1 /* vi: set sw=4 ts=4: */
2 /*
3 * Mini su implementation for busybox
4 *
5 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
6 */
7 //config:config SU
8 //config: bool "su"
9 //config: default y
10 //config: select FEATURE_SYSLOG
11 //config: help
12 //config: su is used to become another user during a login session.
13 //config: Invoked without a username, su defaults to becoming the super user.
14 //config:
15 //config: Note that Busybox binary must be setuid root for this applet to
16 //config: work properly.
17 //config:
18 //config:config FEATURE_SU_SYSLOG
19 //config: bool "Enable su to write to syslog"
20 //config: default y
21 //config: depends on SU
22 //config:
23 //config:config FEATURE_SU_CHECKS_SHELLS
24 //config: bool "Enable su to check user's shell to be listed in /etc/shells"
25 //config: depends on SU
26 //config: default y
27
28 //applet:/* Needs to be run by root or be suid root - needs to change uid and gid: */
29 //applet:IF_SU(APPLET(su, BB_DIR_BIN, BB_SUID_REQUIRE))
30
31 //kbuild:lib-$(CONFIG_SU) += su.o
32
33 //usage:#define su_trivial_usage
34 //usage: "[-lmp] [-] [-s SH] [USER [SCRIPT ARGS / -c 'CMD' ARG0 ARGS]]"
35 //usage:#define su_full_usage "\n\n"
36 //usage: "Run shell under USER (by default, root)\n"
37 //usage: "\n -,-l Clear environment, go to home dir, run shell as login shell"
38 //usage: "\n -p,-m Do not set new $HOME, $SHELL, $USER, $LOGNAME"
39 //usage: "\n -c CMD Command to pass to 'sh -c'"
40 //usage: "\n -s SH Shell to use instead of user's default"
41
42 #include "libbb.h"
43 #include <syslog.h>
44
45 #if ENABLE_FEATURE_SU_CHECKS_SHELLS
46 /* Return 1 if SHELL is a restricted shell (one not returned by
47 * getusershell), else 0, meaning it is a standard shell. */
restricted_shell(const char * shell)48 static int restricted_shell(const char *shell)
49 {
50 char *line;
51 int result = 1;
52
53 /*setusershell(); - getusershell does it itself*/
54 while ((line = getusershell()) != NULL) {
55 if (/* *line != '#' && */ strcmp(line, shell) == 0) {
56 result = 0;
57 break;
58 }
59 }
60 if (ENABLE_FEATURE_CLEAN_UP)
61 endusershell();
62 return result;
63 }
64 #endif
65
66 #define SU_OPT_mp (3)
67 #define SU_OPT_l (4)
68
69 int su_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
su_main(int argc UNUSED_PARAM,char ** argv)70 int su_main(int argc UNUSED_PARAM, char **argv)
71 {
72 unsigned flags;
73 char *opt_shell = NULL;
74 char *opt_command = NULL;
75 const char *opt_username = "root";
76 struct passwd *pw;
77 uid_t cur_uid = getuid();
78 const char *tty;
79 #if ENABLE_FEATURE_UTMP
80 char user_buf[64];
81 #endif
82 const char *old_user;
83
84 /* Note: we don't use "'+': stop at first non-option" idiom here.
85 * For su, "SCRIPT ARGS" or "-c CMD ARGS" do not stop option parsing:
86 * ARGS starting with dash will be treated as su options,
87 * not passed to shell. (Tested on util-linux 2.28).
88 */
89 flags = getopt32(argv, "mplc:s:", &opt_command, &opt_shell);
90 argv += optind;
91
92 if (argv[0] && LONE_DASH(argv[0])) {
93 flags |= SU_OPT_l;
94 argv++;
95 }
96
97 /* get user if specified */
98 if (argv[0]) {
99 opt_username = argv[0];
100 argv++;
101 }
102
103 if (ENABLE_FEATURE_SU_SYSLOG) {
104 /* The utmp entry (via getlogin) is probably the best way to
105 * identify the user, especially if someone su's from a su-shell.
