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Posted Apr 17, 2004
Authored by Bugghy | Site vaida.bogdan.googlepages.com

Mandatory Access Control tutorial to correctly and safely install, configure, and use MAC policies. Written for FreeBSD users. Full step by step details given with explanations.

tags | paper
systems | unix, freebsd
SHA-256 | 207406b5776a26ca845f149070406f74a77aefe86a8bb83717528be689f9754b


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| TrustedBSD Mandatory Access Control framework |

Written by: Bugghy
E-mail: bugghy@rootshell.be
URL: rootshell.be/~bugghy

Comments/errors/bugs/jokes > bugghy@rootshell.be


1. Introduction
2. Simple policies
3. Advanced policies
4. Full example
5. Notes
6. Link of the day
7. Last words


1. Introduction

I've written this tutorial because FreeBSD's handbook
(http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/index.html) didn't offer enough
information on how to correctly/safely install/configure/use MAC on a workstation. Please
refer to the man pages and handbook for detailed explanation on how everything works.

MAC (mandatory access control) is used to introduce system security modules in order to fortify
the default lack of security policies in most unices. This paper discusses the
instalation/configuration and basic use of the following policies:

mac_seeotheruids, mac_bsdextended, mac_ifoff, mac_portacl, mac_test, mac_none, mac_stub
mac_partition, mac_mls, mac_biba, mac_lomac

Compile your kernel with the new policies by adding the following line in the kernel conf. file:
options MAC

2. Simple policies

These policies work without the use of the labelling feature:

A. mac_seeotheruids (man mac_seeotheruids)

$ ps ax |wc -l
# kldload mac_seeotheruids
# sysctl security.mac.seeotheruids.enabled=1
^^^ this is the default behaviour (use sysctl.conf to make permanent changes)
$ ps ax | wc -l

You can exempt a groupd ID from the policy:

$ id -G
$ ps ax | wc -l
# sysctl security.mac.seeotheruids.specificgid_enabled=1
# sysctl security.mac.seeotheruids.specificgid=2000
$ id -G
$ ps ax | wc -l

Or even let users see their primary groups processes: (remember to set seeotheruids.specificgid_enabled to 0)

# sysctl security.mac.seeotheruids.primarygroup_enabled=1
$ id -G
# ps ax | wc -l
(my 30 processes + 5 others owned by the same group)

B. mac_bsdextended

Ever used ipfw ? This is fsfw (file system firewall).

# kldload mac_bsdextended
# ugidfw list
0 slots, 0 rules

# cat rc.mac_bsdextended
while [ ${i} -le 4 ]
ugidfw remove ${i}
i=`expr ${i} + 1`
ugidfw set 0 subject uid new object uid root mode rsx
ugidfw set 1 subject uid new object gid wheel mode rsx
# yes, /bin/ls works now
ugidfw set 2 subject uid new object uid bugghy mode n
ugidfw set 3 subject uid new object gid bugghy mode n
# owned by bugghy == private :)
ugidfw set 4 subject uid new object gid nobody mode n
# new can't "locate | grep /home/bugghy" anymore <-- BIG security risk
# you can deny other groups (from /etc/group) or users (/etc/passwd)

$ id -u -nr
$ echo sex > /tmp/bug; chmod a+rwx /tmp/bug; ls -l /tmp/bug
-rwxrwxrwx 1 bugghy wheel 4 Apr 5 20:05 bug*

$ id -u -nr
$ ls -l /home
ls: bugghy: Permission denied
total 4
drwxr-xr-x 2 new new 512 Mar 28 15:09 new
$ ls /tmp/bug
ls: /tmp/bug: Permission denied

C. mac_ifoff

# kldload mac_ifoff
$ ping -c 1
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=64 time=0.203 ms

--- ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.203/0.203/0.203/0.000 ms
# sysctl security.mac.ifoff.lo_enabled=0
$ ping -c 1
PING ( 56 data bytes

--- ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss

# sysctl security.mac.ifoff.other_enabled=1
^^^ we enable external interface (which is disabled by default)
You can write a script that runs aide (with a proper config file) and if it finds modified
files in the protected directories it won't allow external network traffic.
# sysctl security.mac.ifoff.bpfrecv_enabled=1
^^^ allows Berkeley Packet Filter traffic (man 4 bpf)

