+/*
+ * Run the login shell or command as the given user in a new pty to prevent
+ * terminal injection attacks.
+ *
+ * Copyright (C) 2016 Simon Ruderich
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#define _GNU_SOURCE
+
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <grp.h>
+#include <limits.h>
+#include <poll.h>
+#include <pwd.h>
+#include <signal.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <unistd.h>
+
+
+static void die(const char *s) {
+ perror(s);
+ exit(EXIT_FAILURE);
+}
+static void die_msg(const char *fmt, ...) {
+ va_list ap;
+
+ va_start(ap, fmt);
+ vfprintf(stderr, fmt, ap);
+ va_end(ap);
+
+ exit(EXIT_FAILURE);
+}
+
+static void open_pty_or_die(int *pty_master, int *pty_slave, uid_t uid) {
+ char *slave_path;
+
+ *pty_master = posix_openpt(O_RDWR | O_NOCTTY);
+ if (*pty_master == -1) {
+ die("posix_openpt");
+ }
+ slave_path = ptsname(*pty_master);
+ if (!slave_path) {
+ die("ptsname");
+ }
+ if (grantpt(*pty_master) != 0) {
+ die("grantpt");
+ }
+ if (unlockpt(*pty_master) != 0) {
+ die("unlockpt");
+ }
+
+ *pty_slave = open(slave_path, O_RDWR | O_NOCTTY);
+ if (*pty_slave == -1) {
+ die("open slave tty");
+ }
+ /* The user must be able to write to the new TTY. Normally grantpt() would
+ * do this for us, but we don't trust the user and thus don't want to pass
+ * the pty_master to a process running under that uid. */
+ // TODO: is this a problem?
+ if (chown(slave_path, uid, (gid_t)-1) != 0) {
+ die("chown slave tty");
+ }
+}
+
+static void close_or_die(int fd) {
+ if (close(fd) != 0) {
+ die("close");
+ }
+}
+static void dup2_or_die(int oldfd, int newfd) {
+ if (dup2(oldfd, newfd) != newfd) {
+ die("dup2");
+ }
+}
+static int snprintf_or_assert(char *str, size_t size, const char *format, ...) {
+ int ret;
+ va_list ap;
+
+ va_start(ap, format);
+ ret = vsnprintf(str, size, format, ap);
+ assert(size <= (size_t)INT_MAX);
+ assert(ret < (int)size); /* assert output fit into buffer */
+ va_end(ap);
+
+ return ret;
+}
+
+static void drop_privileges_or_die(uid_t uid, gid_t gid) {
+ /* Drop all supplementary group IDs. */
+ if (setgroups(0, NULL) != 0) {
+ die("setgroups");
+ }
+ if (getgroups(0, NULL) != 0) {
+ die_msg("failed to drop all groups");
+ }
+
+ /* Dropping groups may require privileges, do that first. */
+ if (setresgid(gid, gid, gid) != 0) {
+ die("setresgid");
+ }
+ if (setresuid(uid, uid, uid) != 0) {
+ die("setresuid");
+ }
+
+ /* Ensure we dropped all privileges. */
+ {
+ uid_t ruid, euid, suid;
+ gid_t rgid, egid, sgid;
+
+ if (getresuid(&ruid, &euid, &suid) != 0) {
+ die("getresuid");
+ }
+ if (getresgid(&rgid, &egid, &sgid) != 0) {
+ die("getresgid");
+ }
+ if ( uid != ruid || uid != euid || uid != suid
+ || gid != rgid || gid != egid || gid != sgid) {
+ die_msg("failed to drop privileges");
+ }
+ }
+ /* Just to be safe. */
+ if (setuid(0) != -1) {
+ die_msg("failed to drop privileges (setuid)");
+ }
+}
+
+static void quit_with_matching_code(int status) {
+ if (WIFEXITED(status)) {
+ exit(WEXITSTATUS(status));
+ } else if (WIFSIGNALED(status)) {
+ kill(getpid(), WTERMSIG(status));
+ /* Fall-through, should not happen. */
+ }
+ abort(); /* Should never happen, die painfully. */
+}
+
+static bool read_from_write_to(int from, int to) {
+ char buf[4096];
+
+ ssize_t r = read(from, buf, sizeof(buf));
+ if (r < 0) {
+ return false;
+ }
+
+ size_t left = (size_t)r;
+ char *data = buf;
+
+ while (left > 0) {
+ ssize_t w = write(to, data, left);
+ if (w < 0) {
+ if (errno == EINTR) {
+ continue;
+ }
+ return false;
+ }
+ left -= (size_t)w;
+ data += (size_t)w;
+ }
+
+ return true;
+}
+
+static void proxy_input_between_ttys(int pty_master, int ctty, volatile pid_t *pid_to_wait_for) {
+ struct pollfd fds[] = {
+ { /* 0 */
+ .fd = pty_master,
+ .events = POLLIN,
+ },
+ { /* 1 */
+ .fd = ctty,
+ .events = POLLIN,
+ },
+ };
+
+ sigset_t sigset, sigset_old;
+ sigemptyset(&sigset);
+ sigaddset(&sigset, SIGCHLD);
+ if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
+ die("sigprocmask block sigchld proxy");
+ }
+
+ /* Proxy data until our child has terminated. */
+ while (*pid_to_wait_for != 0) {
+ /*
+ * If a signal happens here _and_ the child hasn't closed pty_slave,
+ * we will hang in poll(); therefore ppoll() is requred.
