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 // TODO: is this a problem?
83 if (chown(slave_path, uid, (gid_t)-1) != 0) {
84 die("chown slave tty");
88 static void close_or_die(int fd) {
93 static void dup2_or_die(int oldfd, int newfd) {
94 if (dup2(oldfd, newfd) != newfd) {
98 static int snprintf_or_assert(char *str, size_t size, const char *format, ...) {
102 va_start(ap, format);
103 ret = vsnprintf(str, size, format, ap);
104 assert(size <= (size_t)INT_MAX);
105 assert(ret < (int)size); /* assert output fit into buffer */
111 static void drop_privileges_or_die(uid_t uid, gid_t gid) {
112 /* Drop all supplementary group IDs. */
113 if (setgroups(0, NULL) != 0) {
116 if (getgroups(0, NULL) != 0) {
117 die_fmt("failed to drop all groups");
120 /* Dropping groups may require privileges, do that first. */
121 if (setresgid(gid, gid, gid) != 0) {
124 if (setresuid(uid, uid, uid) != 0) {
128 /* Ensure we dropped all privileges. */
130 uid_t ruid, euid, suid;
131 gid_t rgid, egid, sgid;
133 if (getresuid(&ruid, &euid, &suid) != 0) {
136 if (getresgid(&rgid, &egid, &sgid) != 0) {
139 if ( uid != ruid || uid != euid || uid != suid
140 || gid != rgid || gid != egid || gid != sgid) {
141 die_fmt("failed to drop privileges");
144 /* Just to be safe. */
145 if (setuid(0) != -1) {
146 die_fmt("failed to drop privileges (setuid)");
150 static void quit_with_matching_code(int status) {
151 if (WIFEXITED(status)) {
152 exit(WEXITSTATUS(status));
153 } else if (WIFSIGNALED(status)) {
154 kill(getpid(), WTERMSIG(status));
155 /* Fall-through, should not happen. */
157 abort(); /* Should never happen, die painfully. */
160 static bool read_from_write_to(int from, int to) {
163 ssize_t r = read(from, buf, sizeof(buf));
168 size_t left = (size_t)r;
172 ssize_t w = write(to, data, left);
174 if (errno == EINTR) {
186 static void proxy_input_between_ttys(int pty_master, int ctty, volatile pid_t *pid_to_wait_for) {
187 struct pollfd fds[] = {
198 sigset_t sigset, sigset_old;
199 sigemptyset(&sigset);
200 sigaddset(&sigset, SIGCHLD);
201 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
202 die("sigprocmask block sigchld proxy");
205 /* Proxy data until our child has terminated. */
206 while (*pid_to_wait_for != 0) {
208 * If a signal happens here _and_ the child hasn't closed pty_slave,
209 * we will hang in poll(); therefore ppoll() is requred.
211 nfds_t nfds = sizeof(fds)/sizeof(*fds);
212 if (ppoll(fds, nfds, NULL /* no timeout */, &sigset_old) == -1) {
213 if (errno == EAGAIN || errno == EINTR) {
221 /* Handle errors first. */
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 /* Then 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");
257 /* Not sig_atomic_t but I don't know how to do that any other way. */
258 static volatile pid_t pid_to_wait_for;
259 static int pid_to_wait_for_status;
261 static void sigchld_handler() {
265 while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
266 if (pid == pid_to_wait_for) {
267 /* Mark that our child has died and we should exit as well. */
269 /* We must exit like our child, save status. */
270 pid_to_wait_for_status = status;
276 int main(int argc, char **argv) {
277 char *exec_argv_shell[] = { NULL, NULL }; /* filled below */
278 char **exec_argv = NULL;
281 /* exec_argv set below */
282 } else if (argc > 2) {
283 exec_argv = argv + 2;
285 die_fmt("%s <user> [<cmd>...]\n", argv[0]);
288 const char *user = argv[1];
290 struct passwd *passwd = getpwnam(user);
292 die_fmt("unknown user name '%s'\n", user);
295 uid_t uid = passwd->pw_uid;
296 gid_t gid = passwd->pw_gid;