D. mac_portacl
To enable mac policies on sockets "sysctl security.mac.enforce_socket=1": (default value)

# kldload mac_portacl

# sysctl net.inet.ip.portrange.reservedlow=0
sysctl net.inet.ip.portrange.reservedlow: 0 -> 0
# sysctl net.inet.ip.portrange.reservedhigh=1000
sysctl net.inet.ip.portrange.reservedhigh: 1023 -> 1000
# sysctl security.mac.portacl.port_high=1010
security.mac.portacl.port_high: 1000 -> 1010

# sysctl security.mac.portacl.suser_exempt=0
^^^ rules apply for root too

$ id -u
$ nc -l -p 1000
Can't grab with bind : Operation not permitted
^^^ the ip.portrange.reservedhigh limit works
$ nc -l -p 1010
Can't grab with bind : Operation not permitted
^^^ the mac.portacl.port_high limit works too

# sysctl security.mac.portacl.rules=uid:2000:tcp:1000,uid:2000:tcp:1010
^^^ we enforce 2 rules (the first tries to bypass ip.portrange.reservedhigh
and the 2nd tries to bypass mac.portacl.port_high)

$ nc -l -p 1000
Can't grab with bind : Permission denied
^^^ mac.portacl allows port 1000 binding while ip.portrange.reservedhigh doesn't
$ nc -l -p 1010
^^^ works due to our firewall rule

NOTE: A basic security policy would be:
# cat rc.mac_portacl
# allow uid 2000 to bind to port 79 and 80

sysctl net.inet.ip.portrange.reservedlow=0
sysctl net.inet.ip.portrange.reservedhigh=50
# first 50 ports are accessible only by root
sysctl security.mac.portacl.port_high=1023
# our policy works for 50 -> 1023
sysctl security.mac.portacl.suser_exempt=1
# root doesn't need the policy
sysctl security.mac.portacl.rules=$rules

E. mac_test

Tests the mac framework, finds corrupt labels amongst other things.

# kldload mac_test
# sysctl security.mac.test
security.mac.test.enabled: 1
security.mac.test.slot: 2
security.mac.test.init_count_bpfdesc: 0
security.mac.test.init_count_cred: 1920
security.mac.test.init_count_devfsdirent: 0
security.mac.test.init_count_ifnet: 0
(big output)

F. mac_none

No effect.

# kldload mac_none

G. mac_stub

Sample policy that does nothing (man 4 mac_stub)

# kldload mac_stub

3. Advanced policies

These policies need labelling. (man 7 maclabel, man 4 mac)

setfmac, setfsmac - set file system labels
setpmac - set process mac
ifconfig - set network interface label
login.conf - set tty/user label

I. login.conf labelling:

Example for the mac_partition and mac_mls policy:

:path=~/bin /bin /usr/bin /usr/local/bin:\
:manpath=/usr/share/man /usr/local/man:\

^^^ We create a new label in login.conf named insecure
(don't forget to run cap_mkdb /etc/login.conf after that)

# pw user mod new -L insecure
^^^ we set the user's label to insecure

II. ifconfig labelling:

# ifconfig rl0 maclabel 'biba/high(low-high)'
^^^ set high for incomming packets and all for outgoing

III. setfmac, setfsmac labelling:

Boot to single user to enable multilabelling: (man 8 tunefs)

# tunefs -l enable /; tunefs -l enable /home

Exit single user and test:

# ls -lZa test
-rw-r--r-- 1 root new mls/low 0 Apr 6 16:01 test
# setfmac mls/equal test
# getfmac test
test: mls/equal
# tail -n 1 mls-policy.txt
/home/new/test mls/high
# setfsmac -f mls-policy.txt test
setfsmac: mls-policy.txt: read 23 specifications
# getfmac test
test: mls/high

We are set:

$ pw user show new | awk -F\: '{ print $5 }'
^^^ user new's label is insecure
$ id -P
new:*:2000:2000:insecure:0:0:User &:/home/new:/bin/sh

