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>

3 files changed, 31 insertions, 6 deletions

diff --git a/loginutils/login.c b/loginutils/login.c
index 94b6c45db..52abc1886 100644
--- a/loginutils/login.c
+++ b/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 */
 }
diff --git a/loginutils/su.c b/loginutils/su.c
index 3c0e8c100..24ffbde86 100644
--- a/loginutils/su.c
+++ b/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 */
 }
diff --git a/loginutils/sulogin.c b/loginutils/sulogin.c
index 6befea933..2e32e2bbd 100644
--- a/loginutils/sulogin.c
+++ b/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);
 }