platform_system_core/init/mount_namespace.cpp

/*
 * Copyright (C) 2019 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 "mount_namespace.h"

#include <sys/mount.h>

#include <string>
#include <vector>

#include <android-base/file.h>
#include <android-base/logging.h>
#include <android-base/properties.h>
#include <android-base/result.h>
#include <android-base/unique_fd.h>

#include "util.h"

namespace android {
namespace init {
namespace {

static bool BindMount(const std::string& source, const std::string& mount_point) {
    if (mount(source.c_str(), mount_point.c_str(), nullptr, MS_BIND | MS_REC, nullptr) == -1) {
        PLOG(ERROR) << "Failed to bind mount " << source;
        return false;
    }
    return true;
}

static bool ChangeMount(const std::string& mount_point, unsigned long mountflags) {
    if (mount(nullptr, mount_point.c_str(), nullptr, mountflags, nullptr) == -1) {
        PLOG(ERROR) << "Failed to remount " << mount_point << " as " << std::hex << mountflags;
        return false;
    }
    return true;
}

static int OpenMountNamespace() {
    int fd = open("/proc/self/ns/mnt", O_RDONLY | O_CLOEXEC);
    if (fd < 0) {
        PLOG(ERROR) << "Cannot open fd for current mount namespace";
    }
    return fd;
}

static std::string GetMountNamespaceId() {
    std::string ret;
    if (!android::base::Readlink("/proc/self/ns/mnt", &ret)) {
        PLOG(ERROR) << "Failed to read namespace ID";
        return "";
    }
    return ret;
}

// In case we have two sets of APEXes (non-updatable, updatable), we need two separate mount
// namespaces.
static bool NeedsTwoMountNamespaces() {
    if (IsRecoveryMode()) return false;
    // In microdroid, there's only one set of APEXes in built-in directories include block devices.
    if (IsMicrodroid()) return false;
    return true;
}

static android::base::unique_fd bootstrap_ns_fd;
static android::base::unique_fd default_ns_fd;

static std::string bootstrap_ns_id;
static std::string default_ns_id;

}  // namespace

bool SetupMountNamespaces() {
    // Set the propagation type of / as shared so that any mounting event (e.g.
    // /data) is by default visible to all processes. When private mounting is
    // needed for /foo/bar, then we will make /foo/bar as a mount point (by
    // bind-mounting by to itself) and set the propagation type of the mount
    // point to private.
    if (!ChangeMount("/", MS_SHARED | MS_REC)) return false;

    // /apex is a private mountpoint to give different sets of APEXes for
    // the bootstrap and default mount namespaces. The processes running with
    // the bootstrap namespace get APEXes from the read-only partition.
    if (!(ChangeMount("/apex", MS_PRIVATE))) return false;

    // /linkerconfig is a private mountpoint to give a different linker configuration
    // based on the mount namespace. Subdirectory will be bind-mounted based on current mount
    // namespace
    if (!(ChangeMount("/linkerconfig", MS_PRIVATE))) return false;

    // The two mount namespaces present challenges for scoped storage, because
    // vold, which is responsible for most of the mounting, lives in the
    // bootstrap mount namespace, whereas most other daemons and all apps live
    // in the default namespace.  Scoped storage has a need for a
    // /mnt/installer view that is a slave bind mount of /mnt/user - in other
    // words, all mounts under /mnt/user should automatically show up under
    // /mnt/installer. However, additional mounts done under /mnt/installer
    // should not propagate back to /mnt/user. In a single mount namespace
    // this is easy to achieve, by simply marking the /mnt/installer a slave
    // bind mount. Unfortunately, if /mnt/installer is only created and
    // bind mounted after the two namespaces are created below, we end up
    // with the following situation:
    // /mnt/user and /mnt/installer share the same peer group in both the
    // bootstrap and default namespaces. Marking /mnt/installer slave in either
    // namespace means that it won't propagate events to the /mnt/installer in
    // the other namespace, which is still something we require - vold is the
    // one doing the mounting under /mnt/installer, and those mounts should
    // show up in the default namespace as well.
    //
    // The simplest solution is to do the bind mount before the two namespaces
    // are created: the effect is that in both namespaces, /mnt/installer is a
    // slave to the /mnt/user mount, and at the same time /mnt/installer in the
    // bootstrap namespace shares a peer group with /mnt/installer in the
    // default namespace.
    // /mnt/androidwritable is similar to /mnt/installer but serves for
    // MOUNT_EXTERNAL_ANDROID_WRITABLE apps.
    if (!mkdir_recursive("/mnt/user", 0755)) return false;
    if (!mkdir_recursive("/mnt/installer", 0755)) return false;
    if (!mkdir_recursive("/mnt/androidwritable", 0755)) return false;
    if (!(BindMount("/mnt/user", "/mnt/installer"))) return false;
    if (!(BindMount("/mnt/user", "/mnt/androidwritable"))) return false;
    // First, make /mnt/installer and /mnt/androidwritable a slave bind mount
    if (!(ChangeMount("/mnt/installer", MS_SLAVE))) return false;
    if (!(ChangeMount("/mnt/androidwritable", MS_SLAVE))) return false;
    // Then, make it shared again - effectively creating a new peer group, that
    // will be inherited by new mount namespaces.
    if (!(ChangeMount("/mnt/installer", MS_SHARED))) return false;
    if (!(ChangeMount("/mnt/androidwritable", MS_SHARED))) return false;

