path: root/src/daemon.cpp

/* An implementation of a SPICE streaming agent
 *
 * \copyright
 * Copyright 2016-2017 Red Hat Inc. All rights reserved.
 */

/* This file implement the daemon part required to
 * avoid permission issues.
 * The daemon will listen to a socket for clients.
 * Clients simply will send informations about new display.
 * Daemon will monitor current terminal and launch a proper
 * agent with information passed.
 */

#include <config.h>
#include "daemon.hpp"
#include "eventfd.hpp"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <syslog.h>
#include <poll.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <pwd.h>
#include <grp.h>
#include <systemd/sd-daemon.h>
#include <string>
#include <map>
#include <stdexcept>

/*
 * There are 3 "roles" for the agent:
 * - main agent;
 * - daemon
 * - helper
 * The role of the agent is to handle a given graphical session
 * capturing and sending video stream to SPICE server.
 * The role of the daemon is to listen to information from helpers
 * collecting Unix session information from the helpers and from
 * system and managing agents.
 * The helper just send session information to the daemon. These
 * information are used to be able to connect to the display server
 * (like X).
 * The agent is a child (forked) of the daemon.
 * This schema is used for different reasons:
 * - the daemon can be run as root having access to the streaming
 *   device file;
 * - the daemon can control the live of the agent making possible to
 *   switch between sessions;
 * - running agents directly launched from a graphical session cause
 *   some issue with SELinux, launching outside a Unix session allows
 *   the process to have less restrictions.
 */

/*
 * The protocol between helper and daemon is pretty simple.
 * Helper connects to daemon and send all information needed to
 * connect to the display server.
 * The helper send a single message which is pretty small (should be at
 * most 1K) through a Unix socket.
 * This:
 * - allows to pass credentials;
 * - a single small message prevent the helper to consume memory on the
 *   daemon side;
 * - allows dynamic activation using SystemD;
 * - writing the client is really easy and can be written in a script
 *   language.
 *
 * Message is:
 * - 1 byte. Version. Has to be 1;
 * - a set of strings, each:
 *   - 1 byte type field, currently:
 *     1) DISPLAY environment;
 *     2) XAUTHORITY environment;
 *   - 1 byte for length
 *   - data
 * - DISPLAY content. The DISPLAY should be local like ":1";
 * - XAUTHORITY environment content (a filename).
 *
 * Note: Linux allows to read the peer credentials (user/group) and
 * PID which we need. If we would need to extent to other systems
 * (like *BSD/Mac OS X) these allows to pass these informations using
 * an ancilliary message and SCM_CREDS (Linux also has a similar
 * SCM_CREDENTIALS).
 */

static const char daemon_socket_name[] = "@spice-streaming-agent-daemon";

struct TerminalInfo
{
    TerminalInfo() = default;
    TerminalInfo(const uint8_t *msg, size_t msg_len);

    std::string display;
    std::string xauthority;
    uid_t uid;
};

/**
 * Parse a message from a client.
 * Throw an exception if the message content is invalid.
 */
TerminalInfo::TerminalInfo(const uint8_t *msg, size_t msg_len)
{
    if (msg_len < 1 || msg[0] != 1) {
        throw std::runtime_error("Invalid message header");
    }

    auto msg_end = msg + msg_len;
    ++msg;
    while (msg_end - msg >= 2) {
        uint8_t type = *msg++;
        uint8_t len = *msg++;
        if (msg_end - msg < len) {
            throw std::runtime_error("Invalid field header");
        }

        switch (type) {
        case 1:
            display = std::string((const char*) msg, len);
            break;
        case 2:
            xauthority = std::string((const char*) msg, len);
            break;
        default:
            throw std::runtime_error("Invalid field");
        }

        msg += len;
    }
    if (msg != msg_end) {
        throw std::runtime_error("Message not terminated correctly");
    }
}

static void api_err(const char *msg)
{
    syslog(LOG_ERR, "%s: %m", msg);
    exit(1);
}

/**
 * Get terminal number from PID.
 * This function is supposed to be quite low level.
 * We don't log any error to avoid possibles DoS.
 * The caller can use errno.
 * @returns terminal or -1 if error
 */
static int get_terminal(pid_t pid)
{
    char fn[128];
    sprintf(fn, "/proc/%u/stat", pid);

