2 * Run the login shell or command as the given user in a new pty to prevent
3 * terminal injection attacks.
5 * Copyright (C) 2016-2017 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");
85 /* The user must be able to write to the new TTY. Normally grantpt() would
86 * do this for us, but we don't trust the user and thus don't want to pass
87 * the pty_master to a process running under that uid. */
88 if (chown(slave_path, uid, (gid_t)-1) != 0) {
89 die("chown slave tty");
93 static void close_or_die(int fd) {
98 static void dup2_or_die(int oldfd, int newfd) {
99 if (dup2(oldfd, newfd) != newfd) {
103 static int snprintf_or_assert(char *str, size_t size, const char *format, ...) {
107 va_start(ap, format);
108 ret = vsnprintf(str, size, format, ap);
109 assert(size <= (size_t)INT_MAX);
110 assert(ret < (int)size); /* assert output fit into buffer */
116 static void drop_privileges_or_die(uid_t uid, gid_t gid) {
117 /* Drop all supplementary group IDs. */
118 if (setgroups(0, NULL) != 0) {
121 if (getgroups(0, NULL) != 0) {
122 die_fmt("failed to drop all supplementary groups");
125 /* Dropping groups may require privileges, do that first. */
126 if (setresgid(gid, gid, gid) != 0) {
129 if (setresuid(uid, uid, uid) != 0) {
133 /* Ensure we dropped all privileges. */
135 uid_t ruid, euid, suid;
136 gid_t rgid, egid, sgid;
138 if (getresuid(&ruid, &euid, &suid) != 0) {
141 if (getresgid(&rgid, &egid, &sgid) != 0) {
144 if ( uid != ruid || uid != euid || uid != suid
145 || gid != rgid || gid != egid || gid != sgid) {
146 die_fmt("failed to drop privileges");
149 /* Just to be safe. */
150 if (setuid(0) != -1) {
151 die_fmt("failed to drop privileges (setuid)");
155 static void quit_with_matching_code(int status) {
156 if (WIFEXITED(status)) {
157 exit(WEXITSTATUS(status));
158 } else if (WIFSIGNALED(status)) {
159 kill(getpid(), WTERMSIG(status));
160 /* Fall-through, should not happen. */
162 abort(); /* Should never happen, die painfully. */
165 static bool read_from_write_to(int from, int to) {
168 ssize_t r = read(from, buf, sizeof(buf));
173 size_t left = (size_t)r;
177 ssize_t w = write(to, data, left);
179 if (errno == EINTR) {
191 static void proxy_input_between_ttys(int pty_master, int ctty, volatile pid_t *pid_to_wait_for) {
192 struct pollfd fds[] = {
203 sigset_t sigset, sigset_old;
204 sigemptyset(&sigset);
205 sigaddset(&sigset, SIGCHLD);
206 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
207 die("sigprocmask block sigchld proxy");
210 /* Proxy data until our child has terminated. */
211 while (*pid_to_wait_for != 0) {
213 * If a signal happens here _and_ the child hasn't closed pty_slave,
214 * we would hang in poll(); therefore ppoll() is necessary.
216 nfds_t nfds = sizeof(fds)/sizeof(*fds);
217 if (ppoll(fds, nfds, NULL /* no timeout */, &sigset_old) == -1) {
218 if (errno == EAGAIN || errno == EINTR) {
226 * Handle errors first. (Data available before the error occurred
227 * might be dropped, but shouldn't matter here.)
229 if (fds[0].revents & (POLLERR | POLLNVAL)) {
230 fprintf(stderr, "poll: error on master: %d\n", fds[0].revents);
233 if (fds[1].revents & (POLLERR | POLLNVAL)) {
234 fprintf(stderr, "poll: error on ctty: %d\n", fds[1].revents);
238 /* Read data if available. */
239 if (fds[0].revents & POLLIN) {
240 if (!read_from_write_to(pty_master, ctty)) {
241 perror("read from master write to ctty");
245 if (fds[1].revents & POLLIN) {
246 if (!read_from_write_to(ctty, pty_master)) {
247 perror("read from ctty write to master");
252 /* Finally we are done if either side of the pty has disconnected. */
253 if ((fds[0].revents & POLLHUP) || (fds[1].revents & POLLHUP)) {
258 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
259 die("sigprocmask setmask proxy");
265 * Not sig_atomic_t (as required by POSIX) but I don't know how to do that any
268 static volatile pid_t pid_to_wait_for;
269 static int pid_to_wait_for_status;
271 static void sigchld_handler() {
275 while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
276 if (pid == pid_to_wait_for) {
277 /* Mark that our child has died and we should exit as well. */
279 /* We must exit like our child, save status. */
280 pid_to_wait_for_status = status;
286 int main(int argc, char **argv) {
287 char *exec_argv_shell[] = { NULL, NULL }; /* filled below */
288 char **exec_argv = NULL;
