apparmor/parser/apparmor.pod

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=pod

=head1 NAME

AppArmor - kernel enhancement to confine programs to a limited set of resources.

=head1 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 I<profiles> loaded into the kernel
via apparmor_parser(8), typically through the F<apparmor.service>
systemd unit, which is used like this:

	# systemctl start apparmor
	# systemctl reload apparmor

AppArmor can operate in two modes: I<enforcement>, and I<complain or learning>:

=over 4

=item *

I<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.  

=item *

I<complain> - Profiles loaded in  C<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. 


=back

Profiles are traditionally stored in files in F</etc/apparmor.d/>
under filenames with the convention of replacing the B</> in pathnames
with B<.> (except for the root B</>) so profiles are easier to manage
(e.g. the F</usr/sbin/nscd> profile would be named F<usr.sbin.nscd>).

Profiles are applied to a process at exec(3) time (as seen through the
execve(2) system call): once a profile is loaded for a program, that
program will be confined on the next exec(3). If a process is already
running under a profile, when one replaces that profile in the kernel,
the updated profile is applied immediately to that process.
On the other hand, a process that is already running unconfined cannot
be confined.

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)

=head2 Complain mode

Instead of denying access to resources the profile does not have a rule for
AppArmor can "allow" the access and log a message for the operation
that triggers it. This is called I<complain mode>. It is important to
note that rules that are present in the profile are still applied, so
allow rules will still quiet or force audit messages, and deny rules
will still result in denials and quieting of denial messages (see
I<Turn off deny audit quieting> if this is a problem).

Complain mode can be used to develop profiles incrementally as an
application is exercised. The logged accesses can be added to the
profile and then can the application further exercised to discover further
additions that are needed. Because AppArmor allows the accesses the
application will behave as it would if AppArmor was not confining it.

B<Warning> complain mode does not provide any security, only
auditing, while it is enabled. It should not be used in a hostile
environment or bad behaviors may be logged and added to the profile
as if they are resource accesses that should be used by the
application.

B<Note> complain mode can be very noisy with new or empty profiles,
but with developed profiles might not log anything if the profile
covers the application behavior well. See I<Audit Rate Limiting> if
complain mode is generating too many log messages.

To set a profile and any children or hat profiles the profile may contain
into complain mode use

	aa-complain /etc/apparmor.d/<the-application>

To manually set a specific profile in complain mode, add the
C<complain> flag, and then manually reload the profile:

	profile foo flags=(complain) { ... }

Note that the C<complain> flag must also be added manually to any
hats or children profiles of the profile or they will continue to
use the previous mode.

To enable complain mode globally, run:

	echo -n complain > /sys/module/apparmor/parameters/mode

or to set it on boot add:

	apparmor.mode=complain

as a kernel boot parameter.

B<Warning> Setting complain mode globally disables all apparmor
security protections. It can be useful during debugging or profile
development, but setting it selectively on a per profile basis is
safer.

=head1 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(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 F</var/log/audit/audit.log> or F</var/log/messages>,
depending upon local configuration.

=head1 DEBUGGING

AppArmor provides a few facilities to log more information,
which can help debugging profiles.

=head2 Enable debug mode

When debug mode is enabled, AppArmor will log a few extra messages to
dmesg (not via the audit subsystem). For example, the logs will state when
ld.so(8) secure-execution mode has been applied in a profile transition.

To enable debug mode, run:

	echo 1 > /sys/module/apparmor/parameters/debug

or to set it on boot add:

	apparmor.debug=1

as a kernel boot parameter.

=head2 Turn off deny audit quieting

By default, operations that trigger C<deny> rules are not logged.
This is called I<deny audit quieting>.

To turn off deny audit quieting, run:

	echo -n noquiet >/sys/module/apparmor/parameters/audit

or to set it on boot add:

	apparmor.audit=noquiet

as a kernel boot parameter.

=head2 Force audit mode

AppArmor can log a message for every operation that triggers a rule
configured in the policy. This is called I<force audit mode>.

B<Warning!> Force audit mode can be extremely noisy even for a single profile,
let alone when enabled globally.

To set a specific profile in force audit mode, add the C<audit> flag:

	profile foo flags=(audit) { ... }

To enable force audit mode globally, run:

	echo -n all > /sys/module/apparmor/parameters/audit

or to set it on boot add:

	apparmor.audit=all

as a kernel boot parameter.

B<Audit Rate Limiting>

If auditd is not running, to avoid losing too many of the extra log
messages, you will likely have to turn off rate limiting by doing:

	echo 0 > /proc/sys/kernel/printk_ratelimit

But even then the kernel ring buffer may overflow and you might
lose messages.

Else, if auditd is running, see auditd(8) and auditd.conf(5).

=head1 FILES

=over 4

=item F</etc/apparmor.d/>

=item F</var/cache/apparmor/>

=item F</var/log/audit/audit.log>

=item F</var/log/messages>

=back

=head1 SEE ALSO

apparmor_parser(8), aa_change_hat(2), apparmor.d(5),
aa-autodep(1), clean(1),
auditd(8),
aa-unconfined(8), aa-enforce(1), aa-complain(1), and
L<https://wiki.apparmor.net>.

=cut