Mandatory Access Control with AppArmor
Monitoring Changes
The process described in the preceding section works well for a constant script. If any changes are made to the script or any program, AppArmor will restrict the functioning of the modified code, since those changes were not defined in the profile.
The test script creates, copies, then removes a file. I will now modify the script by adding a soft link to the previously created test.txt
file. If you run the program again, you will notice that the ln -s test.txt
command has been denied permission (Listing 4).
Listing 4
Permission Denied
01 shashwat@shash-nix:~/appar_t$ ./aatest 02 This is a test for apparmor capabilities. 03 ./aatest: line 6: /bin/ln: Permission denied 04 File created 05 File Copied 06 File has been deleted 07 Test successful
To allow this new function, you have to modify the previously created AppArmor profile. Simply, use the command aa-logprof
, and a new Utility access prompt will appear, asking you to grant new additional privileges to the profile (Listing 5).
Listing 5
aa-logprof Command
01 sudo aa-logprof 02 [sudo] password for shashwat: 03 Reading log entries from /var/log/syslog. 04 Updating AppArmor profiles in /etc/apparmor.d. 05 06 Profile: /home/shashwat/appar_t/aatest 07 Execute: /bin/ln 08 Severity: unknown 09 10 (I)nherit / (C)hild / (N)amed / (X) ix On / (D)eny / Abo(r)t / (F)inish
Once you grant the additional privileges, if you run the program again, you will not face any issues.
Predefined Profiles
To explore AppArmor on an app level, I will use a simple example of a generic app with a pre-configured profile and check how insufficient security on a generic app can cause a big vulnerability. Ubuntu used to default to ntpd for syncing time. Newer releases have shifted to timesyncd, but for more precise time changes, NTP is still the preferred service. The profile for NTP doesn't come bundled with AppArmor profiles, but it comes with the NTP package itself.
NTP requires root access with write privileges on root files, along with the need for an open port to get data from an online server. The combination of root privileges and open port access is a recipe for disaster, and in case of a bug, it can provide an opportunity for a system-wide exploit.
AppArmor curtails NTP's capabilities to limit POSIX permission and file access. The profile limits the capability to write access to a limited number of files and practically creates a whitelist (Listing 6). Even if the process is compromised, the hacker can only do so much, and the scope of the exploits is limited to permissions granted within the profile; in this case, it is limited to certain time-related files.
Listing 6
NTP Profile
01 capability ipc_lock, 02 capability net_bind_service, 03 capability setgid, 04 capability setuid, 05 capability sys_chroot, 06 capability sys_resource, 07 capability sys_time, 08 capability sys_nice, 09 10 /var/lib/ntp/*drift rw, 11 /var/lib/ntp/*drift.TEMP rw, 12 /var/log/ntp w, 13 /var/log/ntp.log w, 14 /var/log/ntpd w,
Removing Profiles
In most cases, it is not advisable to recuse an app or script, but if the need arises, AppArmor provides a provision to circumvent the scanning process. To remove a profile, you can either delete the profile present in /etc/apparmor.d/
or put the profile or its soft link under /etc/apparmor.d/disable
.
After you make the change, notify AppArmor using the following command:
apparmor_parser -R profile_path
To remove the ntpd process from the scanning, use the following command:
apparmor_parse -R /etc/apparmor.d/usr.sbin.ntpd
If you are deleting a profile, make sure you restart the AppArmor process.
« Previous 1 2 3 Next »
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Linux Servers Targeted by Akira Ransomware
A group of bad actors who have already extorted $42 million have their sights set on the Linux platform.
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.
-
XZ Gets the All-Clear
The back door xz vulnerability has been officially reverted for Fedora 40 and versions 38 and 39 were never affected.
-
Canonical Collaborates with Qualcomm on New Venture
This new joint effort is geared toward bringing Ubuntu and Ubuntu Core to Qualcomm-powered devices.
-
Kodi 21.0 Open-Source Entertainment Hub Released
After a year of development, the award-winning Kodi cross-platform, media center software is now available with many new additions and improvements.
-
Linux Usage Increases in Two Key Areas
If market share is your thing, you'll be happy to know that Linux is on the rise in two areas that, if they keep climbing, could have serious meaning for Linux's future.
-
Vulnerability Discovered in xz Libraries
An urgent alert for Fedora 40 has been posted and users should pay attention.
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.
-
New Pentesting Distribution to Compete with Kali Linux
SnoopGod is now available for your testing needs