2 * Run the login shell or command as the given user in a new pty to prevent
3 * terminal injection attacks.
5 * Copyright (C) 2016-2018 Simon Ruderich
7 * This program is free software: you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License as published by
9 * the Free Software Foundation, either version 3 of the License, or
10 * (at your option) any later version.
12 * This program is distributed in the hope that it will be useful,
13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 * GNU General Public License for more details.
17 * You should have received a copy of the GNU General Public License
18 * along with this program. If not, see <http://www.gnu.org/licenses/>.
36 #include <sys/ioctl.h>
37 #include <sys/types.h>
42 /* Default PATH for new process.*/
43 #ifndef PTYAS_DEFAULT_PATH
44 /* Default user PATH from Debian's /etc/profile, change as needed. */
45 # define PTYAS_DEFAULT_PATH "/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games"
49 static void die(const char *s) {
53 static void die_fmt(const char *fmt, ...) {
57 vfprintf(stderr, fmt, ap);
63 static void open_pty_or_die(int *pty_master, int *pty_slave, uid_t uid) {
66 *pty_master = posix_openpt(O_RDWR | O_NOCTTY);
67 if (*pty_master == -1) {
70 slave_path = ptsname(*pty_master);
74 if (grantpt(*pty_master) != 0) {
77 if (unlockpt(*pty_master) != 0) {
81 *pty_slave = open(slave_path, O_RDWR | O_NOCTTY);
82 if (*pty_slave == -1) {
83 die("open slave tty");
86 * The user must be able to write to the new TTY. Normally grantpt() would
87 * do this for us, but we don't trust the user and thus don't want to pass
88 * the pty_master to a process running under that uid.
90 if (chown(slave_path, uid, (gid_t)-1) != 0) {
91 die("chown slave tty");
95 static void close_or_die(int fd) {
100 static void dup2_or_die(int oldfd, int newfd) {
101 if (dup2(oldfd, newfd) != newfd) {
105 static int snprintf_or_assert(char *str, size_t size, const char *format, ...) {
109 va_start(ap, format);
110 ret = vsnprintf(str, size, format, ap);
111 assert(size <= (size_t)INT_MAX);
112 assert(ret < (int)size); /* assert output fit into buffer */
118 static void drop_privileges_or_die(uid_t uid, gid_t gid) {
119 /* Drop all supplementary group IDs. */
120 if (setgroups(0, NULL) != 0) {
123 if (getgroups(0, NULL) != 0) {
124 die_fmt("failed to drop all supplementary groups");
127 /* Dropping groups may require privileges, do that first. */
128 if (setresgid(gid, gid, gid) != 0) {
131 if (setresuid(uid, uid, uid) != 0) {
135 /* Ensure we dropped all privileges. */
137 uid_t ruid, euid, suid;
138 gid_t rgid, egid, sgid;
140 if (getresuid(&ruid, &euid, &suid) != 0) {
143 if (getresgid(&rgid, &egid, &sgid) != 0) {
146 if ( uid != ruid || uid != euid || uid != suid
147 || gid != rgid || gid != egid || gid != sgid) {
148 die_fmt("failed to drop privileges");
151 /* Just to be safe. */
152 if (setuid(0) != -1) {
153 die_fmt("failed to drop privileges (setuid)");
157 static void quit_with_matching_code(int status) {
158 if (WIFEXITED(status)) {
159 exit(WEXITSTATUS(status));
160 } else if (WIFSIGNALED(status)) {
161 kill(getpid(), WTERMSIG(status));
162 /* Fall-through, should not happen. */
164 abort(); /* Should never happen, die painfully. */
167 static bool read_from_write_to(int from, int to) {
170 ssize_t r = read(from, buf, sizeof(buf));
175 size_t left = (size_t)r;
179 ssize_t w = write(to, data, left);
181 if (errno == EINTR) {
193 static void proxy_input_between_ttys(int pty_master, int ctty, volatile pid_t *pid_to_wait_for) {
194 struct pollfd fds[] = {
205 sigset_t sigset, sigset_old;
206 sigemptyset(&sigset);
207 sigaddset(&sigset, SIGCHLD);
208 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
209 die("sigprocmask block sigchld proxy");
212 /* Proxy data until our child has terminated. */
213 while (*pid_to_wait_for != 0) {
215 * If a signal happens here _and_ the child hasn't closed pty_slave,
216 * we would hang in poll(); therefore ppoll() is necessary.
