apparmor

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, 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 CWcomplain 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) mount(2) umount(2) ptrace(2) reboot(2) setdomainname(2) sethostname(2) swapoff(2) swapon(2) sysctl(2)

A confined process can not call mknod(2) to create character or block devices.

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(1148420912.879:96): REJECTING x access to /bin/uname (sh(6646) profile /tmp/sh active /tmp/sh)

audit(1148420912.879:97): REJECTING r access to /bin/uname (sh(6646) profile /tmp/sh active /tmp/sh)

audit(1148420944.837:98): REJECTING access to capability 'dac_override' (sh(6641) profile /tmp/sh active /tmp/sh)

The permissions requested by the process are immediately after \s-1REJECTING\s0. The “name” and process id of the running program are reported, as well as the profile name and any “hat” that may be active. (“Name” is in quotes, because the process name is limited to 15 bytes; it is the same as reported through the Berkeley process accounting.) If no hat is active (see change_hat(2)) then the profile name is printed for “active”.

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(1146868287.904:237): PERMITTING r access to /etc/apparmor.d/tunables (du(3811) profile /usr/bin/du active /usr/bin/du)

audit(1146868287.904:238): PERMITTING r access to /etc/apparmor.d (du(3811) profile /usr/bin/du active /usr/bin/du)

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 \s-1KERN\s0 facility. Thus, \s-1REJECTING\s0 and \s-1PERMITTING\s0 messages may go to either /var/log/audit/audit.log or /var/log/messages, depending upon local configuration.

"/etc/init.d/apparmor"

"/etc/apparmor.d/"

"/var/lib/apparmor/"

"/var/log/audit/audit.log"

"/var/log/messages"

apparmor_parser(8), change_hat(2), apparmor.d(5), subdomain.conf(5), autodep(1), clean(1), auditd(8), unconfined(8), enforce(1), complain(1), and <http://forge.novell.com/modules/xfmod/project/?apparmor>.