A. mac_partition

# kldload mac_partition
# top

$ id -u
$ ps -Za|grep top
^^^ we can't see top as the insecure user

# killall -9 top

# setpmac partition/13 top
^^^ we label top to our label

$ ps -Z
partition/13 4701 v1 SL 0:00.07 -su (sh)
partition/13 4783 v1 RL+ 0:00.00 ps -Z
$ ps -ZU root
partition/13 976 p3 S+ 0:00.02 top
^^^ we can see top now (even if owned by root)

You can disable all services from /etc/rc.conf and make a script to launch them
manually with proper labelling. (Why should an insecure user see cron running?)
Or even mess with the login scripts:

# setpmac partition/50 bash
# id -u
# pw user show root
root:*:0:0::0:0:Charlie &:/root:/usr/local/bin/bash
# ps Zax
partition/50 1136 p3 S 0:00.06 bash
partition/50 1169 p3 R+ 0:00.00 ps Zax
^^^ even root can only see his own partition processes

B. mac_mls

mac_mls prevents the downward flow of information

Set default's label to "mls/equal(equal-equal)" and insecure's label to "mls/5(5-5)"
in /etc/login.conf (Do: cap_mkdb /etc/login.conf). Add "mac_mls_load="YES"" to
/boot/loader.conf. Reboot.

$ id -u
$ getpmac
$ ls -lZ /dev/kmem
ls: /dev/kmem: Permission denied
^^^ filesystem protection is in place

# echo s > test1; echo e > test2; echo x > test3
# getfmac test2
test: mls/equal
# setfmac mls/1 test1; setfmac mls/10 test3
# chown new:new test?

Observation test:

$ ls test?
ls: test3: Permission denied
test1 test2
^^^ we can't observe higher clearance level

Read test:

$ cat test?
cat: test3: Permission denied
^^^ higher clearance level dissallows read

Write test:

$ echo 1 > test1
cannot create test1: Permission denied
$ echo 1 > test2
$ echo 1 > test3

$ cat test?
cat: test3: Permission denied
# cat test3
^^^ we can write to equal or higher, but not lower

NOTE: lower clearance can't observe higher clearance processes
A basic policy would be to enforce mls/high on everything not to be
read (even if it needs to be written) mls/low on everything not to be
written (even if it needs to be read) and mls/equal on the rest. Any
insecure users should be labelled mls/low.

C. mac_biba

mac_biba prevents the upward flow of information

For this, the default label in /etc/login.conf will be "biba/equal(equal-equal)",
insecure's label will be "biba/5". Run "cap_mkdb /etc/login.conf" also add
mac_biba_load="YES" to loader.conf. Reboot.

$ id -u
$ getpmac
$ ls -lZ /dev/kmem
crw-r----- 1 root kmem biba/high 2, 1 Apr 7 08:23 /dev/kmem
^^^ filesystem protection is in place

Let the tests begin:

# echo s > test1; echo e > test2; echo x > test3; echo o > test4; echo r > test5
# getfmac test2
test2: biba/high
# setfmac biba/2 test1; setfmac biba/4 test2; setfmac biba/5 test3; setfmac biba/6 test4; setfmac biba/9 test5
# chown new:new test?

Observation test:

$ ls test?; echo; cat test?
ls: test1: Permission denied
ls: test2: Permission denied
test3 test4 test5

cat: test1: Permission denied
cat: test2: Permission denied
^^^ a higher integrity subject can't observe or read a lower integrity object

Write test:

$ echo 1 > test1
$ echo 1 > test2
$ echo 1 > test3
$ echo 1 > test4
cannot create test4: Permission denied
$ echo 1 > test5
cannot create test5: Permission denied
$ cat test?
cat: test1: Permission denied
cat: test2: Permission denied
^^^ a lower integrity subject can't write to a higher integrity subject

D. mac_lomac (man 4 mac_lomac)

While mac_biba denies access to lower integrity objects, mac_lomac
permits access to them, but downgrades the integrity level thus not
violating the integrity rules. (yes I've taken this from the man page)

See section 5. (Notes) part IV. for details about why I didn't explain this policy.