    // use C style FILE, the kernel document this file syntax using
    // scanf format specification
    FILE *f = fopen(fn, "re");
    if (!f) {
        return -1;
    }

    char line[1024*2];
    if (fgets(line, sizeof(line), f) == NULL) {
        fclose(f);
        return -1;
    }
    fclose(f);

    int terminal = -1, tty = -1;
    const char *end_exename = strstr(line, ") ");
    if (end_exename && sscanf(end_exename+2, "%*c %*d %*d %*d %d", &tty) > 0) {
        // check tty is a physical one (just looks at major/minor)
        int major = tty >> 8;
        int minor = tty & 0xff;
        if (major == 4 && minor > 0 && minor < 64) {
            terminal = minor;
        }
    }
    return terminal;
}

static bool check_xauthority(const std::string& fn, uid_t uid)
{
    // TODO timeout on check, could have passed a weird NFS
    // impersonate uid
    // file must be present
    // file must be small
    // read file
    // check for keys (memmem)
    //   MIT-MAGIC-COOKIE-1
    //   XDM-AUTHORIZATION-1
    //   MIT-KERBEROS-5
    //   SUN-DES-1
    return true;
}

/**
 * Check if a given file descriptor is the daemon socket we should
 * accept requests from.
 * In case the daemon is launched form inetd or systemd the file
 * descriptor is inherited from the parent.
 */
static bool fd_is_agent(int fd)
{
    // must be a socket
    struct stat st;
    if (fstat(fd, &st) != 0) {
        return false;
    }
    if ((st.st_mode & S_IFMT) != S_IFSOCK) {
        return false;
    }

    // must have our address
    struct sockaddr_un address;
    socklen_t len = sizeof(address);
    if (getsockname(fd, (sockaddr *) &address, &len) != 0) {
        return false;
    }
    if (address.sun_family != AF_UNIX) {
        return false;
    }
    if (address.sun_path[0] != 0) {
        return false;
    }
    address.sun_path[0] = '@';
    if (len != SUN_LEN(&address) || strcmp(address.sun_path, daemon_socket_name) != 0) {
        return false;
    }

    // TODO must be in listening, but this is mainly a paranoia check,
    // the file descriptor can come only from a trusted source

    return true;
}

class Daemon
{
public:
    Daemon(const char *stream_port_name);
    ~Daemon();
    void loop(int &streamfd);
private:
    void remove_client(unsigned n);
    void data_from_client(unsigned n);
    void handle_new_fd(int fd);
    void handle_fd_events(unsigned n, unsigned events);
    bool check_agent(int &streamfd);

    enum { max_clients = 63 };
    enum {
        LISTEN_FD,
        WAKEUP_FD,
        VT_FD,
        FIXED_FDS
    };
    spice::streaming_agent::EventFD wakeup_loop;
    struct pollfd fds[max_clients+FIXED_FDS];
    struct pollfd * const client_fds = fds+FIXED_FDS;
    unsigned num_clients = 0;
    pid_t child_pid = -1;
    int working_tty = -1;
    const char *stream_port_name;

    std::map<int, TerminalInfo> terminal_info;

    static void handle_sigchild(int);
    static Daemon *current;
};

Daemon *Daemon::current = nullptr;

Daemon::Daemon(const char *stream_port_name):
    stream_port_name(stream_port_name)
{
    int ret;

    fds[WAKEUP_FD].fd = wakeup_loop.raw_fd();
    fds[WAKEUP_FD].events = POLLIN;

    int fd;
    if (fd_is_agent(0)) {
        // inetd style socket passed
        fd = 0;
    } else if (fd_is_agent(SD_LISTEN_FDS_START)) {
        // systemd style socket passed
        fd = SD_LISTEN_FDS_START;
    } else {
        // open socket
        fd = socket(PF_UNIX, SOCK_STREAM|SOCK_NONBLOCK|SOCK_CLOEXEC, 0);
        if (fd < 0) {
            api_err("Unable to open daemon socket");
        }

        struct sockaddr_un address;
        memset(&address, 0, sizeof(address));
        address.sun_family = AF_UNIX;
        strcpy(address.sun_path, daemon_socket_name);
        int len = SUN_LEN(&address);
        address.sun_path[0] = 0;
        ret = bind(fd, (struct sockaddr *)&address, len);
        if (ret < 0) {
            // TODO remove fd leak
            api_err("Unable to bind daemon socket");
        }
        // listen to socket
        ret = listen(fd, 5);
        if (ret < 0) {
            // TODO remove fd leak
            api_err("Unable to listen to daemon socket");
        }
    }

    fds[LISTEN_FD].fd = fd;