291 /* exec_argv set below */
292 } else if (argc > 2) {
293 exec_argv = argv + 2;
295 die_fmt("%s <user> [<cmd>...]\n", argv[0]);
298 const char *user = argv[1];
300 struct passwd *passwd = getpwnam(user);
302 die_fmt("unknown user name '%s'\n", user);
305 uid_t uid = passwd->pw_uid;
306 gid_t gid = passwd->pw_gid;
310 exec_argv_shell[0] = passwd->pw_shell;
311 exec_argv = exec_argv_shell;
314 int pty_master, pty_slave;
316 open_pty_or_die(&pty_master, &pty_slave, uid);
318 int ctty = open("/dev/tty", O_RDWR | O_NOCTTY); /* controlling TTY */
320 die("open /dev/tty");
323 sigset_t sigset, sigset_old;
324 sigemptyset(&sigset);
325 sigaddset(&sigset, SIGCHLD);
326 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
327 die("sigprocmask block sigchld");
333 } else if (pid == 0) {
334 /* child, will become a session leader */
336 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
337 die("sigprocmask setmask child");
341 if (ioctl(ctty, TIOCGWINSZ, &size) == -1) {
342 die("ioctl TIOCGWINSZ");
345 close_or_die(pty_master);
348 /* Start a new session and attach controlling TTY. */
349 if (setsid() == -1) {
352 if (ioctl(pty_slave, TIOCSCTTY, 0) == -1) {
353 die("ioctl TIOCSCTTY");
356 if (ioctl(pty_slave, TIOCSWINSZ, &size) == -1) {
357 die("ioctl TIOCSWINSZ");
363 } else if (pid == 0) {
365 * Drop the privileges just now so that the other user doesn't get
366 * access to the master TTY or the session leader (which might
367 * have additional privileges).
369 drop_privileges_or_die(uid, gid);
371 dup2_or_die(pty_slave, STDIN_FILENO);
372 dup2_or_die(pty_slave, STDOUT_FILENO);
373 dup2_or_die(pty_slave, STDERR_FILENO);
374 close_or_die(pty_slave);
376 const char *term_orig = getenv("TERM");
377 const char *term = term_orig;
379 term = ""; /* for strlen() below */
381 const char *home = passwd->pw_dir;
384 * Ignore errors here as we don't want to die on non-existent home
385 * directories to allow running as any user (think "/nonexistent"
386 * as home) and an error message will be annoying to ignore when
387 * running this command in scripts.
391 char envp_user[strlen("USER=") + strlen(user) + 1];
392 char envp_home[strlen("HOME=") + strlen(home) + 1];
393 char envp_term[strlen("TERM=") + strlen(term) + 1];
394 snprintf_or_assert(envp_user, sizeof(envp_user), "USER=%s", user);
395 snprintf_or_assert(envp_home, sizeof(envp_home), "HOME=%s", home);
396 snprintf_or_assert(envp_term, sizeof(envp_term), "TERM=%s", term);
398 char *exec_envp[] = {
399 "PATH=" PTYAS_DEFAULT_PATH,
402 term_orig ? envp_term : NULL,
406 execve(exec_argv[0], exec_argv, exec_envp);
409 close_or_die(pty_slave);
410 close_or_die(STDIN_FILENO);
411 close_or_die(STDOUT_FILENO);
412 close_or_die(STDERR_FILENO);
416 if (waitpid(pid, &status, 0) <= 0) {
417 die("waitpid child");
419 quit_with_matching_code(status);
421 close_or_die(pty_slave);
423 pid_to_wait_for = pid;
424 struct sigaction action = {
425 .sa_handler = sigchld_handler,
427 if (sigaction(SIGCHLD, &action, NULL) != 0) {
431 if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
432 die("sigprocmask setmask parent");
435 struct termios old_term, term;
437 /* Change terminal to raw mode. */
438 if (tcgetattr(ctty, &old_term) != 0) {
442 /* From man 3 cfmakeraw; cfmakeraw is non-standard so set it manually. */
443 term.c_iflag &= ~(tcflag_t)(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
444 term.c_oflag &= ~(tcflag_t)(OPOST);
445 term.c_lflag &= ~(tcflag_t)(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
446 term.c_cflag &= ~(tcflag_t)(CSIZE | PARENB);
448 if (tcsetattr(ctty, TCSADRAIN, &term) != 0) {
452 proxy_input_between_ttys(pty_master, ctty, &pid_to_wait_for);
454 /* Restore terminal mode. */
455 if (tcsetattr(ctty, TCSADRAIN, &old_term) != 0) {
456 die("tcsetattr restore");
460 * Wait until we got the status code from our child. poll() might already
461 * exit after POLLHUP while we haven't collected the child yet.
463 if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
464 die("sigprocmask block sigchld loop");
466 while (pid_to_wait_for != 0) {
467 sigsuspend(&sigset_old);
468 if (errno != EINTR) {
472 if (sigprocmask(SIG_SETMASK, &sigset, &sigset_old) != 0) {
473 die("sigprocmask setmask sigchld loop");
476 /* Try to exit the same way as the spawned process. */
477 if (pid_to_wait_for == 0) {
478 quit_with_matching_code(pid_to_wait_for_status);