1 /*
2  * Copyright (c) 2021, 2022 Omar Polo <op@omarpolo.com>
3  *
4  * Permission to use, copy, modify, and distribute this software for any
5  * purpose with or without fee is hereby granted, provided that the above
6  * copyright notice and this permission notice appear in all copies.
7  *
8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15  */
16 
17 #include <sys/types.h>
18 #include <sys/stat.h>
19 #include <sys/socket.h>
20 #include <sys/wait.h>
21 
22 #include <ctype.h>
23 #include <errno.h>
24 #include <event.h>
25 #include <fcntl.h>
26 #include <limits.h>
27 #include <netdb.h>
28 #include <signal.h>
29 #include <stdio.h>
30 #include <stdlib.h>
31 #include <string.h>
32 #include <syslog.h>
33 #include <unistd.h>
34 
35 #include "log.h"
36 
37 #ifndef SSH_PATH
38 #define SSH_PATH "/usr/bin/ssh"
39 #endif
40 
41 #define MAXSOCK 32
42 #define BACKOFF 1
43 #define RETRIES 16
44 
45 #ifndef __OpenBSD__
46 #define pledge(p, e) 0
47 #endif
48 
49 const char	*addr;		/* our addr */
50 const char	*ssh_tflag;
51 const char	*ssh_dest;
52 
53 char		 ssh_host[256];
54 char		 ssh_port[16];
55 
56 struct event	 sockev[MAXSOCK];
57 int		 socks[MAXSOCK];
58 int		 nsock;
59 
60 int		 debug;
61 int		 verbose;
62 
63 struct event	 sighupev;
64 struct event	 sigintev;
65 struct event	 sigtermev;
66 struct event	 sigchldev;
67 struct event	 siginfoev;
68 
69 struct timeval	 timeout = {600, 0}; /* 10 minutes */
70 struct event	 timeoutev;
71 
72 pid_t		 ssh_pid = -1;
73 
74 int		 conn;
75 
76 struct conn {
77 	int			 ntentative;
78 	struct timeval		 retry;
79 	struct event		 waitev;
80 	int			 source;
81 	struct bufferevent	*sourcebev;
82 	int			 to;
83 	struct bufferevent	*tobev;
84 };
85 
86 static void
87 sig_handler(int sig, short event, void *data)
88 {
89 	int status;
90 
91 	switch (sig) {
92 	case SIGHUP:
93 	case SIGINT:
94 	case SIGTERM:
95 		event_loopbreak();
96 		break;
97 	case SIGCHLD:
98 		if (waitpid(ssh_pid, &status, WNOHANG) == -1)
99 			fatal("waitpid");
100 		ssh_pid = -1;
101 		break;
102 #ifdef SIGINFO
103 	case SIGINFO:
104 #else
105 	case SIGUSR1:
106 #endif
107 		log_info("connections: %d", conn);
108 	}
109 }
110 
111 static void
112 spawn_ssh(void)
113 {
114 	log_debug("spawning ssh");
115 
116 	switch (ssh_pid = fork()) {
117 	case -1:
118 		fatal("fork");
119 	case 0:
120 		execl(SSH_PATH, "ssh", "-L", ssh_tflag, "-NTq", ssh_dest,
121 		    NULL);
122 		fatal("exec");
123 	default:
124 		return;
125 	}
126 }
127 
128 static void
129 conn_free(struct conn *c)
130 {
131 	if (c->sourcebev != NULL)
132 		bufferevent_free(c->sourcebev);
133 	if (c->tobev != NULL)
134 		bufferevent_free(c->tobev);
135 
136 	if (evtimer_pending(&c->waitev, NULL))
137 		evtimer_del(&c->waitev);
138 
139 	close(c->source);
140 	close(c->to);
141 
142 	free(c);
143 }
144 
145 static void
146 killing_time(int fd, short event, void *data)
147 {
148 	if (ssh_pid == -1)
149 		return;
150 
151 	log_debug("timeout expired, killing ssh (%d)", ssh_pid);
152 	kill(ssh_pid, SIGTERM);
153 	ssh_pid = -1;
154 }
155 
156 static void
157 nopcb(struct bufferevent *bev, void *d)
158 {
159 	return;
160 }
161 
162 static void
163 sreadcb(struct bufferevent *bev, void *d)
