Page 2 of 2 The chroot() function can be a powerful mechanism to secure your system, but only if
used correctly. Anton provides a good foundation for implementing it in your programs and services running on your system.
Chroot command and chroot system call might sound
like a good security
measure - one command executed and plain old UNIX "cd /" no longer
transports you to a root directory of the system. Instead, you are
bound to stay in the restricted part of the filesystem, surrounded
only by files chosen by a paranoid system administrator. In fact, that
is how it should be.
Is it possible to break out of chroot solitary confinement? Yes, if
certain conditions are met. In this paper we will analyze what chroot
is good for and also what chroot is bad for.
First, how does it work? When one types /sbin/chroot directory_name
on the UNIX system command line one sees that the new root is now
'directory_name' (the /bin/ls / command produces the listing of
files from 'directory_name' presuming that you have an 'ls' command
located within your new root). Chroot shell command changes the root
directory for a process, goes into this directory and then starts a
shell or a user-specified command.
Chroot command uses a chroot() system call. The command and the system
call have an important difference between them: unlike the shell
command, chroot() call does not change your working directory to the
one inside chrooted jail. The source of chroot.c (shell command, in
Linux part of sh-utils) shows the following sequence of system calls:
chroot (argv[1]);
chdir ("/");
As will be seen further, it will allow for easy chroot jail breaking.
Chroot is often used as a security measure. If one has ever used an
anonymous ftp server, one has used chroot. Ftp server chroots itself
into a special directory upon the anonymous ftp login. DNS (Domain
Name System) daemon bind is often chrooted as well. People also
suggested chrooting telnet/ssh remote shell users into their
corresponding home directories, so they can only update their web
pages. Web servers can be run chrooted too. Smap secure email wrapper
from FWTK firewall tool kit runs chrooted to the mail spool directory.
When chroot is implemented, programs running inside cannot access any
system resources on the outside. Thus all system libraries,
configuration files and even devices files should be recreated within
the chroot jail.
What daemons can be chrooted? If a daemon has to access files that are
not easily collectible in one place, chrooting it will be hard. For
example, sendmail needs mail spool (/var/spool/mail), other files in
spool (such as mqueue), user's home directories (to check for .forward
files) and system configuration files in /etc. There is no place on
the filesystem where sendmail can be effectively confined. Of course,
some makeshift solution is possible, but it is not clear that it will
add to security, but rather cause it to lax. If sendmail functionality
is separated into spool daemon and mail transfer program (like done in
FWTK smap and smapd), than chrooting is entirely possible.
Chrooting shell users is possible if there is a business need to keep
them in some particular directory. Suggestions for doing this with
ssh2 are provided in the SSH FAQ, and for OpenSSH in the Linux From Scratch archives and
SF Linux archives.
However, it might involve copying multiple system libraries, files and
other resources such as Linux Pluggable Authentication Modules (PAM)
architecture, used by most modern Linux distributions.
Anything else such as bind, apache, squid can be chrooted, but
sometimes the benefits are unclear, especially for daemons that run as
root. This URL (http://www.networkdweebs.com/chroot.html) provides a
nice list of daemons that its author chrooted successfully.
"What daemon should be chrooted?" is an entirely different question
from "What daemons can be chrooted?" Before we answer it, lets analyze
how attackers break out of chroot.
First, the more software is deployed within chroot environment, the
more dangerous it becomes, since it is hard to keep track of programs
that can be used by the attacker to elevate permission and escape.
Second, the number of ways that root user can break out of chroot is
huge. Starting from simple use of a chroot() call with no chdir() [see
code below] to esoteric methods as the creation of your own /dev/hda
or /dev/kmem devices, injection code into the running kernel
(http://www.big.net.au/~silvio/runtime-kernel-kmem-patching.txt),
using open directory handles outside chroot or chroot-breaking buffer
overflows. While system capabilities can be used to render inoperable
many of these methods, new ones will likely be found by smart
attackers.
Sample code to break out of chroot:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
int main(void)
{
int i;
mkdir("breakout", 0700);
chroot("breakout");
for (i = 0; i < 100; i++)
chdir("..") ;
chroot(".");
execl("/bin/sh", "/bin/sh",NULL);
}
compile statically (using "gcc -static") and run within chrooted
directory (after doing "chroot ." or similar from shell prompt) to
escape.
Third, if there is no root user defined within the chroot environment,
no SUID binaries, no devices, and the daemon itself dropped root
privileges right after calling chroot() call (like in the code below),
breaking out of chroot appears to be impossible. In other words, if
there is no way to gain root shell or perform actions that only root
can usually perform (e.g. create devices, or access raw memory)
breaking chroot is not clearly possible. Ideally, if the custom
software uses chroot for security the sequence of calls should be:
chdir("/home/safedir");
chroot("/home/safedir");
setuid(500);
Keep in mind, that after these lines are executed there will be no way
for the program to regain root privileges.
Fourth, in some cases attackers might not be able to break (i.e. run
processes outside of chrooted directory), but instead will be able to
somewhat affect such processes. For example, if bind is chrooted,
several devices should be created. One of them is /dev/log, necessary
for logging bind messages into the regular system logs. By crafting a
malicious log message and sending it into /dev/log from within the
chrooted directory attacker will influence the behavior of syslog
daemon running outside the chroot. If there is a buffer overflow in
syslog (which runs as root), additional privileges can be obtained.
What daemons can be chrooted but with no valuable security outcome? In
light of the above, chrooting programs that do not drop root
privileges while running, or programs that provide root shell access
(sshd, telnet with a root account within chrooted directory) does not
provide any extra security.
To conclude, chroot() is a good way to increase the security of the
software provided that secure programming guidelines are utilized and
chroot() system call limitations are taken into account. Chrooting
will prevent an attacker from reading files outside the chroot jail
and will prevent many local UNIX attacks (such as SUID abuse and /tmp
race conditions).
References
- Secure Programming HOWTO
-- This book provides a set of design and implementation guidelines for
writing secure programs for Linux and Unix systems. Such programs
include
application programs used as viewers of remote data, web applications
(including CGI scripts), network servers, and setuid/setgid programs.
Specific
guidelines for C, C++, Java, Perl, PHP, Python, TCL, and Ada95 are
included.
- Chroot-BIND HOWTO
-- This document describes installing the BIND 8 nameserver to run in a
chroot jail and as a non-root user, to provide added security and
minimise the
potential effects of a security compromise.
- Linux Security QuickReference Card
-- This Quick Reference Guide is intended to provide a starting point
for improving the security of your system. Contained within include
references to security resources around the net, tips on securing your
Linux box, and general security information.
- Linux Security Documentation -- Linux Security documentation starting point.
- WWW Security FAQ -- Steps on chrooting the Apache Web server.
- Securing and Configuring Postfix HOWTO -- General steps on securing the postfix mail
server, including how to configure it to run in a chroot.
- Debian Security HOWTO -- Debian security checklist including programs to be chrooted.
- Chrooting All Services in Linux -- Good starting point to
explain the basics of using
chroot() but be sure to heed
the instructions provided in this article before chrooting all services
running on your system.
About the Author
Anton Chuvakin, Ph.D. is a Senior Security Analyst with netForensics, a security information
management company that provides real-time network security monitoring
solutions.
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