platform_system_core/init/snapuserd_transition.cpp

/*
 * Copyright (C) 2020 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 "snapuserd_transition.h"

#include <sys/mman.h>
#include <sys/socket.h>
#include <sys/syscall.h>
#include <sys/xattr.h>
#include <unistd.h>

#include <filesystem>
#include <string>
#include <string_view>

#include <android-base/file.h>
#include <android-base/logging.h>
#include <android-base/parseint.h>
#include <android-base/strings.h>
#include <android-base/unique_fd.h>
#include <cutils/sockets.h>
#include <libsnapshot/snapshot.h>
#include <libsnapshot/snapuserd_client.h>
#include <private/android_filesystem_config.h>
#include <procinfo/process_map.h>
#include <selinux/android.h>

#include "block_dev_initializer.h"
#include "service_utils.h"
#include "util.h"

namespace android {
namespace init {

using namespace std::string_literals;

using android::base::unique_fd;
using android::snapshot::SnapshotManager;
using android::snapshot::SnapuserdClient;

static constexpr char kSnapuserdPath[] = "/system/bin/snapuserd";
static constexpr char kSnapuserdFirstStagePidVar[] = "FIRST_STAGE_SNAPUSERD_PID";
static constexpr char kSnapuserdFirstStageFdVar[] = "FIRST_STAGE_SNAPUSERD_FD";
static constexpr char kSnapuserdLabel[] = "u:object_r:snapuserd_exec:s0";
static constexpr char kSnapuserdSocketLabel[] = "u:object_r:snapuserd_socket:s0";

void LaunchFirstStageSnapuserd() {
    SocketDescriptor socket_desc;
    socket_desc.name = android::snapshot::kSnapuserdSocket;
    socket_desc.type = SOCK_STREAM;
    socket_desc.perm = 0660;
    socket_desc.uid = AID_SYSTEM;
    socket_desc.gid = AID_SYSTEM;

    // We specify a label here even though it technically is not needed. During
    // first_stage_mount there is no sepolicy loaded. Once sepolicy is loaded,
    // we bypass the socket entirely.
    auto socket = socket_desc.Create(kSnapuserdSocketLabel);
    if (!socket.ok()) {
        LOG(FATAL) << "Could not create snapuserd socket: " << socket.error();
    }

    pid_t pid = fork();
    if (pid < 0) {
        PLOG(FATAL) << "Cannot launch snapuserd; fork failed";
    }
    if (pid == 0) {
        socket->Publish();
        char arg0[] = "/system/bin/snapuserd";
        char* const argv[] = {arg0, nullptr};
        if (execv(arg0, argv) < 0) {
            PLOG(FATAL) << "Cannot launch snapuserd; execv failed";
        }
        _exit(127);
    }

    setenv(kSnapuserdFirstStagePidVar, std::to_string(pid).c_str(), 1);

    LOG(INFO) << "Relaunched snapuserd with pid: " << pid;
}

std::optional<pid_t> GetSnapuserdFirstStagePid() {
    const char* pid_str = getenv(kSnapuserdFirstStagePidVar);
    if (!pid_str) {
        return {};
    }

    int pid = 0;
    if (!android::base::ParseInt(pid_str, &pid)) {
        LOG(FATAL) << "Could not parse pid in environment, " << kSnapuserdFirstStagePidVar << "="
                   << pid_str;
    }
    return {pid};
}

static void RelabelLink(const std::string& link) {
    selinux_android_restorecon(link.c_str(), 0);

    std::string path;
    if (android::base::Readlink(link, &path)) {
        selinux_android_restorecon(path.c_str(), 0);
    }
}

static void RelabelDeviceMapper() {
    selinux_android_restorecon("/dev/device-mapper", 0);