4. Full example

I. Preparation:

We will use the following policies to build a secure environment on a FreeBSD 5.2.1 workstation:

mac_seeotheruids, mac_partition, mac_mls, mac_biba

We boot in single user mode and "tunefs -l enable" all partitions.

We add the following modules to loader.conf and then reboot:

# tail -n 6 /boot/loader.conf

# kldstat | grep mac
4 1 0xc070d000 7cdc mac_biba.ko
5 1 0xc0715000 7e5c mac_mls.ko
9 1 0xc21e0000 2000 mac_seeotheruids.ko
12 1 0xc21e9000 2000 mac_partition.ko
^^^ Modules are loaded

We edit /etc/login.conf and add the following lines:

# tail -n 25 /etc/login.conf

:path=~/bin /bin /usr/bin /usr/local/bin:\
:manpath=/usr/share/man /usr/local/man:\

We also label the default class in order not to interfere with us:

# cat /etc/login.conf|grep -A 25 "default:\\\\" | grep label

# cap_mkdb /etc/login.conf

# adduser
Username: new
Full name: test user
Uid (Leave empty for default): 2000
Login group [new]:
Login group is new. Invite new into other groups? []:
Login class [default]: insecure
Shell (sh csh tcsh bash nologin) [sh]: bash
Home directory [/home/new]:
Use password-based authentication? [yes]:
Use an empty password? (yes/no) [no]:
Use a random password? (yes/no) [no]: yes
Lock out the account after creation? [no]:
Username : new
Password : <random>
Full Name : test user
Uid : 2000
Class : insecure
Groups : new
Home : /home/new
Shell : /bin/bash
Locked : no
OK? (yes/no): yes
adduser: INFO: Successfully added (new) to the user database.
adduser: INFO: Password for (new) is: VOdCyK11E2p
Add another user? (yes/no): no

# su -s - new
$ id -u
^^^ from now "$" is the new user and "#" is root
$ pw user show new
new:*:2000:2000:insecure:0:0:test user:/home/new:/bin/bash

II. Implementation and tests:

$ getpmac

# setpmac partition/15,mls/equal top
Note: the top process will be killed before we start another top process.

A. mac_seeotheruids test

$ ps Zax
biba/15(15-15),mls/15(15-15),partition/15 1096 #C: S 0:00.03 -su (bash)
biba/15(15-15),mls/15(15-15),partition/15 1101 #C: R+ 0:00.01 ps Zax
^^^ we can't see processes except our own

B. mac_partition test

# sysctl sysctl security.mac.seeotheruids.enabled=0
^^^ it will remain off for the rest of the example

$ ps Zax
biba/equal(low-high),mls/equal(low-high),partition/15 1122 #C: S+ 0:00.02 top
biba/15(15-15),mls/15(15-15),partition/15 1096 #C: S 0:00.05 -su (bash)
biba/15(15-15),mls/15(15-15),partition/15 1123 #C: R+ 0:00.01 ps Zax
^^^ we can now see all processes in our partition (15)

C. mac_biba and mac_mls test

# setpmac partition/15,mls/equal,biba/high\(high-high\) top

$ ps Zax
biba/high(high-high),mls/equal(low-high),partition/15 1251 #C: S+ 0:00.02 top
biba/15(15-15),mls/15(15-15),partition/15 1096 #C: S 0:00.06 -su (bash)
biba/15(15-15),mls/15(15-15),partition/15 1157 #C: R+ 0:00.00 ps Zax
^^^ biba allows us to read higher labelled objects

# setpmac partition/15,mls/equal,biba/low top

$ ps Zax
biba/15(15-15),mls/15(15-15),partition/15 1096 #C: S 0:00.07 -su (bash)
biba/15(15-15),mls/15(15-15),partition/15 1226 #C: R+ 0:00.01 ps Zax
^^^ biba doesn't allow lower labelled objects to be read (mls does!)

