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Application isolation with AppArmor – part III October 18, 2013

Posted by jdstrand in canonical, security, ubuntu.
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Last time I discussed AppArmor, I gave an overview of how AppArmor is used in Ubuntu. With the release of Ubuntu 13.10, a number of features have been added:

  • Support for fine-grained DBus mediation for bus, binding name, object path, interface and member/method
  • The return of named AF_UNIX socket mediation
  • Integration with several services as part of the ApplicationConfinement work in support of click packages and the Ubuntu appstore
  • Better support for policy generation via the aa-easyprof tool and apparmor-easyprof-ubuntu policy
  • Native AppArmor support in Upstart

DBus mediation

 
Prior to Ubuntu 13.10, access to the DBus system bus was on/off and there was no mediation of the session bus or any other DBus buses, such as the accessibility bus. 13.10 introduces fine-grained DBus mediation. In a nutshell, you define ‘dbus’ rules in your AppArmor policy just like any other rules. When an application that is confined by AppArmor uses DBus, the dbus-daemon queries the kernel on if the application is allowed to perform this action. If it is, DBus proceeds normally, if not, DBus denies the access and logs it to syslog. An example denial is:
 
Oct 18 16:02:50 localhost dbus[3626]: apparmor="DENIED" operation="dbus_method_call" bus="session" path="/ca/desrt/dconf/Writer/user" interface="ca.desrt.dconf.Writer" member="Change" mask="send" name="ca.desrt.dconf" pid=30538 profile="/usr/lib/firefox/firefox{,*[^s][^h]}" peer_pid=3927 peer_profile="unconfined"

We can see that firefox tried to access gsettings (dconf) but was denied.

DBus rules are a bit more involved than most other AppArmor rules, but they are still quite readable and understandable. For example, consider the following rule:
 
dbus (send)
   bus=session
   path=/org/freedesktop/DBus
   interface=org.freedesktop.DBus
   member=Hello
   peer=(name=org.freedesktop.DBus),

This rule says that the application is allowed to use the ‘Hello’ method on the ‘org.freedesktop.DBus’ interface of the ‘/org/freedesktop/DBus’ object for the process bound to the ‘org.freedesktop.DBus’ name on the ‘session’ bus. That is fine-grained indeed!

However, rules don’t have to be that fine-grained. For example, all of the following are valid rules:
 
dbus,
dbus bus=accessibility,
dbus (send) bus=session peer=(name=org.a11y.Bus),

Couple of things to keep in mind:

  • Because dbus-daemon is the one performing the mediation, DBus denials are logged to syslog and not kern.log. Recent versions of Ubuntu log kernel messages to /var/log/syslog, so I’ve gotten in the habit of just looking there for everything
  • The message content of DBus traffic is not examined
  • The userspace tools don’t understand DBus rules yet. That means aa-genprof, aa-logprof and aa-notify don’t work with these new rules. The userspace tools are being rewritten and support will be added in a future release.
  • The less fine-grained the rule, the more access is permitted. So ‘dbus,’ allows unrestricted access to DBus.
  • Responses to messages are implicitly allowed, so if you allow an application to send a message to a service, the service is allowed to respond without needing a corresponding rule.
  • dbus-daemon is considered a trusted helper (it integrates with AppArmor to enforce the mediation) and is not confined by default.

As a transitional step, existing policy for packages in the Ubuntu archive that use DBus will continue to have full access to DBus, but future Ubuntu releases may provide fine-grained DBus rules for this software. See ‘man 5 apparmor.d’ for more information on DBus mediation and AppArmor.

Application confinement

 
Ubuntu will support an app store model where software that has not gone through the traditional Ubuntu archive process is made available to users. While this greatly expands the quantity of quality software available to Ubuntu users, it also introduces new security risks. An important part of addressing these risks is to run applications under confinement. In this manner, apps are isolated from each other and are limited in what they can do on the system. AppArmor is at the heart of the Ubuntu ApplicationConfinement story and is already working on Ubuntu 13.10 for phones in the appstore. A nice introduction for developers on what the Ubuntu trust model is and how apps work within it can be found at http://developer.ubuntu.com.

In essence, a developer will design software with the Ubuntu SDK, then declare what type of application it is (which determines the AppArmor template to use), then declares any addition policy groups that the app needs. The templates and policy groups define AppArmor file, network, DBus and anything other rules that are needed. The software is packaged as a lightwight click package and when it is installed, an AppArmor click hook is run which creates a versioned profile for the application based on the templates and policy groups. On Unity 8, application lifecycle makes sure that the app is launched under confinement via an upstart job. For other desktop environments, a desktop file is generated in ~/.local/share/applications that prepends ‘aa-exec-click’ to the Exec line. The upstart job and ‘aa-exec-click’ not only launch the app under confinement, but also setup the environment (eg, set TMPDIR to an application specific directory). Various APIs have been implemented so apps can access files (eg, Pictures via the gallery app), connect to services (eg, location and online accounts) and work within Unity (eg, the HUD) safely and in a controlled and isolated manner.

The work is not done of course and serveral important features need to be implemented and bugs fixed, but application confinement has already added a very significant security improvement on Ubuntu 13.10 for phones.

14.04

As mentioned, work remains. Some of the things we’d like to do for 14.04 include:

  • Finishing IPC mediation for things like signals, networking and abstract sockets
  • Work on APIs and AppArmor integration of services to work better on the converged device (ie, with traditional desktop applications)
  • Work with the upstream kernel on kdbus so we are ready for when that is available
  • Finish the LXC stacking work to allow different host and container policy for the same binary at the same time
  • While Mir already handles keyboard and mouse sniffing, we’d like to integrate with Mir in other ways where applicable (note, X mediation for keyboard/mouse sniffing, clipboard, screen grabs, drag and drop, and xsettings is not currently scheduled nor is wayland support. Both are things we’d like to have though, so if you’d like to help out here, join us on #apparmor on OFTC to discuss how to contribute)

Until next time, enjoy!

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