/* 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 (19 kb)"
//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:config FEATURE_SU_SYSLOG
//config:       bool "Log to syslog all attempts to use su"
//config:       default y
//config:       depends on SU
//config:
//config:config FEATURE_SU_CHECKS_SHELLS
//config:       bool "If user's shell is not in /etc/shells, disallow -s PROG"
//config:       default y
//config:       depends on SU
//config:
//config:config FEATURE_SU_BLANK_PW_NEEDS_SECURE_TTY
//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: */
//applet:IF_SU(APPLET(su, BB_DIR_BIN, BB_SUID_REQUIRE))

//kbuild:lib-$(CONFIG_SU) += su.o

//usage:#define su_trivial_usage
//usage:       "[-lmp] [-s SH] [-] [USER [FILE 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>

#if ENABLE_FEATURE_SU_CHECKS_SHELLS
/* 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;
                        break;
                }
        }
        if (ENABLE_FEATURE_CLEAN_UP)
                endusershell();
        return result;
}
#endif

#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;
#if ENABLE_FEATURE_UTMP
        char user_buf[64];
#endif
        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;
                argv++;
        }

        /* get user if specified */
        if (argv[0]) {
                opt_username = argv[0];
                argv++;
        }

        tty = xmalloc_ttyname(STDIN_FILENO);
        if (!tty)
                tty = "none";
        tty = skip_dev_pfx(tty);

        if (ENABLE_FEATURE_SU_SYSLOG) {
                /* 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.  */
#if ENABLE_FEATURE_UTMP
                old_user = user_buf;
                if (getlogin_r(user_buf, sizeof(user_buf)) != 0)
#endif
                {
                        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) {
                if (ENABLE_FEATURE_SU_BLANK_PW_NEEDS_SECURE_TTY
                 && r == CHECKPASS_PW_HAS_EMPTY_PASSWORD
                 && !is_tty_secure(tty)
                ) {
                        goto fail;
                }
                if (ENABLE_FEATURE_SU_SYSLOG)
                        syslog(LOG_NOTICE, "%c %s %s:%s",
                                '+', tty, old_user, opt_username);
        } else {
 fail:
                if (ENABLE_FEATURE_SU_SYSLOG)
                        syslog(LOG_NOTICE, "%c %s %s:%s",
                                '-', tty, old_user, opt_username);
                pause_after_failed_login();
                bb_simple_error_msg_and_die("incorrect password");
        }

        if (ENABLE_FEATURE_CLEAN_UP && ENABLE_FEATURE_SU_SYSLOG) {
                closelog();
        }

        if (!opt_shell && (flags & SU_OPT_mp)) {
                /* -s SHELL is not given, but "preserve env" opt is */
                opt_shell = getenv("SHELL");
        }

#if ENABLE_FEATURE_SU_CHECKS_SHELLS
        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_simple_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. */
#endif
        if (!opt_shell)
                opt_shell = pw->pw_shell;

        change_identity(pw);
        setup_environment(opt_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),
                        pw);
        IF_SELINUX(set_current_security_context(NULL);)

        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 */
        exec_shell(opt_shell, flags & SU_OPT_l, (const char**)argv);

        /* return EXIT_FAILURE; - not reached */
}