* function_map_;
90    const std::string context_;
91    const int init_fd_;
92};
93
94void SubcontextProcess::RunCommand(const SubcontextCommand::ExecuteCommand& execute_command,
95                                   SubcontextReply::ResultMessage* result_message) const {
96    // Need to use ArraySplice instead of this code.
97    auto args = std::vector<std::string>();
98    for (const auto& string : execute_command.args()) {
99        args.emplace_back(string);
100    }
101
102    auto map_result = function_map_->FindFunction(args);
103    Result<Success> result;
104    if (!map_result) {
105        result = Error() << "Cannot find command: " << map_result.error();
106    } else {
107        result = RunBuiltinFunction(map_result->second, args, context_);
108    }
109
110    if (result) {
111        result_message->set_success(true);
112    } else {
113        result_message->set_success(false);
114        result_message->set_error_string(result.error_string());
115        result_message->set_error_errno(result.error_errno());
116    }
117}
118
119void SubcontextProcess::MainLoop() {
120    pollfd ufd[1];
121    ufd[0].events = POLLIN;
122    ufd[0].fd = init_fd_;
123
124    while (true) {
125        ufd[0].revents = 0;
126        int nr = TEMP_FAILURE_RETRY(poll(ufd, arraysize(ufd), -1));
127        if (nr == 0) continue;
128        if (nr < 0) {
129            PLOG(FATAL) << "poll() of subcontext socket failed, continuing";
130        }
131
132        auto init_message = ReadMessage(init_fd_);
133        if (!init_message) {
134            LOG(FATAL) << "Could not read message from init: " << init_message.error();
135        }
136
137        auto subcontext_command = SubcontextCommand();
138        if (!subcontext_command.ParseFromString(*init_message)) {
139            LOG(FATAL) << "Unable to parse message from init";
140        }
141
142        auto reply = SubcontextReply();
143        switch (subcontext_command.command_case()) {
144            case SubcontextCommand::kExecuteCommand: {
145                RunCommand(subcontext_command.execute_command(), reply.mutable_result());
146                break;
147            }
148            default:
149                LOG(FATAL) << "Unknown message type from init: "
150                           << subcontext_command.command_case();
151        }
152
153        if (auto result = SendMessage(init_fd_, reply); !result) {
154            LOG(FATAL) << "Failed to send message to init: " << result.error();
155        }
156    }
157}
158
159}  // namespace
160
161int SubcontextMain(int argc, char** argv, const KeywordFunctionMap* function_map) {
162    if (argc < 4) LOG(FATAL) << "Fewer than 4 args specified to subcontext (" << argc << ")";
163
164    auto context = std::string(argv[2]);
165    auto init_fd = std::atoi(argv[3]);
166
167    SelabelInitialize();
168    auto subcontext_process = SubcontextProcess(function_map, context, init_fd);
169    subcontext_process.MainLoop();
170    return 0;
171}
172
173void Subcontext::Fork() {
174    unique_fd subcontext_socket;
175    if (!Socketpair(AF_UNIX, SOCK_SEQPACKET | SOCK_CLOEXEC, 0, &socket_, &subcontext_socket)) {
176        LOG(FATAL) << "Could not create socket pair to communicate to subcontext";
177        return;
178    }
179
180    auto result = fork();
181
182    if (result == -1) {
183        LOG(FATAL) << "Could not fork subcontext";
184    } else if (result == 0) {
185        socket_.reset();
186
187        // We explicitly do not use O_CLOEXEC here, such that we can reference this FD by number
188        // in the subcontext process after we exec.
189        int child_fd = dup(subcontext_socket);
190        if (child_fd < 0) {
191            PLOG(FATAL) << "Could not dup child_fd";
192        }
193
194        if (setexeccon(context_.c_str()) < 0) {
195            PLOG(FATAL) << "Could not set execcon for '" << context_ << "'";
196        }
197
198        auto init_path = GetExecutablePath();
199        auto child_fd_string = std::to_string(child_fd);
200        const char* args[] = {init_path.c_str(), "subcontext", context_.c_str(),
201                              child_fd_string.c_str(), nullptr};
202        execv(init_path.data(), const_cast<char**>(args));
203
204        PLOG(FATAL) << "Could not execv subcontext init";
205    } else {
206        subcontext_socket.reset();
207        pid_ = result;
208        LOG(INFO) << "Forked subcontext for '" << context_ << "' with pid " << pid_;
209    }
210}
211
212void Subcontext::Restart() {
213    LOG(ERROR) << "Restarting subcontext '" << context_ << "'";
214    if (pid_) {
215        kill(pid_, SIGKILL);
216    }
217    pid_ = 0;
218    socket_.reset();
219    Fork();
220}
221
222Result<Success> Subcontext::Execute(const std::vector<std::string>& args) {
223    auto subcontext_command = SubcontextCommand();
224    std::copy(
225        args.begin(), args.end(),
226        RepeatedPtrFieldBackInserter(subcontext_command.mutable_execute_command()->mutable_args()));
227
228    if (auto result = SendMessage(socket_, subcontext_command); !result) {
229        Restart();
230        return ErrnoError() << "Failed to send message to subcontext";
231    }
232
233    auto subcontext_message = ReadMessage(socket_);
234    if (!subcontext_message) {
235        Restart();
236        return Error() << "Failed to receive result from subcontext: " << subcontext_message.error();
237    }
238
239    auto subcontext_reply = SubcontextReply();
240    if (!subcontext_reply.ParseFromString(*subcontext_message)) {
241        Restart();
242        return Error() << "Unable to parse message from subcontext";
243    }
244
245    switch (subcontext_reply.reply_case()) {
246        case SubcontextReply::kResult: {
247            auto result = subcontext_reply.result();
248            if (result.success()) {
249                return Success();
250            } else {
251                return ResultError(result.error_string(), result.error_errno());
252            }
253        }
254        default:
255            return Error() << "Unknown message type from subcontext: "
256                           << subcontext_reply.reply_case();
257    }
258}
259
260static std::vector<Subcontext> subcontexts;
261
262std::vector<Subcontext>* InitializeSubcontexts() {
263    static const char* const paths_and_secontexts[][2] = {
264        {"/vendor", kVendorContext.c_str()},
265    };
266    for (const auto& [path_prefix, secontext] : paths_and_secontexts) {
267        subcontexts.emplace_back(path_prefix, secontext);
268    }
269    return &subcontexts;
270}
271
272bool SubcontextChildReap(pid_t pid) {
273    for (auto& subcontext : subcontexts) {
274        if (subcontext.pid() == pid) {
275            subcontext.Restart();
276            return true;
277        }
278    }
279    return false;
280}
281
282}  // namespace init
283}  // namespace android
284