164 {
165 	struct conn *c = d;
166 
167 	bufferevent_write_buffer(c->tobev, EVBUFFER_INPUT(bev));
168 }
169 
170 static void
171 treadcb(struct bufferevent *bev, void *d)
172 {
173 	struct conn *c = d;
174 
175 	bufferevent_write_buffer(c->sourcebev, EVBUFFER_INPUT(bev));
176 }
177 
178 static void
179 errcb(struct bufferevent *bev, short event, void *d)
180 {
181 	struct conn *c = d;
182 
183 	log_info("closing connection (event=%x)", event);
184 
185 	conn_free(c);
186 
187 	if (--conn == 0) {
188 		log_debug("scheduling ssh termination (%llds)",
189 		    (long long)timeout.tv_sec);
190 		if (timeout.tv_sec != 0) {
191 			evtimer_set(&timeoutev, killing_time, NULL);
192 			evtimer_add(&timeoutev, &timeout);
193 		}
194 	}
195 }
196 
197 static int
198 connect_to_ssh(void)
199 {
200 	struct addrinfo hints, *res, *res0;
201 	int r, saved_errno, sock;
202 	const char *cause;
203 
204 	memset(&hints, 0, sizeof(hints));
205 	hints.ai_family = AF_UNSPEC;
206 	hints.ai_socktype = SOCK_STREAM;
207 
208 	r = getaddrinfo(ssh_host, ssh_port, &hints, &res0);
209 	if (r != 0) {
210 		log_warnx("getaddrinfo(\"%s\", \"%s\"): %s",
211 		    ssh_host, ssh_port, gai_strerror(r));
212 		return -1;
213 	}
214 
215 	for (res = res0; res; res = res->ai_next) {
216 		sock = socket(res->ai_family, res->ai_socktype,
217 		    res->ai_protocol);
218 		if (sock == -1) {
219 			cause = "socket";
220 			continue;
221 		}
222 
223 		if (connect(sock, res->ai_addr, res->ai_addrlen) == -1) {
224 			cause = "connect";
225 			saved_errno = errno;
226 			close(sock);
227 			errno = saved_errno;
228 			sock = -1;
229 			continue;
230 		}
231 
232 		break;
233 	}
234 
235 	if (sock == -1)
236 		log_warn("%s", cause);
237 
238 	freeaddrinfo(res0);
239 	return sock;
240 }
241 
242 static void
243 try_to_connect(int fd, short event, void *d)
244 {
245 	struct conn *c = d;
246 
247 	/* ssh may die in the meantime */
248 	if (ssh_pid == -1) {
249 		close(c->source);
250 		c->source = -1;
251 		return;
252 	}
253 
254 	c->ntentative++;
255 	log_info("trying to connect to %s:%s (%d/%d)", ssh_host, ssh_port,
256 	    c->ntentative, RETRIES);
257 
258 	if ((c->to = connect_to_ssh()) == -1) {
259 		if (c->ntentative == RETRIES) {
260 			log_warnx("giving up connecting");
261 			close(c->source);
262 			c->source = -1;
263 			return;
264 		}
265 
266 		evtimer_set(&c->waitev, try_to_connect, c);
267 		evtimer_add(&c->waitev, &c->retry);
268 		return;
269 	}
270 
271 	log_info("connected!");
272 
273 	c->sourcebev = bufferevent_new(c->source, sreadcb, nopcb, errcb, c);
274 	c->tobev = bufferevent_new(c->to, treadcb, nopcb, errcb, c);
275 	if (c->sourcebev == NULL || c->tobev == NULL) {
276 		log_warn("bufferevent_new");
277 		conn_free(c);
278 		return;
279 	}
280 
281 	bufferevent_enable(c->sourcebev, EV_READ|EV_WRITE);
282 	bufferevent_enable(c->tobev, EV_READ|EV_WRITE);
283 }
284 
285 static void
286 do_accept(int fd, short event, void *data)
287 {
288 	struct conn *c;
289 	int s;
290 
291 	log_debug("incoming connection");
292 
293 	if (evtimer_pending(&timeoutev, NULL))
294 		evtimer_del(&timeoutev);
295 
296 	if ((s = accept(fd, NULL, 0)) == -1)