    // detect TTY changes
    fd = open("/sys/class/tty/tty0/active", O_RDONLY|O_CLOEXEC);
    if (fd < 0) {
        api_err("Unable to open TTY change file");
    }
    fds[VT_FD].fd = fd;
    fds[VT_FD].events = POLLPRI;
}

Daemon::~Daemon()
{
    // close file descriptors
    // this is executed also on the main streaming agent
    for (unsigned n = 0; n < num_clients+FIXED_FDS; ++n) {
        if (n == WAKEUP_FD) {
            continue;
        }
        close(fds[n].fd);
    }
}

void Daemon::remove_client(unsigned n)
{
    close(client_fds[n].fd);
    client_fds[n].fd = -1;
    if (n != num_clients-1) {
        client_fds[n] = client_fds[num_clients-1];
    }
    --num_clients;
}

// check message, should contain a DISPLAY and XAUTHORITY
// callback when data are received
void Daemon::data_from_client(unsigned n)
{
    const int fd = client_fds[n].fd;

    // get message, the protocol specify a single message
    char msg_buffer[1024*4];
    iovec iov[1];
    iov[0].iov_base = msg_buffer;
    iov[0].iov_len = sizeof(msg_buffer);
    msghdr msg;
    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = iov;
    msg.msg_iovlen = 1;
    ssize_t msg_len = recvmsg(fd, &msg, MSG_CMSG_CLOEXEC|MSG_DONTWAIT);
    if (msg_len < 0 && errno == EAGAIN) {
        return;
    }
    if (msg_len < 0 && errno == EWOULDBLOCK) {
        return;
    }
    if (msg_len <= 0) {
        remove_client(n);
        return;
    }

    // get credentials (uid+pid)
    // the uid is used to be able to run the streaming agent with
    // correct user so we should get it from a secure source
    ucred cred;
    socklen_t cred_length = sizeof(cred);
    if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &cred, &cred_length) < 0) {
        remove_client(n);
        return;
    }

    // get tty terminal from process
    // Don't trust too much the helper sending information, the
    // terminal cannot be changed easily from normal accounts, get
    // this information directly from the kernel
    int num_terminal = get_terminal(cred.pid);

    // send an ack to the helper, we got all the information
    // The helper can now terminate
    remove_client(n);

    if (num_terminal < 0) {
        return;
    }

    // parse message, should contain data and credentials
    TerminalInfo info;
    try {
        info = TerminalInfo((const uint8_t *) msg_buffer, msg_len);
    }
    catch (std::runtime_error& err) {
        // avoid DoS on the logs just ignoring the error
        return;
    }

    // check Xauthority using the uid passed
    // TODO check only name?
    // TODO check just on child to avoid possible DoS
    if (!check_xauthority(info.xauthority, cred.uid)) {
        return;
    }

    // set final informations
    info.uid = cred.uid;
    terminal_info[num_terminal] = info;
}

void Daemon::handle_new_fd(int fd)
{
    // limit number of clients
    if (num_clients >= max_clients) {
        close(fd);
        return;
    }

    // append to loop handlers
    client_fds[num_clients].fd = fd;
    client_fds[num_clients].events = POLLIN;
    client_fds[num_clients].revents = 0;
    ++num_clients;

    // TODO timeout for data
}

void Daemon::handle_fd_events(unsigned n, unsigned events)
{
    if (events == POLLIN) {
        data_from_client(n);
        return;
    }
    // delete client if other events
    if (events) {
        remove_client(n);
    }
}

static int current_tty = -1;

static void handle_vt_change(int fd)
{
    char vt_name[128];
    for (;;) {
        auto len = read(fd, vt_name, sizeof(vt_name));
        if (len < 0 && errno == EINTR) {
            continue;
        }
        if (len < 0) {
            // TODO error
            return;
        }

        unsigned tty_num;
        if (sscanf(vt_name, "tty%u", &tty_num) == 1 && tty_num < 64) {
            current_tty = tty_num;
        }
        lseek(fd, 0, SEEK_SET);
        break;
    }
}

bool Daemon::check_agent(int &streamfd)
{
    syslog(LOG_DEBUG, "tty working %d current %d", working_tty, current_tty);

    pid_t pid;
    int status;
    while ((pid=waitpid(-1, &status, WNOHANG)) != -1 && pid != 0) {
        if (pid == child_pid) {
            child_pid = -1;
        }
    }

    // try to open streamfd if not already opened
    if (child_pid == -1 && streamfd < 0) {
        // open device as soon as possible to make sure is not busy
        streamfd = open(stream_port_name, O_RDWR|O_CLOEXEC);
        if (streamfd < 0) {
            api_err("Unable to open streaming device");
        }
    }

    // TODO kill the child if it doesn't exit after a given timeout?

