Application isolation with AppArmor – part I

A lot of exciting work has been going on with AppArmor and this multipart series will discuss where AppArmor is now, how it is currently used in Ubuntu and how it fits into the larger application isolation story moving forward.

Brief History and Background

AppArmor is a Mandatory Access Control (MAC) system which is a Linux Security Module (LSM) to confine programs to a limited set of resources. AppArmor’s security model is to bind access control attributes to programs rather than to users. AppArmor confinement is provided via profiles loaded into the kernel. AppArmor profiles can be in one of two modes: enforcement and complain. Profiles loaded in enforcement mode will result in enforcement of the policy defined in the profile as well as reporting policy violation attempts (either via syslog or auditd) such that what is not allowed in policy is denied. Profiles in complain mode will not enforce policy but instead report policy violation attempts. AppArmor is typically deployed on systems as a targeted policy where only some (eg high risk) applications have an AppArmor profile defined, but it also supports system wide policy.

Some defining characteristics of AppArmor are that it:

• is root strong
• is path-based
• allows for mixing of enforcement and complain mode profiles
• uses include files to ease development
• is very lightweight in terms of resources
• is easy to learn
• is relatively easy to audit

AppArmor is an established technology first seen in Immunix and later integrated into SUSE and Ubuntu and their derivatives. AppArmor is also available in Debian, Mandriva, Arch, PLD, Pardus and others. Core AppArmor functionality is in the mainline Linux kernel starting with 2.6.36. AppArmor maintenance and development is ongoing.

An example AppArmor profile

Probably the easiest way to describe what AppArmor does and how it works is to look at an example, in this case the profile for tcpdump on Ubuntu 12.04 LTS:

#include <tunables/global>
/usr/sbin/tcpdump {
  #include <abstractions/base>
  #include <abstractions/nameservice>
  #include <abstractions/user-tmp>
  capability net_raw,
  capability setuid,
  capability setgid,
  capability dac_override,

  network raw,
  network packet,

  # for -D
  capability sys_module,
  @{PROC}/bus/usb/ r,
  @{PROC}/bus/usb/** r,

  # for finding an interface
  @{PROC}/[0-9]*/net/dev r,
  /sys/bus/usb/devices/ r,
  /sys/class/net/ r,
  /sys/devices/**/net/* r,

  # for tracing USB bus, which libpcap supports
  /dev/usbmon* r,
  /dev/bus/usb/ r,
  /dev/bus/usb/** r,

  # for init_etherarray(), with -e
  /etc/ethers r,

  # for USB probing (see libpcap-1.1.x/
  # pcap-usb-linux.c:probe_devices())
  /dev/bus/usb/**/[0-9]* w,

  # for -z
  /bin/gzip ixr,
  /bin/bzip2 ixr,

  # for -F and -w
  audit deny @{HOME}/.* mrwkl,
  audit deny @{HOME}/.*/ rw,
  audit deny @{HOME}/.*/** mrwkl,
  audit deny @{HOME}/bin/ rw,
  audit deny @{HOME}/bin/** mrwkl,
  owner @{HOME}/ r,
  owner @{HOME}/** rw,

  # for -r, -F and -w
  /**.[pP][cC][aA][pP] rw,

  # for convenience with -r (ie, read 
  # pcap files from other sources)
  /var/log/snort/*log* r,

  /usr/sbin/tcpdump r,

  # Site-specific additions and overrides. See 
  # local/README for details.
  #include <local/usr.sbin.tcpdump>
}

This profile is representative of traditional AppArmor profiling for a program that processes untrusted input over the network. As can be seen:

• profiles are simple text files
• comments are supported in the profile
• absolute paths as well as file globbing can be used when specifying file access
• various access controls for files are present. From the profile we see ‘r’ (read), ‘w’ (write), ‘m’ (memory map as executable), ‘k’ (file locking), ‘l’ (creation hard links), and ‘ix’ to execute another program with the new program inheriting policy. Other access rules also exists such as ‘Px’ (execute under another profile, after cleaning the environment), ‘Cx’ (execute under a child profile, after cleaning the environment), and ‘Ux’ (execute unconfined, after cleaning the environment)
• access controls for capabilities are present
• access controls for networking are present
• explicit deny rules are supported, to override other allow rules (eg access to @{HOME}/bin/bad.sh is denied with auditing due to ‘audit deny @{HOME}/bin/** mrwkl,’ even though general access to @{HOME} is permitted with ‘@{HOME}/** rw,’)
• include files are supported to ease development and simplify profiles (ie #include <abstractions/base>, #include <abstractions/nameservice>, #include <abstractions/user-tmp>, #include <local/usr.sbin.tcpdump>)
• variables can be defined and manipulated outside the profile (#include <tunables/global> for @{PROC} and @{HOME})
• AppArmor profiles are fairly easy to read and audit

Complete information on the profile language can be found in ‘man 5 apparmor.d’ as well as the AppArmor wiki.

Updating and creating profiles

AppArmor uses the directory heirarchy as described in policy layout, but most of the time, you are either updating an existing profile or creating a new one and so the files you most care about are in /etc/apparmor.d.

The AppArmor wiki has a lot of information on debugging and updating existing profiles. AppArmor denials are logged to /var/log/kern.log (or /var/log/audit/audit.log if auditd is installed). If an application is misbehaving and you think it is because of AppArmor, check the logs first. If there is an AppArmor denial, adjust the policy in /etc/apparmor.d, then reload the policy and restart the program like so:

$ sudo apparmor_parser -R /etc/apparmor.d/usr.bin.foo
$ sudo apparmor_parser -a /etc/apparmor.d/usr.bin.foo
$ <restart application>

Oftentimes it is enough to just reload the policy without unloading/loading the profile or restarting the application:

$ sudo apparmor_parser -r /etc/apparmor.d/usr.bin.foo

Creating a profile can be done either with tools or by hand. Due to the current pace and focus of development, the tools are somewhat behind and lack some features. It is generally recommended that you profile by hand instead.

When profiling, keep in mind that:

• AppArmor provides an additional permission check to traditional Discretionary Access Controls (DAC). DAC is always checked in addition to the AppArmor permission checks. As such, AppArmor cannot override DAC to provide more access than what would be normally allowed.
• AppArmor normalizes path names. It resolves symlinks and considers each hard link as a different access path.
• AppArmor evaluates file access by pathname rather than using on disk labeling. This eases profiling since AppArmor handles all the labelling behind the scenes.
• Deny rules are evaluated after allow rules and cannot be overridden by an allow rule.
• Creation of files requires the create permission (implied by w) on the path to be created. Separate rules for writing to the directory of where the file resides are not required. Deletion works like creation but requires the delete permission (implied by w). Copy requires ‘r’ of the source with create and write at the destination (implied by w). Move is like copy, but also requires delete at source.
• The profile must be loaded before an application starts for the confinement to take effect. You will want to make sure that you load policy during boot before any confined daemons or applications.
• The kernel will rate limit AppArmor denials which can cause problems while profiling. You can avoid this be installing auditd or by adjusting rate limiting in the kernel:

  $ sudo sysctl -w kernel.printk_ratelimit=0

Resources

There is a lot of documentation on AppArmor (though some is still in progress):

• AppArmor upstream documentation
• Debugging AppArmor in Ubuntu
• apparmor.d man page
• apparmor man page
• Ubuntu AppArmor Documentation

Next time I’ll discuss the specifics of how Ubuntu uses AppArmor in the distribution.

Thanks to Seth Arnold and John Johansen for their review.