218 nfds_t nfds = sizeof(fds)/sizeof(*fds);
219 if (ppoll(fds, nfds, NULL /* no timeout */, &sigset_old) == -1) {
220 if (errno == EAGAIN || errno == EINTR) {
228 * Handle errors first. (Data available before the error occurred
229 * might be dropped, but shouldn't matter here.)
231 if (fds[0].revents & (POLLERR | POLLNVAL)) {
232 fprintf(stderr, "poll: error on master: %d\n", fds[0].revents);
235 if (fds[1].revents & (POLLERR | POLLNVAL)) {
236 fprintf(stderr, "poll: error on ctty: %d\n", fds[1].revents);
240 /* Read data if available. */
241 if (fds[0].revents & POLLIN) {
242 if (!read_from_write_to(pty_master, ctty)) {
243 perror("read from master write to ctty");
247 if (fds[1].revents & POLLIN) {
248 if (!read_from_write_to(ctty, pty_master)) {
249 perror("read from ctty write to master");
254 /* Finally we are done if either side of the pty has disconnected. */
255 if ((fds[0].revents & POLLHUP) || (fds[1].revents & POLLHUP)) {
260 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
261 die("sigprocmask setmask proxy");
267 * Not sig_atomic_t (as required by POSIX) but I don't know how to do that any
270 static volatile pid_t pid_to_wait_for;
271 static int pid_to_wait_for_status;
273 static void sigchld_handler(int signal) {
279 while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
280 if (pid == pid_to_wait_for) {
281 /* Mark that our child has died and we should exit as well. */
283 /* We must exit like our child, save status. */
284 pid_to_wait_for_status = status;
290 * SIGWINCH handler to handle resizes of the outer terminal.
292 * Errors are ignored without message because printing in signal handlers is
293 * problematic (no FILE * usable due to locks) and there's not much we can do
296 static int sigwinch_ctty = -1;
297 static int sigwinch_slave = -1;
299 static void sigwinch_handler(int signal) {
303 if (ioctl(sigwinch_ctty, TIOCGWINSZ, &size) == -1) {
306 if (ioctl(sigwinch_slave, TIOCSWINSZ, &size) == -1) {
312 int main(int argc, char **argv) {
313 char *exec_argv_shell[] = { NULL, NULL }; /* filled below */
314 char **exec_argv = NULL;
317 /* exec_argv set below */
318 } else if (argc > 2) {
319 exec_argv = argv + 2;
321 die_fmt("%s <user> [<cmd>...]\n", argv[0]);
324 const char *user = argv[1];
326 struct passwd *passwd = getpwnam(user);
328 die_fmt("unknown user name '%s'\n", user);
331 uid_t uid = passwd->pw_uid;
332 gid_t gid = passwd->pw_gid;
336 exec_argv_shell[0] = passwd->pw_shell;
337 exec_argv = exec_argv_shell;
340 int pty_master, pty_slave;
342 open_pty_or_die(&pty_master, &pty_slave, uid);
344 int ctty = open("/dev/tty", O_RDWR | O_NOCTTY); /* controlling TTY */
346 die("open /dev/tty");
349 sigset_t sigset, sigset_old;
350 sigemptyset(&sigset);
351 sigaddset(&sigset, SIGCHLD);
352 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
353 die("sigprocmask block sigchld");
359 } else if (pid == 0) {
360 /* child, will become a session leader */
362 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
363 die("sigprocmask setmask child");
367 if (ioctl(ctty, TIOCGWINSZ, &size) == -1) {
368 die("ioctl TIOCGWINSZ");
371 close_or_die(pty_master);
374 /* Start a new session and attach controlling TTY. */
375 if (setsid() == -1) {
378 if (ioctl(pty_slave, TIOCSCTTY, 0) == -1) {
379 die("ioctl TIOCSCTTY");
382 if (ioctl(pty_slave, TIOCSWINSZ, &size) == -1) {
383 die("ioctl TIOCSWINSZ");
389 } else if (pid == 0) {
391 * Drop the privileges just now so that the other user doesn't get
392 * access to the master TTY or the session leader (which might
393 * have additional privileges).