    std::error_code ec;
    for (auto& iter : std::filesystem::directory_iterator("/dev/block", ec)) {
        const auto& path = iter.path();
        if (android::base::StartsWith(path.string(), "/dev/block/dm-")) {
            selinux_android_restorecon(path.string().c_str(), 0);
        }
    }
}

static std::optional<int> GetRamdiskSnapuserdFd() {
    const char* fd_str = getenv(kSnapuserdFirstStageFdVar);
    if (!fd_str) {
        return {};
    }

    int fd;
    if (!android::base::ParseInt(fd_str, &fd)) {
        LOG(FATAL) << "Could not parse fd in environment, " << kSnapuserdFirstStageFdVar << "="
                   << fd_str;
    }
    return {fd};
}

void RestoreconRamdiskSnapuserd(int fd) {
    if (fsetxattr(fd, XATTR_NAME_SELINUX, kSnapuserdLabel, strlen(kSnapuserdLabel) + 1, 0) < 0) {
        PLOG(FATAL) << "fsetxattr snapuserd failed";
    }
}

SnapuserdSelinuxHelper::SnapuserdSelinuxHelper(std::unique_ptr<SnapshotManager>&& sm, pid_t old_pid)
    : sm_(std::move(sm)), old_pid_(old_pid) {
    // Only dm-user device names change during transitions, so the other
    // devices are expected to be present.
    sm_->SetUeventRegenCallback([this](const std::string& device) -> bool {
        if (android::base::StartsWith(device, "/dev/dm-user/")) {
            return block_dev_init_.InitDmUser(android::base::Basename(device));
        }
        return true;
    });
}

static void LockAllSystemPages() {
    bool ok = true;
    auto callback = [&](const android::procinfo::MapInfo& map) -> void {
        if (!ok || android::base::StartsWith(map.name, "/dev/") ||
            !android::base::StartsWith(map.name, "/")) {
            return;
        }
        auto start = reinterpret_cast<const void*>(map.start);
        auto len = map.end - map.start;
        if (!len) {
            return;
        }
        if (mlock(start, len) < 0) {
            LOG(ERROR) << "mlock failed, " << start << " for " << len << " bytes.";
            ok = false;
        }
    };

    if (!android::procinfo::ReadProcessMaps(getpid(), callback) || !ok) {
        LOG(FATAL) << "Could not process /proc/" << getpid() << "/maps file for init, "
                   << "falling back to mlockall().";
        if (mlockall(MCL_CURRENT) < 0) {
            LOG(FATAL) << "mlockall failed";
        }
    }
}

void SnapuserdSelinuxHelper::StartTransition() {
    LOG(INFO) << "Starting SELinux transition of snapuserd";

    // The restorecon path reads from /system etc, so make sure any reads have
    // been cached before proceeding.
    auto handle = selinux_android_file_context_handle();
    if (!handle) {
        LOG(FATAL) << "Could not create SELinux file context handle";
    }
    selinux_android_set_sehandle(handle);

    // We cannot access /system after the transition, so make sure init is
    // pinned in memory.
    LockAllSystemPages();

    argv_.emplace_back("snapuserd");
    argv_.emplace_back("-no_socket");
    if (!sm_->DetachSnapuserdForSelinux(&argv_)) {
        LOG(FATAL) << "Could not perform selinux transition";
    }

    // Make sure the process is gone so we don't have any selinux audits.
    KillFirstStageSnapuserd(old_pid_);
}

void SnapuserdSelinuxHelper::FinishTransition() {
    RelabelLink("/dev/block/by-name/super");
    RelabelDeviceMapper();

    selinux_android_restorecon("/dev/null", 0);
    selinux_android_restorecon("/dev/urandom", 0);
    selinux_android_restorecon("/dev/kmsg", 0);
    selinux_android_restorecon("/dev/dm-user", SELINUX_ANDROID_RESTORECON_RECURSE);

