blob: 4cce82405883f3ab15c5e79cee4988ed2a8c92fc [file] [log] [blame]
/* vi: set sw=4 ts=4: */
* Mini su implementation for busybox
* Licensed under GPLv2 or later, see file LICENSE in this source tree.
//config:config SU
//config: bool "su"
//config: default y
//config: select FEATURE_SYSLOG
//config: help
//config: su is used to become another user during a login session.
//config: Invoked without a username, su defaults to becoming the super user.
//config: Note that busybox binary must be setuid root for this applet to
//config: work properly.
//config:config FEATURE_SU_SYSLOG
//config: bool "Log to syslog all attempts to use su"
//config: default y
//config: depends on SU
//config: bool "If user's shell is not in /etc/shells, disallow -s PROG"
//config: default y
//config: depends on SU
//config: bool "Allow blank passwords only on TTYs in /etc/securetty"
//config: default n
//config: depends on SU
//applet:/* Needs to be run by root or be suid root - needs to change uid and gid: */
//kbuild:lib-$(CONFIG_SU) += su.o
//usage:#define su_trivial_usage
//usage: "[-lmp] [-] [-s SH] [USER [SCRIPT ARGS / -c 'CMD' ARG0 ARGS]]"
//usage:#define su_full_usage "\n\n"
//usage: "Run shell under USER (by default, root)\n"
//usage: "\n -,-l Clear environment, go to home dir, run shell as login shell"
//usage: "\n -p,-m Do not set new $HOME, $SHELL, $USER, $LOGNAME"
//usage: "\n -c CMD Command to pass to 'sh -c'"
//usage: "\n -s SH Shell to use instead of user's default"
#include "libbb.h"
#include <syslog.h>
/* Return 1 if SHELL is a restricted shell (one not returned by
* getusershell), else 0, meaning it is a standard shell. */
static int restricted_shell(const char *shell)
char *line;
int result = 1;
/*setusershell(); - getusershell does it itself*/
while ((line = getusershell()) != NULL) {
if (/* *line != '#' && */ strcmp(line, shell) == 0) {
result = 0;
return result;
#define SU_OPT_mp (3)
#define SU_OPT_l (4)
int su_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int su_main(int argc UNUSED_PARAM, char **argv)
unsigned flags;
char *opt_shell = NULL;
char *opt_command = NULL;
const char *opt_username = "root";
struct passwd *pw;
uid_t cur_uid = getuid();
const char *tty;
char user_buf[64];
const char *old_user;
int r;
/* Note: we don't use "'+': stop at first non-option" idiom here.
* For su, "SCRIPT ARGS" or "-c CMD ARGS" do not stop option parsing:
* ARGS starting with dash will be treated as su options,
* not passed to shell. (Tested on util-linux 2.28).
flags = getopt32(argv, "mplc:s:", &opt_command, &opt_shell);
argv += optind;
if (argv[0] && LONE_DASH(argv[0])) {
flags |= SU_OPT_l;
/* get user if specified */
if (argv[0]) {
opt_username = argv[0];
tty = xmalloc_ttyname(STDIN_FILENO);
if (!tty)
tty = "none";
tty = skip_dev_pfx(tty);
/* The utmp entry (via getlogin) is probably the best way to
* identify the user, especially if someone su's from a su-shell.
* But getlogin can fail -- usually due to lack of utmp entry.
* in this case resort to getpwuid. */
old_user = user_buf;
if (getlogin_r(user_buf, sizeof(user_buf)) != 0)
pw = getpwuid(cur_uid);
old_user = pw ? xstrdup(pw->pw_name) : "";
openlog(applet_name, 0, LOG_AUTH);
pw = xgetpwnam(opt_username);
r = 1;
if (cur_uid != 0)
r = ask_and_check_password(pw);
if (r > 0) {
&& !is_tty_secure(tty)
) {
goto fail;
syslog(LOG_NOTICE, "%c %s %s:%s",
'+', tty, old_user, opt_username);
} else {
syslog(LOG_NOTICE, "%c %s %s:%s",
'-', tty, old_user, opt_username);
bb_error_msg_and_die("incorrect password");
if (!opt_shell && (flags & SU_OPT_mp)) {
/* -s SHELL is not given, but "preserve env" opt is */
opt_shell = getenv("SHELL");
if (opt_shell && cur_uid != 0 && pw->pw_shell && restricted_shell(pw->pw_shell)) {
/* The user being su'd to has a nonstandard shell, and so is
* probably a uucp account or has restricted access. Don't
* compromise the account by allowing access with a standard
* shell. */
bb_error_msg("using restricted shell");
opt_shell = NULL; /* ignore -s PROG */
/* else: user can run whatever he wants via "su -s PROG USER".
* This is safe since PROG is run under user's uid/gid. */
if (!opt_shell)
opt_shell = pw->pw_shell;
((flags & SU_OPT_l) / SU_OPT_l * SETUP_ENV_CLEARENV)
+ (!(flags & SU_OPT_mp) * SETUP_ENV_CHANGEENV)
+ (!(flags & SU_OPT_l) * SETUP_ENV_NO_CHDIR),
if (opt_command) {
*--argv = opt_command;
*--argv = (char*)"-c";
/* A nasty ioctl exists which can stuff data into input queue:
* #include <sys/ioctl.h>
* int main() {
* const char *msg = "echo $UID\n";
* while (*msg) ioctl(0, TIOCSTI, *msg++);
* return 0;
* }
* With "su USER -c EXPLOIT" run by root, exploit can make root shell
* read as input and execute arbitrary command.
* It's debatable whether we need to protect against this
* (root may hesitate to run unknown scripts interactively).
* Some versions of su run -c CMD in a different session:
* ioctl(TIOCSTI) works only on the controlling tty.
/* Never returns */
run_shell(opt_shell, flags & SU_OPT_l, (const char**)argv);
/* return EXIT_FAILURE; - not reached */