tmate-ssh-server/tmate-ssh-server.c

#include <libssh/libssh.h>
#include <libssh/server.h>
#include <libssh/callbacks.h>
#include <sys/socket.h>
#include <netinet/tcp.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#include <sched.h>
#include <stdio.h>
#include <event.h>
#include <arpa/inet.h>

#include "tmate.h"

#define SSH_GRACE_PERIOD 60

static void on_keepalive_timer(evutil_socket_t fd, short what, void *arg)
{
	struct tmate_ssh_client *client = arg;
	ssh_send_keepalive(client->session);
	tmate_start_keepalive_timer(client);
}

void tmate_start_keepalive_timer(struct tmate_ssh_client *client)
{
	struct timeval tv;
	tv.tv_sec = TMATE_KEEPALIVE;
	tv.tv_usec = 0;

	evtimer_assign(&client->ev_keepalive_timer, ev_base,
		       on_keepalive_timer, client);
	evtimer_add(&client->ev_keepalive_timer, &tv);
}

static int pty_request(ssh_session session, ssh_channel channel,
		       const char *term, int width, int height,
		       int pxwidth, int pwheight, void *userdata)
{
	struct tmate_ssh_client *client = userdata;

	client->winsize_pty.ws_col = width;
	client->winsize_pty.ws_row = height;

	return 0;
}

static int shell_request(ssh_session session, ssh_channel channel,
			 void *userdata)
{
	struct tmate_ssh_client *client = userdata;

	client->role = TMATE_ROLE_CLIENT;

	return 0;
}

static int subsystem_request(ssh_session session, ssh_channel channel,
			     const char *subsystem, void *userdata)
{
	struct tmate_ssh_client *client = userdata;

	if (!strcmp(subsystem, "tmate"))
		client->role = TMATE_ROLE_SERVER;

	return 0;
}

static struct ssh_channel_callbacks_struct ssh_channel_cb = {
    .channel_pty_request_function = pty_request,
    .channel_shell_request_function = shell_request,
    .channel_subsystem_request_function = subsystem_request,
};

static ssh_channel channel_open_request_cb(ssh_session session, void *userdata)
{
	struct tmate_ssh_client *client = userdata;

	if (!client->username) {
		/* The authentication did not go through yet */
		return NULL;
	}

	if (client->channel) {
		/* We already have a channel, returning NULL means we are unhappy */
		return NULL;
	}

	client->channel = ssh_channel_new(session);
	if (!client->channel)
		tmate_fatal("Error getting channel");

	ssh_channel_cb.userdata = client;
	ssh_callbacks_init(&ssh_channel_cb);
	ssh_set_channel_callbacks(client->channel, &ssh_channel_cb);

	return client->channel;
}

static int auth_pubkey_cb(ssh_session session, const char *user,
			  struct ssh_key_struct *pubkey,
			  char signature_state, void *userdata)
{
	struct tmate_ssh_client *client = userdata;

	switch (signature_state) {
	case SSH_PUBLICKEY_STATE_VALID:
		client->username = xstrdup(user);

		if (ssh_pki_export_pubkey_base64(pubkey, &client->pubkey) != SSH_OK)
			tmate_fatal("error getting public key");
		return SSH_AUTH_SUCCESS;
	case SSH_PUBLICKEY_STATE_NONE:
		return SSH_AUTH_SUCCESS;
	default:
		return SSH_AUTH_DENIED;
	}
}

static struct ssh_server_callbacks_struct ssh_server_cb = {
	.auth_pubkey_function = auth_pubkey_cb,
	.channel_open_request_session_function = channel_open_request_cb,
};

static void client_bootstrap(struct tmate_ssh_client *client)
{
	int auth = 0;
	int grace_period = SSH_GRACE_PERIOD;
	ssh_event mainloop;
	ssh_session session = client->session;
	ssh_message msg;

	/* new process group, we don't want to die with our parent (upstart) */
	setpgid(0, 0);

	int flag = 1;
	setsockopt(ssh_get_fd(session), IPPROTO_TCP, TCP_NODELAY,
		   &flag, sizeof(flag));

	alarm(grace_period);

	ssh_server_cb.userdata = client;
	ssh_callbacks_init(&ssh_server_cb);
	ssh_set_server_callbacks(client->session, &ssh_server_cb);

	ssh_options_set(session, SSH_OPTIONS_TIMEOUT, &grace_period);
	ssh_options_set(session, SSH_OPTIONS_COMPRESSION, "yes");

	ssh_set_auth_methods(client->session, SSH_AUTH_METHOD_PUBLICKEY);

	tmate_debug("Exchanging DH keys");
	if (ssh_handle_key_exchange(session) < 0)
		tmate_fatal("Error doing the key exchange");

	mainloop = ssh_event_new();
	ssh_event_add_session(mainloop, session);

	while (!client->role) {
		if (ssh_event_dopoll(mainloop, -1) == SSH_ERROR)
			tmate_fatal("Error polling ssh socket: %s", ssh_get_error(session));
	}

	alarm(0);
	tmate_spawn_slave(client);
	/* never reached */
}

static void handle_sigchld(void)
{
	siginfo_t si;

