su: expand help; simplify passing of -c CMD to run_shell()

Also, added a comment about bug 9401 (TIOCSTI input injection).

function                                             old     new   delta
packed_usage                                       30909   30932     +23
su_main                                              470     487     +17
sulogin_main                                         260     258      -2
run_applet_and_exit                                  681     678      -3
run_shell                                            166     126     -40

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1341,7 +1341,7 @@ char *bb_simplify_abs_path_inplace(char *path) FAST_FUNC;
 #endif
 extern void bb_do_delay(int seconds) FAST_FUNC;
 extern void change_identity(const struct passwd *pw) FAST_FUNC;
-extern void run_shell(const char *shell, int loginshell, const char *command, const char **additional_args) NORETURN FAST_FUNC;
+extern void run_shell(const char *shell, int loginshell, const char **args) NORETURN FAST_FUNC;
 
 /* Returns $SHELL, getpwuid(getuid())->pw_shell, or DEFAULT_SHELL.
  * Note that getpwuid result might need xstrdup'ing

libbb/executable.c

@@ -97,5 +97,5 @@ void FAST_FUNC exec_prog_or_SHELL(char **argv)
 	if (argv[0]) {
 		BB_EXECVP_or_die(argv);
 	}
-	run_shell(getenv("SHELL"), /*login:*/ 1, NULL, NULL);
+	run_shell(getenv("SHELL"), /*login:*/ 1, NULL);
 }

libbb/run_shell.c

@@ -50,19 +50,17 @@ void FAST_FUNC set_current_security_context(security_context_t sid)
 #endif
 
 /* Run SHELL, or DEFAULT_SHELL if SHELL is "" or NULL.
- * If COMMAND is nonzero, pass it to the shell with the -c option.
+ * If ADDITIONAL_ARGS is not NULL, pass them to the shell.
- * If ADDITIONAL_ARGS is nonzero, pass it to the shell as more
+ */
- * arguments.  */
+void FAST_FUNC run_shell(const char *shell, int loginshell, const char **additional_args)
-void FAST_FUNC run_shell(const char *shell, int loginshell, const char *command, const char **additional_args)
 {
 	const char **args;
-	int argno;
-	int additional_args_cnt = 0;
 
-	for (args = additional_args; args && *args; args++)
+	args = additional_args;
-		additional_args_cnt++;
+	while (args && *args)
+		args++;
 
-	args = xmalloc(sizeof(char*) * (4 + additional_args_cnt));
+	args = xmalloc(sizeof(char*) * (2 + (args - additional_args)));
 
 	if (!shell || !shell[0])
 		shell = DEFAULT_SHELL;
@@ -70,16 +68,13 @@ void FAST_FUNC run_shell(const char *shell, int loginshell, const char *command,
 	args[0] = bb_get_last_path_component_nostrip(shell);
 	if (loginshell)
 		args[0] = xasprintf("-%s", args[0]);
-	argno = 1;
+	args[1] = NULL;
-	if (command) {
-		args[argno++] = "-c";
-		args[argno++] = command;
-	}
 	if (additional_args) {
-		for (; *additional_args; ++additional_args)
+		int cnt = 1;
-			args[argno++] = *additional_args;
+		for (;;)
+			if ((args[cnt++] = *additional_args++) == NULL)
+				break;
 	}
-	args[argno] = NULL;
 
 #if ENABLE_SELINUX
 	if (current_sid)

loginutils/login.c

@@ -618,7 +618,7 @@ int login_main(int argc UNUSED_PARAM, char **argv)
 	signal(SIGINT, SIG_DFL);
 
 	/* Exec login shell with no additional parameters */
-	run_shell(pw->pw_shell, 1, NULL, NULL);
+	run_shell(pw->pw_shell, 1, NULL);
 
 	/* return EXIT_FAILURE; - not reached */
 }

loginutils/su.c

@@ -31,10 +31,10 @@
 //kbuild:lib-$(CONFIG_SU) += su.o
 
 //usage:#define su_trivial_usage
-//usage:       "[OPTIONS] [-] [USER]"
+//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, run shell as login shell"
+//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"
@@ -81,8 +81,12 @@ int su_main(int argc UNUSED_PARAM, char **argv)
 #endif
 	const char *old_user;
 
+	/* 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);
-	//argc -= optind;
 	argv += optind;
 
 	if (argv[0] && LONE_DASH(argv[0])) {
@@ -162,8 +166,29 @@ int su_main(int argc UNUSED_PARAM, char **argv)
 			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 */
-	run_shell(opt_shell, flags & SU_OPT_l, opt_command, (const char**)argv);
+	run_shell(opt_shell, flags & SU_OPT_l, (const char**)argv);
 
 	/* return EXIT_FAILURE; - not reached */
 }

loginutils/sulogin.c

@@ -89,5 +89,5 @@ int sulogin_main(int argc UNUSED_PARAM, char **argv)
 		shell = pwd->pw_shell;
 
 	/* Exec login shell with no additional parameters. Never returns. */
-	run_shell(shell, 1, NULL, NULL);
+	run_shell(shell, 1, NULL);
 }