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 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>
43 static void die(const char *s) {
47 static void die_fmt(const char *fmt, ...) {
51 vfprintf(stderr, fmt, ap);
57 static void open_pty_or_die(int *pty_master, int *pty_slave, uid_t uid) {
60 *pty_master = posix_openpt(O_RDWR | O_NOCTTY);
61 if (*pty_master == -1) {
64 slave_path = ptsname(*pty_master);
68 if (grantpt(*pty_master) != 0) {
71 if (unlockpt(*pty_master) != 0) {
75 *pty_slave = open(slave_path, O_RDWR | O_NOCTTY);
76 if (*pty_slave == -1) {
77 die("open slave tty");
79 /* The user must be able to write to the new TTY. Normally grantpt() would
80 * do this for us, but we don't trust the user and thus don't want to pass
81 * the pty_master to a process running under that uid. */
82 if (chown(slave_path, uid, (gid_t)-1) != 0) {
83 die("chown slave tty");
87 static void close_or_die(int fd) {
92 static void dup2_or_die(int oldfd, int newfd) {
93 if (dup2(oldfd, newfd) != newfd) {
97 static int snprintf_or_assert(char *str, size_t size, const char *format, ...) {
101 va_start(ap, format);
102 ret = vsnprintf(str, size, format, ap);
103 assert(size <= (size_t)INT_MAX);
104 assert(ret < (int)size); /* assert output fit into buffer */
110 static void drop_privileges_or_die(uid_t uid, gid_t gid) {
111 /* Drop all supplementary group IDs. */
112 if (setgroups(0, NULL) != 0) {
115 if (getgroups(0, NULL) != 0) {
116 die_fmt("failed to drop all groups");
119 /* Dropping groups may require privileges, do that first. */
120 if (setresgid(gid, gid, gid) != 0) {
123 if (setresuid(uid, uid, uid) != 0) {
127 /* Ensure we dropped all privileges. */
129 uid_t ruid, euid, suid;
130 gid_t rgid, egid, sgid;
132 if (getresuid(&ruid, &euid, &suid) != 0) {
135 if (getresgid(&rgid, &egid, &sgid) != 0) {
138 if ( uid != ruid || uid != euid || uid != suid
139 || gid != rgid || gid != egid || gid != sgid) {
140 die_fmt("failed to drop privileges");
143 /* Just to be safe. */
144 if (setuid(0) != -1) {
145 die_fmt("failed to drop privileges (setuid)");
149 static void quit_with_matching_code(int status) {
150 if (WIFEXITED(status)) {
151 exit(WEXITSTATUS(status));
152 } else if (WIFSIGNALED(status)) {
153 kill(getpid(), WTERMSIG(status));
154 /* Fall-through, should not happen. */
156 abort(); /* Should never happen, die painfully. */
159 static bool read_from_write_to(int from, int to) {
162 ssize_t r = read(from, buf, sizeof(buf));
167 size_t left = (size_t)r;
171 ssize_t w = write(to, data, left);
173 if (errno == EINTR) {
185 static void proxy_input_between_ttys(int pty_master, int ctty, volatile pid_t *pid_to_wait_for) {
186 struct pollfd fds[] = {
197 sigset_t sigset, sigset_old;
198 sigemptyset(&sigset);
199 sigaddset(&sigset, SIGCHLD);
200 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
201 die("sigprocmask block sigchld proxy");
204 /* Proxy data until our child has terminated. */
205 while (*pid_to_wait_for != 0) {
207 * If a signal happens here _and_ the child hasn't closed pty_slave,
208 * we would hang in poll(); therefore ppoll() is necessary.