297 		fatal("accept");
298 
299 	conn++;
300 
301 	if (ssh_pid == -1)
302 		spawn_ssh();
303 
304 	if ((c = calloc(1, sizeof(*c))) == NULL) {
305 		log_warn("calloc");
306 		close(s);
307 		return;
308 	}
309 
310 	c->source = s;
311 	c->retry.tv_sec = BACKOFF;
312 	evtimer_set(&c->waitev, try_to_connect, c);
313 	evtimer_add(&c->waitev, &c->retry);
314 }
315 
316 static const char *
317 copysec(const char *s, char *d, size_t len)
318 {
319 	const char *c;
320 
321 	if ((c = strchr(s, ':')) == NULL)
322 		return NULL;
323 	if ((size_t)(c - s) >= len-1)
324 		return NULL;
325 	memset(d, 0, len);
326 	memcpy(d, s, c - s);
327 	return c;
328 }
329 
330 static void
331 bind_socket(void)
332 {
333 	struct addrinfo hints, *res, *res0;
334 	int v, r, saved_errno;
335 	char host[64];
336 	const char *c, *h, *port, *cause;
337 
338 	if ((c = strchr(addr, ':')) == NULL) {
339 		h = NULL;
340 		port = addr;
341 	} else {
342 		if ((c = copysec(addr, host, sizeof(host))) == NULL)
343 			fatalx("name too long: %s", addr);
344 
345 		h = host;
346 		port = c+1;
347 	}
348 
349 	memset(&hints, 0, sizeof(hints));
350 	hints.ai_family = AF_UNSPEC;
351 	hints.ai_socktype = SOCK_STREAM;
352 	hints.ai_flags = AI_PASSIVE;
353 
354 	r = getaddrinfo(h, port, &hints, &res0);
355 	if (r != 0)
356 		fatalx("getaddrinfo(%s): %s", addr, gai_strerror(r));
357 
358 	for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
359 		socks[nsock] = socket(res->ai_family, res->ai_socktype,
360 		    res->ai_protocol);
361 		if (socks[nsock] == -1) {
362 			cause = "socket";
363 			continue;
364 		}
365 
366 		if (bind(socks[nsock], res->ai_addr, res->ai_addrlen) == -1) {
367 			cause = "bind";
368 			saved_errno = errno;
369 			close(socks[nsock]);
370 			errno = saved_errno;
371 			continue;
372 		}
373 
374 		v = 1;
375 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEADDR, &v,
376 		    sizeof(v)) == -1)
377 			fatal("setsockopt(SO_REUSEADDR)");
378 
379 		v = 1;
380 		if (setsockopt(socks[nsock], SOL_SOCKET, SO_REUSEPORT, &v,
381 		    sizeof(v)) == -1)
382 			fatal("setsockopt(SO_REUSEPORT)");
383 
384 		listen(socks[nsock], 5);
385 
386 		nsock++;
387 	}
388 	if (nsock == 0)
389 		fatal("%s", cause);
390 
391 	freeaddrinfo(res0);
392 }
393 
394 static void
395 parse_tflag(void)
396 {
397 	const char *c;
398 
399 	if (isdigit((unsigned char)*ssh_tflag)) {
400 		strlcpy(ssh_host, "localhost", sizeof(ssh_host));
401 		if (copysec(ssh_tflag, ssh_port, sizeof(ssh_port)) == NULL)
402 			goto err;
403 		return;
404 	}
405 
406 	if ((c = copysec(ssh_tflag, ssh_host, sizeof(ssh_host))) == NULL)
407 		goto err;
408 	if (copysec(c+1, ssh_port, sizeof(ssh_port)) == NULL)
409 		goto err;
410 	return;
411 
412 err:
413 	fatal("wrong value for -B");
414 }
415 
416 static void __dead
417 usage(void)
418 {
419 	fprintf(stderr, "usage: %s [-dv] -B sshaddr -b addr [-t timeout]"
420 	    " destination\n", getprogname());
421 	exit(1);
422 }
423 
424 int
425 main(int argc, char **argv)
426 {
427 	int ch, i, fd;
428 	const char *errstr;
429 	struct stat sb;
430 
431 	/*
432 	 * Ensure we have fds 0-2 open so that we have no issue with
433 	 * calling bind_socket before daemon(3).