+ */
+ nfds_t nfds = sizeof(fds)/sizeof(*fds);
+ if (ppoll(fds, nfds, NULL /* no timeout */, &sigset_old) == -1) {
+ if (errno == EAGAIN || errno == EINTR) {
+ continue;
+ } else {
+ perror("poll");
+ }
+ break;
+ }
+
+ /* Handle errors first. */
+ if (fds[0].revents & (POLLERR | POLLNVAL)) {
+ fprintf(stderr, "poll: error on master: %d\n", fds[0].revents);
+ break;
+ }
+ if (fds[1].revents & (POLLERR | POLLNVAL)) {
+ fprintf(stderr, "poll: error on ctty: %d\n", fds[1].revents);
+ break;
+ }
+
+ /* Then read data if available. */
+ if (fds[0].revents & POLLIN) {
+ if (!read_from_write_to(pty_master, ctty)) {
+ perror("read from master write to ctty");
+ break;
+ }
+ }
+ if (fds[1].revents & POLLIN) {
+ if (!read_from_write_to(ctty, pty_master)) {
+ perror("read from ctty write to master");
+ break;
+ }
+ }
+
+ /* Finally we are done if either side of the pty has disconnected. */
+ if ((fds[0].revents & POLLHUP) || (fds[1].revents & POLLHUP)) {
+ break;
+ }
+ }
+
+ if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
+ die("sigprocmask setmask proxy");
+ }
+}
+
+
+/* Not sig_atomic_t but I don't know how to do that any other way. */
+static volatile pid_t pid_to_wait_for;
+static int pid_to_wait_for_status;
+
+static void sigchld_handler() {
+ int status;
+ pid_t pid;
+
+ while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
+ if (pid == pid_to_wait_for) {
+ /* Mark that our child has died and we should exit as well. */
+ pid_to_wait_for = 0;
+ /* We must exit like our child, save status. */
+ pid_to_wait_for_status = status;
+ }
+ }
+}
+
+
+int main(int argc, char **argv) {
+ char *exec_argv_shell[] = { NULL, NULL }; /* filled below */
+ char **exec_argv = NULL;
+
+ if (argc == 2) {
+ /* exec_argv set below */
+ } else if (argc > 2) {
+ exec_argv = argv + 2;
+ } else {
+ die_msg("%s <user> [<cmd>...]\n", argv[0]);
+ }
+
+ const char *user = argv[1];
+
+ struct passwd *passwd = getpwnam(user);
+ if (!passwd) {
+ die_msg("unknown user name '%s'\n", user);
+ }
+
+ uid_t uid = passwd->pw_uid;
+ gid_t gid = passwd->pw_gid;
+
+ if (!exec_argv) {
+ assert(argc == 2);
+ exec_argv_shell[0] = passwd->pw_shell;
+ exec_argv = exec_argv_shell;
+ }
+
+ int pty_master, pty_slave;
+
+ open_pty_or_die(&pty_master, &pty_slave, uid);
+
+ int ctty = open("/dev/tty", O_RDWR | O_NOCTTY); /* controlling TTY */
+ if (ctty == -1) {
+ die("open /dev/tty");
+ }
+
+ sigset_t sigset, sigset_old;
+ sigemptyset(&sigset);
+ sigaddset(&sigset, SIGCHLD);
+ if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
+ die("sigprocmask block sigchld");
+ }
+
+ pid_t pid = fork();
+ if (pid == -1) {
+ die("fork parent");
+ } else if (pid == 0) {
+ if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
+ die("sigprocmask setmask child");
+ }
+
+ struct winsize size;
+ if (ioctl(ctty, TIOCGWINSZ, &size) == -1) {
+ die("ioctl TIOCGWINSZ");
+ }
+
+ close_or_die(pty_master);
+ close_or_die(ctty);
+
+ /* Start a new session and attach controlling TTY. */
+ if (setsid() == -1) {
+ die("setsid");
+ }
+ if (ioctl(pty_slave, TIOCSCTTY, 0) == -1) {
+ die("ioctl TIOCSCTTY");
+ }
+
