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Application isolation with AppArmor – part II February 8, 2013

Posted by jdstrand in canonical, security, ubuntu.
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Last time I discussed AppArmor, gave some background and some information on how it can be used. This time I’d like to discuss how AppArmor is used within Ubuntu. The information in this part applies to Ubuntu 12.10 and later, and unless otherwise noted, Ubuntu 12.04 LTS (and because we also push our changes into Debian, much of this will apply to Debian as well).

/etc/apparmor.d

  • Ubuntu follows the upstream policy layout and naming conventions and uses the abstractions extensively. A few abstractions are Ubuntu-specific and they are prefixed with ‘ubuntu’. Eg /etc/apparmor.d/abstractions/ubuntu-browsers.
  • Ubuntu encourages the use of the include files in /etc/apparmor.d/local in any shipped profiles. This allows administrators to make profile additions and apply overrides without having to change the shipped profile (will need to reload the profile with apparmor_parser, see /etc/apparmor.d/local/README for more information)
  • All profiles in Ubuntu use ‘#include <tunables/global>’, which pulls in a number of tunables: ‘@{PROC}’ from ‘tunables/proc’, ‘@{HOME}’ and ‘@{HOMEDIRS}’ from ‘tunables/home’ and @{multiarch} from ‘tunables/multiarch’
  • In addition to ‘tunables/home’, Ubuntu utilizes the ‘tunables/home.d/ubuntu’ file so that ‘@{HOMEDIRS}’ is preseedable at installation time, or adjustable via ‘sudo dpkg-reconfigure apparmor’
  • Binary caches in /etc/apparmor.d/cache are used to speed up boot time.
  • Ubuntu uses the /etc/apparmor.d/disable and /etc/apparmor.d/force-complain directories. Touching (or symlinking) a file with the same name as the profile in one of these directories will cause AppArmor to either skip policy load (eg disable/usr.sbin.rsyslogd) or load in complain mode (force-complain/usr.bin.firefox)

Boot

Policy load happens in 2 stages during boot:

  1. within the job of a package with an Upstart job file
  2. via the SysV initscript (/etc/init.d/apparmor)

For packages with an Upstart job and an AppArmor profile (eg, cups), the job file must load the AppArmor policy prior to execution of the confined binary. As a convenience, the /lib/init/apparmor-profile-load helper is provided to simplify Upstart integration. For packages that ship policy but do not have a job file (eg, evince), policy must be loaded sometime before application launch, which is why stage 2 is needed. Stage 2 will (re)load all policy. Binary caches are used in both stages unless it is determined that policy must be recompiled (eg, booting a new kernel).

dhclient is a corner case because it needs to have its policy loaded very early in the boot process (ie, before any interfaces are brought up). To accommodate this, the /etc/init/network-interface-security.conf upstart job file is used.

For more information on why this implementation is used, please see the upstart mailing list.

Packaging

AppArmor profiles are shipped as Debian conffiles (ie, the package manager treats them specially during upgrades). AppArmor profiles in Ubuntu should use an include directive to include a file from /etc/apparmor.d/local so that local site changes can be made without modifying the shipped profile. When a package ships an AppArmor profile, it is added to /etc/apparmor.d then individually loaded into the kernel (with caching enabled). On package removal, any symlinks/files in /etc/apparmor.d/disable, /etc/apparmor.d/force-complain and /etc/apparmor.d are cleaned up. Developers can use dh_apparmor to aid in shipping profiles with their packages.

Profiles and applications

Ubuntu ships a number of AppArmor profiles. The philosophy behind AppArmor profiles on Ubuntu is that the profile should add a meaningful security benefit while at the same time not introduce regressions in default or common functionality. Because it is all too easy for a security mechanism to be turned off completely in order to get work done, Ubuntu won’t ship overly restrictive profiles. If we can provide a meaningful security benefit with an AppArmor profile in the default install while still maintaining functionality for the vast majority of users, we may ship an enforcing profile. Unfortunately, because applications are not designed to run under confinement or are designed to do many things, it can be difficult to confine these applications while still maintaining usability. Sometimes we will ship disabled-by-default profiles so that people can opt into them if desired. Usually profiles are shipped in the package that provides the confined binary (eg, tcpdump ships its own AppArmor profile). Some in progress profiles are also offered in the apparmor-profiles package and are in complain mode by default. When filing AppArmor bugs in Ubuntu, it is best to file the bug against the application that ships the profile.

In addition to shipped profiles, some applications have AppArmor integration built in or have AppArmor confinement applied in a non-standard way.

Libvirt

An AppArmor sVirt driver is provided and enabled by default for libvirt managed QEMU virtual machines. This provides strong guest isolation and userspace host protection for virtual machines. AppArmor profiles are dynamically generated in /etc/apparmor.d/libvirt and usually you won’t have to worry about AppArmor confinement. If needed, profiles for the individual machines can be adjusted in /etc/apparmor.d/libvirt/libvirt-<uuid> or in all virtual machines in /etc/apparmor.d/abstractions/libvirt-qemu. See /usr/share/doc/libvirt-bin/README.Debian.gz for details.

