AppArmor − kernel enhancement to confine programs to a limited set of resources.
AppArmor is a kernel enhancement to confine programs to a limited set of resources. AppArmor’s unique security model is to bind access control attributes to programs rather than to users.
AppArmor confinement is provided via profiles loaded into the kernel via apparmor_parser(8), typically through the /etc/init.d/apparmor SysV initscript (on Ubuntu, see UBUNTU POLICY LOAD, below), which is used like this:
# /etc/init.d/apparmor start # /etc/init.d/apparmor stop # /etc/init.d/apparmor restart
AppArmor can operate in two modes: enforcement, and complain or learning:
enforcement − Profiles loaded in enforcement mode will result in enforcement of the policy defined in the profile as well as reporting policy violation attempts to syslogd.
complain − Profiles loaded in "complain" mode will not enforce policy. Instead, it will report policy violation attempts. This mode is convenient for developing profiles. To manage complain mode for individual profiles the utilities aa−complain(8) and aa−enforce(8) can be used. These utilities take a program name as an argument.
Profiles are traditionally stored in files in /etc/apparmor.d/ under filenames with the convention of replacing the / in pathnames with . (except for the root /) so profiles are easier to manage (e.g. the /usr/sbin/nscd profile would be named usr.sbin.nscd).
Profiles are applied to a process at exec(3) time (as seen through the execve(2) system call); an already running process cannot be confined. However, once a profile is loaded for a program, that program will be confined on the next exec(3).
AppArmor supports the Linux kernel’s securityfs filesystem, and makes available the list of the profiles currently loaded; to mount the filesystem:
# mount −tsecurityfs securityfs /sys/kernel/security $ cat /sys/kernel/security/apparmor/profiles /usr/bin/mutt /usr/bin/gpg ...
Normally, the initscript will mount securityfs if it has not already been done.
AppArmor also restricts what privileged operations a confined process may execute, even if the process is running as root. A confined process cannot call the following system calls:
create_module(2) delete_module(2) init_module(2) ioperm(2) iopl(2) ptrace(2) reboot(2) setdomainname(2) sethostname(2) swapoff(2) swapon(2) sysctl(2)
Ubuntu systems use upstart(8) instead of a traditional SysV init system. Because upstart is an event-driven init system and understanding that policy must be loaded before execution, Ubuntu loads policy in two ways:
1. via upstart jobs for services started during the boot process 2. via the AppArmor upstart job for any remaining policy
The AppArmor upstart job is configured to make sure all policy is loaded before any user sessions start. When developing policy it is important to know how your application is started and if policy load should be handled specially.
The upstart job may be call with or without arguments, like so:
$ sudo start apparmor $ sudo start apparmor ACTION=clear # clear policy cache $ sudo start apparmor ACTION=teardown # unload all policy $ sudo start apparmor ACTION=reload # reload policy $ sudo start apparmor ACTION=force−reload # same as 'reload'
Because the job is an upstart(8) task, use ’start apparmor ACTION=teardown’ to unload all policy.
In general, nothing extra has to be done for applications not started during boot or those that start after AppArmor’s upstart job.
If the confined application has an Upstart job, adjust the job to call /lib/init/apparmor−profile−load with the filename of the policy file (relative to /etc/apparmor.d/). For example:
pre−start script /lib/init/apparmor−profile−load usr.bin.foo end script
If the confined application does not have an Upstart job but it starts before AppArmor’s second stage initscript, then add a symlink from the policy file in /etc/apparmor.d to /etc/apparmor/init/network−interface−security/. For example:
$ cd /etc/apparmor/init/network−interface−security/ $ sudo ln −s /etc/apparmor.d/usr.bin.foo .
The network-interface-security Upstart job will load all the symlinked policy files in /etc/apparmor/init/network−interface−security/ before any network interfaces come up. Because network interfaces come up very early in the boot process, this will help ensure that AppArmor policy is loaded before the confined application starts.
In addition, AppArmor on Ubuntu stores policy in two places:
1. /etc/apparmor.d for system policy 2. /var/lib/apparmor/profiles for click policy
See apparmor_parser(8) and aa−clickhook(1) for details.
When a confined process tries to access a file it does not have permission to access, the kernel will report a message through audit, similar to:
audit(1386511672.612:238): apparmor="DENIED" operation="exec" parent=7589 profile="/tmp/sh" name="/bin/uname" pid=7605 comm="sh" requested_mask="x" denied_mask="x" fsuid=0 ouid=0 audit(1386511672.613:239): apparmor="DENIED" operation="open" parent=7589 profile="/tmp/sh" name="/bin/uname" pid=7605 comm="sh" requested_mask="r" denied_mask="r" fsuid=0 ouid=0 audit(1386511772.804:246): apparmor="DENIED" operation="capable" parent=7246 profile="/tmp/sh" pid=7589 comm="sh" pid=7589 comm="sh" capability=2 capname="dac_override"
The permissions requested by the process are described in the operation= and denied_mask= (for files − capabilities etc. use a slightly different log format). The "name" and process id of the running program are reported, as well as the profile name including any "hat" that may be active, separated by "//". ("Name" is in quotes, because the process name is limited to 15 bytes; it is the same as reported through the Berkeley process accounting.)
For confined processes running under a profile that has been loaded in complain mode, enforcement will not take place and the log messages reported to audit will be of the form:
audit(1386512577.017:275): apparmor="ALLOWED" operation="open" parent=8012 profile="/usr/bin/du" name="/etc/apparmor.d/tunables/" pid=8049 comm="du" requested_mask="r" denied_mask="r" fsuid=1000 ouid=0 audit(1386512577.017:276): apparmor="ALLOWED" operation="open" parent=8012 profile="/usr/bin/du" name="/etc/apparmor.d/tunables/" pid=8049 comm="du" requested_mask="r" denied_mask="r" fsuid=1000 ouid=0
If the userland auditd is not running, the kernel will send audit events to klogd; klogd will send the messages to syslog, which will log the messages with the KERN facility. Thus, REJECTING and PERMITTING messages may go to either /var/log/audit/audit.log or /var/log/messages, depending upon local configuration.
apparmor_parser(8), aa_change_hat(2), apparmor.d(5), subdomain.conf(5), aa−autodep(1), clean(1), auditd(8), aa−unconfined(8), aa−enforce(1), aa−complain(1), and <http://wiki.apparmor.net>.