$ ifconfig rl0 | grep maclabel
maclabel biba/low(low-low),mls/low(low-low)
$ ping -c 1
PING ( 56 data bytes
ping: sendto: Permission denied
^^^ everyone pings yahoo.com
You can set the default interface label to an insecure one (for testing purposes)
Add security.mac.biba.trust_all_interfaces=1 to sysctl.conf
This is caused due to the default policy label in the biba policy. Taken from:

# ifconfig rl0 maclabel biba/equal\(low-high\),mls/equal\(low-high\)
$ ping -c 1
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=50 time=204.455 ms

--- ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 204.455/204.455/204.455/0.000 ms
^^^ pinging works now

# touch test1 test2 test3 test4 test5
# getfmac test1
test1: biba/equal,mls/equal
# setfmac biba/low test1 test2; setfmac biba/high test4 test5; setfmac mls/low test1 test3; setfmac mls/high test2 test4
^^^ can you keep up? :)
# setfmac mls/equal,biba/equal test3
# getfmac test?
test1: biba/low,mls/low
test2: biba/low,mls/high
test3: biba/equal,mls/equal
test4: biba/high,mls/high
test5: biba/high,mls/equal
# chown new:new test?
^^^ owned by our user

Observation/read test:

$ ls
test1 test2 test3 test4 test5
$ ls test?
ls: test1: Permission denied
ls: test2: Permission denied
ls: test4: Permission denied
test3 test5
^^^ we can't observe pairs (biba/low,mls/low) (biba/low,mls/high) and (biba/high,mls/high)
(and of course, we can't read them)

Writting test:

$ for i in `echo test*`; do echo 1 > $i; done
-su: test1: Permission denied
-su: test4: Permission denied
-su: test5: Permission denied
^^^ we can write to pairs (biba/low,mls/high) and (biba/equal,mls/equal)
$ cat test?
cat: test1: Permission denied
cat: test2: Permission denied
cat: test4: Permission denied
# cat test2
^^^ yep, worked

III. Conclusion:
A good security system will have good biba/lomac/mls policies, see
http://www.watson.org/~robert/freebsd/lomac-policy.contexts for an example.
Download file, edit it to suid your needs and then run:

# setfsmac -ef lomac-policy.contexts /

5. Notes

I. I had problem when unloading module mac_partition after playing with labelling:

module_register_init: MOD_LOAD (mac_partition, 0xc04c3480, 0xc2114e20) error 12

^^^ after this I can't load the module anymore.

II. Running startx as with as mls/equal(equal-equal) (biba/equal, lomac/equal) allows us to su
into a lowclass/highclass user: (run from xterm)

# getpmac
# su - new
$ getpmac
# su -s - new
$ getpmac

III. If subjects can read an object, they can also execute it.
# echo ls > test1; echo ls > test2; echo ls > test3; echo ls > test4; echo ls > test5
# setfmac biba/2 test1; setfmac biba/4 test2; setfmac biba/5 test3; setfmac biba/6 test4; setfmac biba/9 test5
# chmod +x test?

$ getpmac
$ ./test1
./test1: Permission denied
$ ./test2
./test2: Permission denied
$ ./test3
test1 test2 test3 test4 test5
$ ./test4
test1 test2 test3 test4 test5
$ ./test5
test1 test2 test3 test4 test5

IV. mac_lomac
I wasn't able to load this policy, so I couldn't test it.
In login.conf I've set: "lomac/equal" to default class and
"lomac/15" to insecure class. In messages I get:

Apr 7 09:47:12 illusion kernel: Preloaded elf module "/boot/kernel/mac_lomac.ko" at 0xc077a4bc.
Apr 7 09:47:12 illusion kernel: Security policy loaded: TrustedBSD MAC/LOMAC (mac_lomac)

The module is loaded:

# kldstat | grep mac_lomac
6 1 0xc071d000 951c mac_lomac.ko

Files don't have default labelling:

# getfmac /dev/kmem
/dev/kmem: mls/high

And I can't label files:

# setfmac lomac/equal test2; getfmac test2
test2: mls/equal

6. Links of the day

TrustedBSD: http://www.trustedbsd.org
FreeBSD security: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/security.html

7. Last words

This tutorial is in alpha state so please send me comments to bugghy@rootshell.be

- ---------------------
This file is for [of course] informational purposes only. <giggle> I
don't take responsibility for anything anyone does after reading this file.


man pages
http://www.freebsd.org (FBSD site)
trial and error
own experience
my own mind (yeah ... sure)

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