434 	 */
435 	for (i = 0; i < 3; ++i) {
436 		if (fstat(i, &sb) == -1) {
437 			if ((fd = open("/dev/null", O_RDWR)) != -1) {
438 				if (dup2(fd, i) == -1)
439 					exit(1);
440 				if (fd > i)
441 					close(fd);
442 			} else
443 				exit(1);
444 		}
445 	}
446 
447 	log_init(1, LOG_DAEMON);
448 	log_setverbose(1);
449 
450 	while ((ch = getopt(argc, argv, "B:b:dt:v")) != -1) {
451 		switch (ch) {
452 		case 'B':
453 			ssh_tflag = optarg;
454 			parse_tflag();
455 			break;
456 		case 'b':
457 			addr = optarg;
458 			break;
459 		case 'd':
460 			debug = 1;
461 			break;
462 		case 't':
463 			timeout.tv_sec = strtonum(optarg, 0, INT_MAX, &errstr);
464 			if (errstr != NULL)
465 				fatalx("timeout is %s: %s", errstr, optarg);
466 			break;
467 		case 'v':
468 			verbose = 1;
469 			break;
470 		default:
471 			usage();
472 		}
473 	}
474 	argc -= optind;
475 	argv += optind;
476 
477 	if (argc != 1 || addr == NULL || ssh_tflag == NULL)
478 		usage();
479 
480 	ssh_dest = argv[0];
481 
482 	bind_socket();
483 
484 	log_init(debug, LOG_DAEMON);
485 	log_setverbose(verbose);
486 
487 	if (!debug)
488 		daemon(1, 0);
489 
490 	signal(SIGPIPE, SIG_IGN);
491 
492 	event_init();
493 
494 	/* initialize the timer */
495 	evtimer_set(&timeoutev, killing_time, NULL);
496 
497 	signal_set(&sighupev, SIGHUP, sig_handler, NULL);
498 	signal_set(&sigintev, SIGINT, sig_handler, NULL);
499 	signal_set(&sigtermev, SIGTERM, sig_handler, NULL);
500 	signal_set(&sigchldev, SIGCHLD, sig_handler, NULL);
501 #ifdef SIGINFO
502 	signal_set(&siginfoev, SIGINFO, sig_handler, NULL);
503 #else
504 	signal_set(&siginfoev, SIGUSR1, sig_handler, NULL);
505 #endif
506 
507 	signal_add(&sighupev, NULL);
508 	signal_add(&sigintev, NULL);
509 	signal_add(&sigtermev, NULL);
510 	signal_add(&sigchldev, NULL);
511 	signal_add(&siginfoev, NULL);
512 
513 	for (i = 0; i < nsock; ++i) {
514 		event_set(&sockev[i], socks[i], EV_READ|EV_PERSIST,
515 		    do_accept, NULL);
516 		event_add(&sockev[i], NULL);
517 	}
518 
519 	/*
520 	 * dns, inet: bind the socket and connect to the childs.
521 	 * proc, exec: execute ssh on demand.
522 	 */
523 	if (pledge("stdio dns inet proc exec", NULL) == -1)
524 		fatal("pledge");
525 
526 	log_info("starting");
527 	event_dispatch();
528 
529 	if (ssh_pid != -1)
530 		kill(ssh_pid, SIGINT);
531 
532 	return 0;
533 }