210 nfds_t nfds = sizeof(fds)/sizeof(*fds);
211 if (ppoll(fds, nfds, NULL /* no timeout */, &sigset_old) == -1) {
212 if (errno == EAGAIN || errno == EINTR) {
220 /* Handle errors first. (Data available before the error occurred
221 * might be skipped, but shouldn't matter here.) */
222 if (fds[0].revents & (POLLERR | POLLNVAL)) {
223 fprintf(stderr, "poll: error on master: %d\n", fds[0].revents);
226 if (fds[1].revents & (POLLERR | POLLNVAL)) {
227 fprintf(stderr, "poll: error on ctty: %d\n", fds[1].revents);
231 /* Read data if available. */
232 if (fds[0].revents & POLLIN) {
233 if (!read_from_write_to(pty_master, ctty)) {
234 perror("read from master write to ctty");
238 if (fds[1].revents & POLLIN) {
239 if (!read_from_write_to(ctty, pty_master)) {
240 perror("read from ctty write to master");
245 /* Finally we are done if either side of the pty has disconnected. */
246 if ((fds[0].revents & POLLHUP) || (fds[1].revents & POLLHUP)) {
251 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
252 die("sigprocmask setmask proxy");
258 * Not sig_atomic_t (as required by POSIX) but I don't know how to do that any
261 static volatile pid_t pid_to_wait_for;
262 static int pid_to_wait_for_status;
264 static void sigchld_handler() {
268 while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
269 if (pid == pid_to_wait_for) {
270 /* Mark that our child has died and we should exit as well. */
272 /* We must exit like our child, save status. */
273 pid_to_wait_for_status = status;
279 int main(int argc, char **argv) {
280 char *exec_argv_shell[] = { NULL, NULL }; /* filled below */
281 char **exec_argv = NULL;
284 /* exec_argv set below */
285 } else if (argc > 2) {
286 exec_argv = argv + 2;
288 die_fmt("%s <user> [<cmd>...]\n", argv[0]);
291 const char *user = argv[1];
293 struct passwd *passwd = getpwnam(user);
295 die_fmt("unknown user name '%s'\n", user);
298 uid_t uid = passwd->pw_uid;
299 gid_t gid = passwd->pw_gid;
303 exec_argv_shell[0] = passwd->pw_shell;
304 exec_argv = exec_argv_shell;
307 int pty_master, pty_slave;
309 open_pty_or_die(&pty_master, &pty_slave, uid);
311 int ctty = open("/dev/tty", O_RDWR | O_NOCTTY); /* controlling TTY */
313 die("open /dev/tty");
316 sigset_t sigset, sigset_old;
317 sigemptyset(&sigset);
318 sigaddset(&sigset, SIGCHLD);
319 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
320 die("sigprocmask block sigchld");
326 } else if (pid == 0) {
327 /* child, will become a session leader */
329 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
330 die("sigprocmask setmask child");
334 if (ioctl(ctty, TIOCGWINSZ, &size) == -1) {
335 die("ioctl TIOCGWINSZ");
338 close_or_die(pty_master);
341 /* Start a new session and attach controlling TTY. */
342 if (setsid() == -1) {
345 if (ioctl(pty_slave, TIOCSCTTY, 0) == -1) {
346 die("ioctl TIOCSCTTY");
349 if (ioctl(pty_slave, TIOCSWINSZ, &size) == -1) {
350 die("ioctl TIOCSWINSZ");
356 } else if (pid == 0) {
357 drop_privileges_or_die(uid, gid);
359 dup2_or_die(pty_slave, STDIN_FILENO);
360 dup2_or_die(pty_slave, STDOUT_FILENO);
361 dup2_or_die(pty_slave, STDERR_FILENO);
362 close_or_die(pty_slave);
364 const char *term_orig = getenv("TERM");
365 const char *term = term_orig;
367 term = ""; /* for strlen() below */
369 const char *home = passwd->pw_dir;
371 char envp_user[strlen("USER=") + strlen(user) + 1];
372 char envp_home[strlen("HOME=") + strlen(home) + 1];
373 char envp_term[strlen("TERM=") + strlen(term) + 1];
374 snprintf_or_assert(envp_user, sizeof(envp_user), "USER=%s", user);
375 snprintf_or_assert(envp_home, sizeof(envp_home), "HOME=%s", home);
376 snprintf_or_assert(envp_term, sizeof(envp_term), "TERM=%s", term);
378 char *exec_envp[] = {
379 "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
382 term_orig ? envp_term : NULL,
386 execve(exec_argv[0], exec_argv, exec_envp);
389 close_or_die(pty_slave);
390 close_or_die(STDIN_FILENO);
391 close_or_die(STDOUT_FILENO);
392 close_or_die(STDERR_FILENO);
396 if (waitpid(pid, &status, 0) <= 0) {
397 die("waitpid child");
399 quit_with_matching_code(status);
401 close_or_die(pty_slave);
403 pid_to_wait_for = pid;
404 struct sigaction action = {
405 .sa_handler = sigchld_handler,
407 if (sigaction(SIGCHLD, &action, NULL) != 0) {
411 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
412 die("sigprocmask setmask parent");
415 struct termios old_term, term;
417 /* Change terminal to raw mode. */
418 if (tcgetattr(ctty, &old_term) != 0) {
422 /* From man 3 cfmakeraw; cfmakeraw is non-standard so set it manually. */
423 term.c_iflag &= ~(tcflag_t)(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
424 term.c_oflag &= ~(tcflag_t)(OPOST);
425 term.c_lflag &= ~(tcflag_t)(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
426 term.c_cflag &= ~(tcflag_t)(CSIZE | PARENB);
428 if (tcsetattr(ctty, TCSADRAIN, &term) != 0) {
432 proxy_input_between_ttys(pty_master, ctty, &pid_to_wait_for);
434 /* Restore terminal mode. */
435 if (tcsetattr(ctty, TCSADRAIN, &old_term) != 0) {
436 die("tcsetattr restore");
439 /* Wait until we got the status code from our child. poll() might also
440 * exit after POLLHUP while we haven't collected the child yet. */
441 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
442 die("sigprocmask block sigchld loop");
444 while (pid_to_wait_for != 0) {
445 sigsuspend(&sigset_old);
446 if (errno != EINTR) {
450 if (sigprocmask(SIG_SETMASK, &sigset, &sigset_old) != 0) {
451 die("sigprocmask setmask sigchld loop");
454 /* Try to exit the same way as the spawned process. */
455 if (pid_to_wait_for == 0) {
456 quit_with_matching_code(pid_to_wait_for_status);