LXC

LXC in Ubuntu uses AppArmor to help make sure files in the container cannot access security-sensitive files on the host. lxc-start is confined by its own restricted profile which allows mounting in the container’s tree and, just before executing the container’s init, transitioning to the container’s own profile. See LXC in precise and beyond for details. Note, currently the libvirt-lxc sVirt driver does not have AppArmor support (confusingly, libvirt-lxc is a different project than LXC, but there are longer term plans to integrate the two and/or add AppArmor support to libvirt-lxc).

Firefox

Ubuntu ships a disabled-by-default profile for Firefox. While it is known to work well in the default Ubuntu installation, Firefox, like all browsers, is a very complex piece of software that can do much more than simply surf web pages so enabling the profile in the default install could affect the overall usability of Firefox in Ubuntu. The goals of the profile are to provide a good usability experience with strong additional protection. The profile allows for the use of plugins and extensions, various helper applications, and access to files in the user’s HOME directory, removable media and network filesystems. The profile prevents execution of arbitrary code, malware, reading and writing to sensitive files such as ssh and gpg keys, and writing to files in the user’s default PATH. It also prevents reading of system and kernel files. All of this provides a level of protection exceeding that of normal UNIX permissions. Additionally, the profile’s use of includes allows for great flexibility for tightly confining firefox. /etc/apparmor.d/usr.bin.firefox is a very restricted profile, but includes both /etc/apparmor.d/local/usr.bin.firefox and /etc/apparmor.d/abstractions/ubuntu-browsers.d/firefox. /etc/apparmor.d/abstractions/ubuntu-browsers.d/firefox contains other include files for tasks such as multimedia, productivity, etc and the file can be manipulated via the aa-update-browser command to add or remove functionality as needed.

Chromium

A complain-mode profile for chromium-browser is available in the apparmor-profiles package. It uses the same methodology as the Firefox profile (see above) with a strict base profile including /etc/apparmor.d/abstractions/ubuntu-browsers.d/chromium-browser whichcan then include any number of additional abstractions (also configurable via aa-update-browser).

Lightdm guest session

The guest session in Ubuntu is protected via AppArmor. When selecting the guest session, Lightdm will transition to a restrictive profile that disallows access to others’ files.

libapache2-mod-apparmor

The libapache2-mod-apparmor package ships a disabled-by-default profile in /etc/apparmor.d/usr.lib.apache2.mpm-prefork.apache2. This profile provides a permissive profile for Apache itself but allows the administrator to add confinement policy for individual web applications as desired via AppArmor’s change_hat() mechanism (note, apache2-mpm-prefork must be used) and an example profile for phpsysinfo is provided in the apparmor-profiles package. For more information on how to use AppArmor with Apache, please see /etc/apparmor.d/usr.lib.apache2.mpm-prefork.apache2 as well as the upstream documentation.

PAM

AppArmor also has a PAM module which allows great flexibility in setting up policies for different users. The idea behind pam_apparmor is simple: when someone uses a confined binary (such as login, su, sshd, etc), that binary will transition to an AppArmor role via PAM. Eg, if ‘su’ is configured for use with pam_apparmor, when a user invokes ‘su’, PAM is consulted and when the PAM session is started, pam_apparmor will change_hat() to a hat that matches the username, the primary group, or the DEFAULT hat (configurable). These hats (typically) provide rudimentary policy which declares the transition to a role profile when the user’s shell is started. The upstream documentation has an example of how to put this all together for ‘su’, unconfined admin users, a tightly confined user, and somewhat confined default users.

aa-notify

aa-notify is a very simple program that can alert you when there are denials on the system. aa-notify will report any new AppArmor denials by consulting /var/log/kern.log (or /var/log/audit/audit.log if auditd is installed). For desktop users who install apparmor-notify, aa-notify is started on session start via /etc/X11/Xsession.d/90apparmor-notify and will watch the logs for any new denials and report them via notify-osd. Server users can add something like ‘/usr/bin/aa-notify -s 1 -v’ to their shell startup files (eg, ~/.profile) to show any AppArmor denials within the last day whenever they login. See ‘man 8 aa-notify’ for details.

Current limitations

For all AppArmor can currently do and all the places it is used in Ubuntu, there are limitations in AppArmor 2.8 and lower (ie, what is in Ubuntu and other distributions). Right now, it is great for servers, network daemons and tools/utilities that process untrusted input. While it can provide a security benefit to client applications, there are currently a number of gaps in this area:

  • Access to the DBus system bus is on/off and there is no mediation of the session bus or any other buses that rely on Unix abstract sockets, such as the accessibility bus
  • AppArmor does not provide environment filtering beyond having the ability to clear the very limited set of glibc secure-exec variables
  • Display management mediation is not present (eg, it doesn’t protect against X snooping)
  • Networking mediation is too coarse-grained (eg you can allow ‘tcp’ but cannot restrict the binding port, integrate with a firewall or utilize secmark)
  • LXC/containers support is functional, but not complete (eg, it doesn’t allow different host and container policy for the same binary at the same time)
  • AppArmor doesn’t currently integrate with other client technologies that might be useful (eg gnome-keyring, signon/gnome-online-accounts and gsettings) and there is no facility to dynamically update a profile via a user prompt like a file/open dialog

Next time I’ll discuss ongoing and future work to address these limitations.

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