+ if (ioctl(pty_slave, TIOCSWINSZ, &size) == -1) {
+ die("ioctl TIOCSWINSZ");
+ }
+
+ pid_t pid = fork();
+ if (pid == -1) {
+ die("fork child");
+ } else if (pid == 0) {
+ drop_privileges_or_die(uid, gid);
+
+ dup2_or_die(pty_slave, STDIN_FILENO);
+ dup2_or_die(pty_slave, STDOUT_FILENO);
+ dup2_or_die(pty_slave, STDERR_FILENO);
+ close_or_die(pty_slave);
+
+ const char *term_orig = getenv("TERM");
+ const char *term = term_orig;
+ if (!term) {
+ term = ""; /* for strlen() below */
+ }
+ const char *home = passwd->pw_dir;
+
+ char envp_user[strlen("USER=") + strlen(user) + 1];
+ char envp_home[strlen("HOME=") + strlen(home) + 1];
+ char envp_term[strlen("TERM=") + strlen(term) + 1];
+ snprintf_or_assert(envp_user, sizeof(envp_user), "USER=%s", user);
+ snprintf_or_assert(envp_home, sizeof(envp_home), "HOME=%s", home);
+ snprintf_or_assert(envp_term, sizeof(envp_term), "TERM=%s", term);
+
+ char *exec_envp[] = {
+ "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
+ envp_user,
+ envp_home,
+ term_orig ? envp_term : NULL,
+ NULL,
+ };
+
+ execve(exec_argv[0], exec_argv, exec_envp);
+ die("execve");
+ }
+ close_or_die(pty_slave);
+ close_or_die(STDIN_FILENO);
+ close_or_die(STDOUT_FILENO);
+ close_or_die(STDERR_FILENO);
+
+ // TODO: EINTR?
+ int status;
+ if (waitpid(pid, &status, 0) <= 0) {
+ die("waitpid child");
+ }
+ quit_with_matching_code(status);
+ }
+ close_or_die(pty_slave);
+
+ pid_to_wait_for = pid;
+ struct sigaction action = {
+ .sa_handler = sigchld_handler,
+ };
+ if (sigaction(SIGCHLD, &action, NULL) != 0) {
+ die("sigaction");
+ }
+
+ if (sigprocmask(SIG_SETMASK, &sigset_old, NULL) != 0) {
+ die("sigprocmask setmask parent");
+ }
+
+ struct termios old_term, term;
+
+ /* Change terminal to raw mode. */
+ if (tcgetattr(ctty, &old_term) != 0) {
+ die("tcgetattr");
+ }
+ term = old_term;
+ /* From man 3 cfmakeraw which is non-standard. */
+ term.c_iflag &= ~(tcflag_t)(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL | IXON);
+ term.c_oflag &= ~(tcflag_t)(OPOST);
+ term.c_lflag &= ~(tcflag_t)(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
+ term.c_cflag &= ~(tcflag_t)(CSIZE | PARENB);
+ term.c_cflag |= CS8;
+ if (tcsetattr(ctty, TCSADRAIN, &term) != 0) {
+ die("tcsetattr");
+ }
+
+ proxy_input_between_ttys(pty_master, ctty, &pid_to_wait_for);
+
+ /* Restore terminal mode. */
+ if (tcsetattr(ctty, TCSADRAIN, &old_term) != 0) {
+ die("tcsetattr restore");
+ }
+
+ /* Wait until we got the status code from our child. poll() might also
+ * exit after POLLHUP while we haven't collected the child yet. */
+ if (sigprocmask(SIG_BLOCK, &sigset, &sigset_old) != 0) {
+ die("sigprocmask block sigchld loop");
+ }
+ while (pid_to_wait_for != 0) {
+ sigsuspend(&sigset_old);
+ if (errno != EINTR) {
+ die("sigsuspend");
+ }
+ }
+ if (sigprocmask(SIG_SETMASK, &sigset, &sigset_old) != 0) {
+ die("sigprocmask setmask sigchld loop");
+ }
+
+ /* Try to exit the same way as the spawned process. */
+ if (pid_to_wait_for == 0) {
+ quit_with_matching_code(pid_to_wait_for_status);
+ }
+ return EXIT_FAILURE;
+}
ruderich.org/simon / ptyas / ptyas.git / commitdiff