395 drop_privileges_or_die(uid, gid);
397 dup2_or_die(pty_slave, STDIN_FILENO);
398 dup2_or_die(pty_slave, STDOUT_FILENO);
399 dup2_or_die(pty_slave, STDERR_FILENO);
400 close_or_die(pty_slave);
402 const char *term_orig = getenv("TERM");
403 const char *term = term_orig;
405 term = ""; /* for strlen() below */
407 const char *home = passwd->pw_dir;
410 * Ignore errors here as we don't want to die on non-existent home
411 * directories to allow running as any user (think "/nonexistent"
412 * as home) and an error message will be annoying to ignore when
413 * running this command in scripts.
417 char envp_user[strlen("USER=") + strlen(user) + 1];
418 char envp_home[strlen("HOME=") + strlen(home) + 1];
419 char envp_term[strlen("TERM=") + strlen(term) + 1];
420 snprintf_or_assert(envp_user, sizeof(envp_user), "USER=%s", user);
421 snprintf_or_assert(envp_home, sizeof(envp_home), "HOME=%s", home);
422 snprintf_or_assert(envp_term, sizeof(envp_term), "TERM=%s", term);
424 char *exec_envp[] = {
425 "PATH=" PTYAS_DEFAULT_PATH,
428 term_orig ? envp_term : NULL,
432 execve(exec_argv[0], exec_argv, exec_envp);
435 close_or_die(pty_slave);
436 close_or_die(STDIN_FILENO);
437 close_or_die(STDOUT_FILENO);
438 close_or_die(STDERR_FILENO);
442 if (waitpid(pid, &status, 0) <= 0) {
443 die("waitpid child");
445 quit_with_matching_code(status);
447 /* Don't close pty_slave here as it's used in sigwinch_handler(). */
449 sigwinch_ctty = ctty;
450 sigwinch_slave = pty_slave;
452 struct sigaction action_sigwinch = {
453 .sa_handler = sigwinch_handler,
455 sigemptyset(&action_sigwinch.sa_mask);
456 if (sigaction(SIGWINCH, &action_sigwinch, NULL) != 0) {
457 die("sigaction SIGWINCH");
460 pid_to_wait_for = pid;
461 struct sigaction action_sigchld = {
462 .sa_handler = sigchld_handler,
464 sigemptyset(&action_sigchld.sa_mask);
465 if (sigaction(SIGCHLD, &action_sigchld, NULL) != 0) {
466 die("sigaction SIGCHLD");
469 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
470 die("sigprocmask setmask parent");
473 struct termios old_term, term;
475 /* Change terminal to raw mode. */
476 if (tcgetattr(ctty, &old_term) != 0) {
480 /* From man 3 cfmakeraw; cfmakeraw is non-standard so set it manually. */
481 term.c_iflag &= ~(tcflag_t)(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
482 term.c_oflag &= ~(tcflag_t)(OPOST);
483 term.c_lflag &= ~(tcflag_t)(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
484 term.c_cflag &= ~(tcflag_t)(CSIZE | PARENB);
486 if (tcsetattr(ctty, TCSADRAIN, &term) != 0) {
490 proxy_input_between_ttys(pty_master, ctty, &pid_to_wait_for);
492 /* Restore terminal mode. */
493 if (tcsetattr(ctty, TCSADRAIN, &old_term) != 0) {
494 die("tcsetattr restore");
498 * Wait until we got the status code from our child. poll() might already
499 * exit after POLLHUP while we haven't collected the child yet.
501 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
502 die("sigprocmask block sigchld loop");
504 while (pid_to_wait_for != 0) {
505 sigsuspend(&sigset_old);
506 if (errno != EINTR) {
510 if (sigprocmask(SIG_SETMASK, &sigset, &sigset_old) != 0) {
511 die("sigprocmask setmask sigchld loop");
514 /* Try to exit the same way as the spawned process. */
515 if (pid_to_wait_for == 0) {
516 quit_with_matching_code(pid_to_wait_for_status);