	/* TODO cleanup the socket when the client dies */
	while (waitid(P_ALL, 0, &si, WEXITED | WNOHANG) >= 0 && si.si_pid) {
		tmate_info("Child %d %s (%d)",
			   si.si_pid,
			   si.si_code == CLD_EXITED ? "exited" : "killed",
			   si.si_status);
	}
}

static void handle_sigalrm(void)
{
	tmate_fatal("Connection grace period (%d) passed", SSH_GRACE_PERIOD);
}

static void handle_sigsegv(void)
{
	tmate_info("CRASH, printing stack trace");
	tmate_print_trace();
	tmate_fatal("CRASHED");
}

static void handle_sigusr1(void)
{
	tmate_reopen_logfile();
}

static void signal_handler(int sig)
{
	switch (sig) {
	case SIGCHLD: handle_sigchld(); break;
	case SIGALRM: handle_sigalrm(); break;
	case SIGSEGV: handle_sigsegv(); break;
	case SIGUSR1: handle_sigusr1(); break;
	}
}

static void setup_signals(void)
{
	signal(SIGCHLD, signal_handler);
	signal(SIGALRM, signal_handler);
	signal(SIGSEGV, signal_handler);
	signal(SIGUSR1, signal_handler);
}

static pid_t namespace_fork(void)
{
	/* XXX we are breaking getpid() libc cache. Bad libc. */
	unsigned long flags;
	flags = CLONE_NEWPID | CLONE_NEWIPC | CLONE_NEWNS | CLONE_NEWNET;
	return syscall(SYS_clone, flags | SIGCHLD, NULL, NULL, NULL);
}

static int get_ip(int fd, char *dst, size_t len)
{
	struct sockaddr sa;
	socklen_t sa_len = sizeof(sa);

	if (getpeername(fd, &sa, &sa_len) < 0)
		return -1;


	switch (sa.sa_family) {
	case AF_INET:
		if (!inet_ntop(AF_INET, &((struct sockaddr_in *)&sa)->sin_addr,
			       dst, len))
			return -1;
		break;
	case AF_INET6:
		if (!inet_ntop(AF_INET6, &((struct sockaddr_in6 *)&sa)->sin6_addr,
			       dst, len))
			return -1;
		break;
	default:
		return -1;
	}

	return 0;
}

struct tmate_ssh_client tmate_client;

static void ssh_log_function(int priority, const char *function,
			     const char *buffer, void *userdata)
{
	tmate_debug("[%d] [%s] %s", priority, function, buffer);
}

static ssh_bind prepare_ssh(const char *keys_dir, int port)
{
	ssh_bind bind;
	char buffer[PATH_MAX];
	int verbosity = SSH_LOG_NOLOG;
	//int verbosity = SSH_LOG_PACKET;

	ssh_set_log_callback(ssh_log_function);

	bind = ssh_bind_new();
	if (!bind)
		tmate_fatal("Cannot initialize ssh");

	ssh_bind_options_set(bind, SSH_BIND_OPTIONS_BINDPORT, &port);
	ssh_bind_options_set(bind, SSH_BIND_OPTIONS_BANNER, TMATE_SSH_BANNER);
	ssh_bind_options_set(bind, SSH_BIND_OPTIONS_LOG_VERBOSITY, &verbosity);

	sprintf(buffer, "%s/ssh_host_dsa_key", keys_dir);
	ssh_bind_options_set(bind, SSH_BIND_OPTIONS_DSAKEY, buffer);

	sprintf(buffer, "%s/ssh_host_rsa_key", keys_dir);
	ssh_bind_options_set(bind, SSH_BIND_OPTIONS_RSAKEY, buffer);

	if (ssh_bind_listen(bind) < 0)
		tmate_fatal("Error listening to socket: %s\n", ssh_get_error(bind));

	tmate_info("Accepting connections on %d", port);

	return bind;
}

void tmate_ssh_server_main(const char *keys_dir, int port)
{
	struct tmate_ssh_client *client = &tmate_client;
	ssh_bind bind;
	pid_t pid;

	setup_signals();
	bind = prepare_ssh(keys_dir, port);

	for (;;) {
		client->session = ssh_new();
		client->channel = NULL;
		if (!client->session)
			tmate_fatal("Cannot initialize session");

		if (ssh_bind_accept(bind, client->session) < 0)
			tmate_fatal("Error accepting connection: %s", ssh_get_error(bind));

		if (get_ip(ssh_get_fd(client->session),
			   client->ip_address, sizeof(client->ip_address)) < 0)
			tmate_fatal("Error getting IP address from connection");

		if ((pid = namespace_fork()) < 0) {
			if (getuid() == 0)
				tmate_fatal("Can't fork in new namespace, are you running a recent kernel?");
			else
				tmate_fatal("Can't fork in new namespace, run me with root priviledges");
		}

		if (pid) {
			tmate_info("Child spawned pid=%d, ip=%s",
				    pid, client->ip_address);
			ssh_free(client->session);
		} else {
			ssh_bind_free(bind);
			tmate_session_token = ".........................";
			client_bootstrap(client);
		}
	}
}