300 exec_argv_shell[0] = passwd->pw_shell;
301 exec_argv = exec_argv_shell;
304 int pty_master, pty_slave;
306 open_pty_or_die(&pty_master, &pty_slave, uid);
308 int ctty = open("/dev/tty", O_RDWR | O_NOCTTY); /* controlling TTY */
310 die("open /dev/tty");
313 sigset_t sigset, sigset_old;
314 sigemptyset(&sigset);
315 sigaddset(&sigset, SIGCHLD);
316 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
317 die("sigprocmask block sigchld");
323 } else if (pid == 0) {
324 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
325 die("sigprocmask setmask child");
329 if (ioctl(ctty, TIOCGWINSZ, &size) == -1) {
330 die("ioctl TIOCGWINSZ");
333 close_or_die(pty_master);
336 /* Start a new session and attach controlling TTY. */
337 if (setsid() == -1) {
340 if (ioctl(pty_slave, TIOCSCTTY, 0) == -1) {
341 die("ioctl TIOCSCTTY");
344 if (ioctl(pty_slave, TIOCSWINSZ, &size) == -1) {
345 die("ioctl TIOCSWINSZ");
351 } else if (pid == 0) {
352 drop_privileges_or_die(uid, gid);
354 dup2_or_die(pty_slave, STDIN_FILENO);
355 dup2_or_die(pty_slave, STDOUT_FILENO);
356 dup2_or_die(pty_slave, STDERR_FILENO);
357 close_or_die(pty_slave);
359 const char *term_orig = getenv("TERM");
360 const char *term = term_orig;
362 term = ""; /* for strlen() below */
364 const char *home = passwd->pw_dir;
366 char envp_user[strlen("USER=") + strlen(user) + 1];
367 char envp_home[strlen("HOME=") + strlen(home) + 1];
368 char envp_term[strlen("TERM=") + strlen(term) + 1];
369 snprintf_or_assert(envp_user, sizeof(envp_user), "USER=%s", user);
370 snprintf_or_assert(envp_home, sizeof(envp_home), "HOME=%s", home);
371 snprintf_or_assert(envp_term, sizeof(envp_term), "TERM=%s", term);
373 char *exec_envp[] = {
374 "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
377 term_orig ? envp_term : NULL,
381 execve(exec_argv[0], exec_argv, exec_envp);
384 close_or_die(pty_slave);
385 close_or_die(STDIN_FILENO);
386 close_or_die(STDOUT_FILENO);
387 close_or_die(STDERR_FILENO);
391 if (waitpid(pid, &status, 0) <= 0) {
392 die("waitpid child");
394 quit_with_matching_code(status);
396 close_or_die(pty_slave);
398 pid_to_wait_for = pid;
399 struct sigaction action = {
400 .sa_handler = sigchld_handler,
402 if (sigaction(SIGCHLD, &action, NULL) != 0) {
406 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
407 die("sigprocmask setmask parent");
410 struct termios old_term, term;
412 /* Change terminal to raw mode. */
413 if (tcgetattr(ctty, &old_term) != 0) {
417 /* From man 3 cfmakeraw which is non-standard. */
418 term.c_iflag &= ~(tcflag_t)(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
419 term.c_oflag &= ~(tcflag_t)(OPOST);
420 term.c_lflag &= ~(tcflag_t)(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
421 term.c_cflag &= ~(tcflag_t)(CSIZE | PARENB);
423 if (tcsetattr(ctty, TCSADRAIN, &term) != 0) {
427 proxy_input_between_ttys(pty_master, ctty, &pid_to_wait_for);
429 /* Restore terminal mode. */
430 if (tcsetattr(ctty, TCSADRAIN, &old_term) != 0) {
431 die("tcsetattr restore");
434 /* Wait until we got the status code from our child. poll() might also
435 * exit after POLLHUP while we haven't collected the child yet. */
436 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
437 die("sigprocmask block sigchld loop");
439 while (pid_to_wait_for != 0) {
440 sigsuspend(&sigset_old);
441 if (errno != EINTR) {
445 if (sigprocmask(SIG_SETMASK, &sigset, &sigset_old) != 0) {
446 die("sigprocmask setmask sigchld loop");
449 /* Try to exit the same way as the spawned process. */
450 if (pid_to_wait_for == 0) {
451 quit_with_matching_code(pid_to_wait_for_status);