    // TODO execute this part also on main loop
    // when should we try again ?
    if (working_tty == current_tty && child_pid != -1) {
        return false;
    }

    syslog(LOG_DEBUG, "trace %d", __LINE__);
    // can we handle a new TTY ?
    if (terminal_info.find(current_tty) == terminal_info.end()) {
        return false;
    }

    // TODO check if the terminal is still valid and a graphic one

    syslog(LOG_DEBUG, "trace %d child pid %d", __LINE__, (int) child_pid);
    // TODO who clear TTY when reset ?
    // TODO switch to text ?

    if (child_pid != -1) {
        return false;
    }

    syslog(LOG_DEBUG, "trace %d", __LINE__);
    // save pid, manage only one agent
    child_pid = fork();
    switch (child_pid) {
    case -1:
        // TODO
        return false;
    case 0:
        // child
        child_pid = -1;
        break;
    default:
        // parent
        close(streamfd);
        streamfd = -1;
        return false;
    }

    // we are the child here, we return to do the stream work

    // TODO handle cases where the child die launching it again
    uid_t uid = terminal_info[current_tty].uid;
    syslog(LOG_DEBUG, "trace %d uid %d", __LINE__, (int) uid);
    passwd *pw = getpwuid(uid);
    if (!pw) {
        api_err("Unable to retrieve user information");
    }
    if (setgid(pw->pw_gid) != 0) {
        api_err("Unable to set group");
    }
    if (initgroups(pw->pw_name, pw->pw_gid) != 0) {
        api_err("Unable to set group list");
    }
    if (setuid(uid) != 0) {
        api_err("Unable to set user");
    }

    setenv("DISPLAY", terminal_info[current_tty].display.c_str(), 1);
    setenv("XAUTHORITY", terminal_info[current_tty].xauthority.c_str(), 1);

    // TODO check if the VM is running on a text terminal we don't
    // keep spawning new childs

    // TODO why on the child? this is doing nothing!
    working_tty = current_tty;
    return true;
}

void Daemon::loop(int &streamfd)
{
    // ignore pipe, prevent signal handling data from file descriptors
    signal(SIGPIPE, SIG_IGN);
    current = this;
    signal(SIGCHLD, handle_sigchild);

    // poll for new events
    while (true) {
        // check if we need to execute the agent
        // this should be done here so if poll get a EINTR
        // for a closed child we check again
        if (check_agent(streamfd)) {
            break;
        }

        // limit clients
        if (num_clients >= max_clients) {
            fds[0].events = 0;
        } else {
            fds[0].events = POLLIN;
        }
        if (poll(fds, num_clients+FIXED_FDS, -1) < 0) {
            // TODO errors
            continue;
        }
        wakeup_loop.ack();
        if ((fds[LISTEN_FD].revents & POLLIN) != 0) {
            // accept
            int new_fd = accept(fds[LISTEN_FD].fd, NULL, NULL);
            if (new_fd < 0) {
                continue;
            }
            handle_new_fd(new_fd);
        }
        if ((fds[VT_FD].revents & POLLPRI) != 0) {
            handle_vt_change(fds[VT_FD].fd);
        }
        for (unsigned n = num_clients; n-- > 0; ) {
            if (client_fds[n].revents) {
                handle_fd_events(n, client_fds[n].revents);
            }
        }
    }

    signal(SIGCHLD, SIG_DFL);
    signal(SIGPIPE, SIG_DFL);
    current = nullptr;
}

void Daemon::handle_sigchild(int)
{
    current->wakeup_loop.signal();
}

void run_daemon(const char *stream_port_name, int &streamfd)
{
    streamfd = -1;

    Daemon daemon(stream_port_name);
    daemon.loop(streamfd);
}