NAME¶
AppArmor - kernel enhancement to confine programs to a limited set of resources.
DESCRIPTION¶
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, also 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) 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.
UBUNTU POLICY LOAD¶
Ubuntu systems use Upstart 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 stages: first via upstart
jobs for binaries that are started in early boot, and then via a SysV
initscript that starts in S37 for all remaining policy. When developing policy
it is important to know how your application is started and if policy load
should be handled specially.
In general, nothing extra has to be done for applications without an initscript
or with an initscript that starts after AppArmor's second stage initscript.
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/
# 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.
ERRORS¶
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 REJECTING. 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
aa_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 KERN facility. Thus, REJECTING and PERMITTING messages may go to
either
/var/log/audit/audit.log or
/var/log/messages, depending
upon local configuration.
FILES¶
- /etc/init.d/apparmor
- /etc/apparmor/init/network-interface-security/
- /etc/apparmor.d/
- /var/lib/apparmor/
- /var/log/audit/audit.log
- /var/log/messages
SEE ALSO¶
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>.