    bootstrap_ns_fd.reset(OpenMountNamespace());
    bootstrap_ns_id = GetMountNamespaceId();

    // When APEXes are updatable (e.g. not-flattened), we create separate mount
    // namespaces for processes that are started before and after the APEX is
    // activated by apexd. In the namespace for pre-apexd processes, small
    // number of essential APEXes (e.g. com.android.runtime) are activated.
    // In the namespace for post-apexd processes, all APEXes are activated.
    bool success = true;
    if (NeedsTwoMountNamespaces()) {
        // Creating a new namespace by cloning, saving, and switching back to
        // the original namespace.
        if (unshare(CLONE_NEWNS) == -1) {
            PLOG(ERROR) << "Cannot create mount namespace";
            return false;
        }
        default_ns_fd.reset(OpenMountNamespace());
        default_ns_id = GetMountNamespaceId();

        if (setns(bootstrap_ns_fd.get(), CLONE_NEWNS) == -1) {
            PLOG(ERROR) << "Cannot switch back to bootstrap mount namespace";
            return false;
        }
    } else {
        // Otherwise, default == bootstrap
        default_ns_fd.reset(OpenMountNamespace());
        default_ns_id = GetMountNamespaceId();
    }

    LOG(INFO) << "SetupMountNamespaces done";
    return success;
}

// Switch the mount namespace of the current process from bootstrap to default OR from default to
// bootstrap. If the current mount namespace is neither bootstrap nor default, keep it that way.
Result<void> SwitchToMountNamespaceIfNeeded(MountNamespace target_mount_namespace) {
    if (IsRecoveryMode()) {
        // we don't have multiple namespaces in recovery mode or if apex is not updatable
        return {};
    }

    const std::string current_namespace_id = GetMountNamespaceId();
    MountNamespace current_mount_namespace;
    if (current_namespace_id == bootstrap_ns_id) {
        current_mount_namespace = NS_BOOTSTRAP;
    } else if (current_namespace_id == default_ns_id) {
        current_mount_namespace = NS_DEFAULT;
    } else {
        // services with `namespace mnt` start in its own mount namespace. So we need to keep it.
        return {};
    }

    // We're already in the target mount namespace.
    if (current_mount_namespace == target_mount_namespace) {
        return {};
    }

    const auto& ns_fd = target_mount_namespace == NS_BOOTSTRAP ? bootstrap_ns_fd : default_ns_fd;
    const auto& ns_name = target_mount_namespace == NS_BOOTSTRAP ? "bootstrap" : "default";
    if (ns_fd.get() != -1) {
        if (setns(ns_fd.get(), CLONE_NEWNS) == -1) {
            return ErrnoError() << "Failed to switch to " << ns_name << " mount namespace.";
        }
    }
    return {};
}

base::Result<MountNamespace> GetCurrentMountNamespace() {
    std::string current_namespace_id = GetMountNamespaceId();
    if (current_namespace_id == "") {
        return Error() << "Failed to get current mount namespace ID";
    }

    if (current_namespace_id == bootstrap_ns_id) {
        return NS_BOOTSTRAP;
    } else if (current_namespace_id == default_ns_id) {
        return NS_DEFAULT;
    }

    return Error() << "Failed to find current mount namespace";
}

}  // namespace init
}  // namespace android