    RelaunchFirstStageSnapuserd();

    if (munlockall() < 0) {
        PLOG(ERROR) << "munlockall failed";
    }
}

void SnapuserdSelinuxHelper::RelaunchFirstStageSnapuserd() {
    auto fd = GetRamdiskSnapuserdFd();
    if (!fd) {
        LOG(FATAL) << "Environment variable " << kSnapuserdFirstStageFdVar << " was not set!";
    }
    unsetenv(kSnapuserdFirstStageFdVar);

    RestoreconRamdiskSnapuserd(fd.value());

    pid_t pid = fork();
    if (pid < 0) {
        PLOG(FATAL) << "Fork to relaunch snapuserd failed";
    }
    if (pid > 0) {
        // We don't need the descriptor anymore, and it should be closed to
        // avoid leaking into subprocesses.
        close(fd.value());

        setenv(kSnapuserdFirstStagePidVar, std::to_string(pid).c_str(), 1);

        LOG(INFO) << "Relaunched snapuserd with pid: " << pid;
        return;
    }

    // Make sure the descriptor is gone after we exec.
    if (fcntl(fd.value(), F_SETFD, FD_CLOEXEC) < 0) {
        PLOG(FATAL) << "fcntl FD_CLOEXEC failed for snapuserd fd";
    }

    std::vector<char*> argv;
    for (auto& arg : argv_) {
        argv.emplace_back(arg.data());
    }
    argv.emplace_back(nullptr);

    int rv = syscall(SYS_execveat, fd.value(), "", reinterpret_cast<char* const*>(argv.data()),
                     nullptr, AT_EMPTY_PATH);
    if (rv < 0) {
        PLOG(FATAL) << "Failed to execveat() snapuserd";
    }
}

std::unique_ptr<SnapuserdSelinuxHelper> SnapuserdSelinuxHelper::CreateIfNeeded() {
    if (IsRecoveryMode()) {
        return nullptr;
    }

    auto old_pid = GetSnapuserdFirstStagePid();
    if (!old_pid) {
        return nullptr;
    }

    auto sm = SnapshotManager::NewForFirstStageMount();
    if (!sm) {
        LOG(FATAL) << "Unable to create SnapshotManager";
    }
    return std::make_unique<SnapuserdSelinuxHelper>(std::move(sm), old_pid.value());
}

void KillFirstStageSnapuserd(pid_t pid) {
    if (kill(pid, SIGTERM) < 0 && errno != ESRCH) {
        LOG(ERROR) << "Kill snapuserd pid failed: " << pid;
    } else {
        LOG(INFO) << "Sent SIGTERM to snapuserd process " << pid;
    }
}

void CleanupSnapuserdSocket() {
    auto socket_path = ANDROID_SOCKET_DIR "/"s + android::snapshot::kSnapuserdSocket;
    if (access(socket_path.c_str(), F_OK) != 0) {
        return;
    }

    // Tell the daemon to stop accepting connections and to gracefully exit
    // once all outstanding handlers have terminated.
    if (auto client = SnapuserdClient::Connect(android::snapshot::kSnapuserdSocket, 3s)) {
        client->DetachSnapuserd();
    }

    // Unlink the socket so we can create it again in second-stage.
    if (unlink(socket_path.c_str()) < 0) {
        PLOG(FATAL) << "unlink " << socket_path << " failed";
    }
}

void SaveRamdiskPathToSnapuserd() {
    int fd = open(kSnapuserdPath, O_PATH);
    if (fd < 0) {
        PLOG(FATAL) << "Unable to open snapuserd: " << kSnapuserdPath;
    }

    auto value = std::to_string(fd);
    if (setenv(kSnapuserdFirstStageFdVar, value.c_str(), 1) < 0) {
        PLOG(FATAL) << "setenv failed: " << kSnapuserdFirstStageFdVar << "=" << value;
    }
}

bool IsFirstStageSnapuserdRunning() {
    return GetSnapuserdFirstStagePid().has_value();
}

}  // namespace init
}  // namespace android