2.1 Breaking a Novell AppArmor Profile into Its Parts

Novell AppArmor profile components are called Novell AppArmor rules. Currently there are two main types of Novell AppArmor rules, path entries and capability entries. Path entries specify what the process can access in the file system and capability entries provide a more fine-grained control over what a confined process is allowed to do through other system calls that require privileges. Includes are a type of meta rule or directives that pull in path and capability entries from other files.

The easiest way of explaining what a profile consists of and how to create one is to show the details of a sample profile. Consider, for example, the following shortened profile for the program /usr/lib/postfix/flush (for the complete version, refer to /etc/apparmor.d/usr.lib.postfix.flush): 

# profile to confine postfix/flush
#include <tunables/global>

/usr/lib/postfix/flush 
{
  #include <abstractions/base>
  ...
  capability setgid,
  ...
  /usr/lib/postfix/flush                      rix,
  /{var/spool/postfix/,}deferred             r,
  ...
  /{var/spool/postfix/,}flush                 rwl,
  ...
  /{var/spool/postfix/,}incoming              r,
  ...
  /{var/spool/postfix/,}public/qmgr           w,
  /etc/mtab                                  r,
  /etc/postfix/main.cf                        r,
  /etc/postfix/virtual.db                     r,
  @{HOME}/.forward                           r,
  /proc/stat                                  r,
  /proc/sys/kernel/ngroups_max                r,
  /var/spool/postfix/pid/unix.flush           rw,
}

A comment naming the program that is confined by this profile. Always precede comments like this with the # sign.

This loads a file containing variable definitions.

The absolute path to the program that is confined.

The curly braces ({}) serve as a container for include statements of other profiles as well as for path and capability entries.

This directive pulls in components of Novell AppArmor profiles to simplify profiles.

Capability entry statements enable each of the 29 POSIX.1e draft capabilities.

The curly braces ({}) make this rule apply to the path both with and without the content enclosed by the braces.

A path entry specifying what areas of the file system the program can access. The first part of a path entry specifies the absolute path of a file (including regular expression globbing) and the second part indicates permissible access modes (r for read, w for write, and x for execute). A whitespace of any kind (spaces or tabs) can precede pathnames or separate the pathname from the access modes. Spaces between the access mode and the trailing comma is optional. Find a comprehensive overview of the available access modes in Section 4.8, File Permission Access Modes.

This variable expands to a value that can be changed without changing the entire profile.

HINT: Using Variables in Profiles

With the current AppArmor tools, variables as presented in the above example can only be used when manually editing and maintaining a profile.

A typical example when variables come in handy are network scenarios in which user home directories are not mounted in the standard location /home/username, but under a custom location. Find the variable definitions for this use case (@{HOME} and @{HOMEDIRS}) in the /etc/apparmor.d/tunables/home file.

When a profile is created for a program, the program can access only the files, modes, and POSIX capabilities specified in the profile. These restrictions are in addition to the native Linux access controls.

Example: To gain the capability CAP_CHOWN, the program must have both access to CAP_CHOWN under conventional Linux access controls (typically, be a root-owned process) and have the capability chown in its profile. Similarly, to be able to write to the file /foo/bar the program must have both the correct user ID and mode bits set in the files attributes (see the chmod and chown man pages) and have /foo/bar w in its profile.

Attempts to violate Novell AppArmor rules are recorded in /var/log/audit/audit.log if the audit package is installed or otherwise in /var/log/messages. In many cases, Novell AppArmor rules prevent an attack from working because necessary files are not accessible and, in all cases, Novell AppArmor confinement restricts the damage that the attacker can do to the set of files permitted by Novell AppArmor.