106 * But getlogin can fail -- usually due to lack of utmp entry.
107 * in this case resort to getpwuid. */
108 #if ENABLE_FEATURE_UTMP
109 old_user = user_buf;
110 if (getlogin_r(user_buf, sizeof(user_buf)) != 0)
111 #endif
112 {
113 pw = getpwuid(cur_uid);
114 old_user = pw ? xstrdup(pw->pw_name) : "";
115 }
116 tty = xmalloc_ttyname(2);
117 if (!tty) {
118 tty = "none";
119 }
120 openlog(applet_name, 0, LOG_AUTH);
121 }
122
123 pw = xgetpwnam(opt_username);
124
125 if (cur_uid == 0 || ask_and_check_password(pw) > 0) {
126 if (ENABLE_FEATURE_SU_SYSLOG)
127 syslog(LOG_NOTICE, "%c %s %s:%s",
128 '+', tty, old_user, opt_username);
129 } else {
130 if (ENABLE_FEATURE_SU_SYSLOG)
131 syslog(LOG_NOTICE, "%c %s %s:%s",
132 '-', tty, old_user, opt_username);
133 bb_do_delay(LOGIN_FAIL_DELAY);
134 bb_error_msg_and_die("incorrect password");
135 }
136
137 if (ENABLE_FEATURE_CLEAN_UP && ENABLE_FEATURE_SU_SYSLOG) {
138 closelog();
139 }
140
141 if (!opt_shell && (flags & SU_OPT_mp)) {
142 /* -s SHELL is not given, but "preserve env" opt is */
143 opt_shell = getenv("SHELL");
144 }
145
146 #if ENABLE_FEATURE_SU_CHECKS_SHELLS
147 if (opt_shell && cur_uid != 0 && pw->pw_shell && restricted_shell(pw->pw_shell)) {
148 /* The user being su'd to has a nonstandard shell, and so is
149 * probably a uucp account or has restricted access. Don't
150 * compromise the account by allowing access with a standard
151 * shell. */
152 bb_error_msg("using restricted shell");
153 opt_shell = NULL; /* ignore -s PROG */
154 }
155 /* else: user can run whatever he wants via "su -s PROG USER".
156 * This is safe since PROG is run under user's uid/gid. */
157 #endif
158 if (!opt_shell)
159 opt_shell = pw->pw_shell;
160
161 change_identity(pw);
162 setup_environment(opt_shell,
163 ((flags & SU_OPT_l) / SU_OPT_l * SETUP_ENV_CLEARENV)
164 + (!(flags & SU_OPT_mp) * SETUP_ENV_CHANGEENV)
165 + (!(flags & SU_OPT_l) * SETUP_ENV_NO_CHDIR),
166 pw);
167 IF_SELINUX(set_current_security_context(NULL);)
168
169 if (opt_command) {
170 *--argv = opt_command;
171 *--argv = (char*)"-c";
172 }
173
174 /* A nasty ioctl exists which can stuff data into input queue:
175 * #include <sys/ioctl.h>
176 * int main() {
177 * const char *msg = "echo $UID\n";
178 * while (*msg) ioctl(0, TIOCSTI, *msg++);
179 * return 0;
180 * }
181 * With "su USER -c EXPLOIT" run by root, exploit can make root shell
182 * read as input and execute arbitrary command.
183 * It's debatable whether we need to protect against this
184 * (root may hesitate to run unknown scripts interactively).
185 *
186 * Some versions of su run -c CMD in a different session:
187 * ioctl(TIOCSTI) works only on the controlling tty.
188 */
189
190 /* Never returns */
191 run_shell(opt_shell, flags & SU_OPT_l, (const char**)argv);
192
193 /* return EXIT_FAILURE; - not reached */
194 }
195