mod_apparmor - fine-grained AppArmor confinement for Apache
An AppArmor profile applies to an executable program; if a portion of
the program needs different access permissions than other portions,
the program can ``change hats'' via aa_change_hat(2) to a different role,
also known as a subprofile. The mod_apparmor Apache module uses the
aa_change_hat(2) mechanism to offer more fine-grained confinement of dynamic
elements within Apache such as individual php and perl scripts, while
still allowing the performance benefits of using mod_php and mod_perl.
To use mod_apparmor with Apache, ensure that mod_apparmor is configured to
be loaded into Apache, either via a2enmod, yast or manual editing of the
apache2(8)/httpd(8) configuration files, and restart Apache. Make sure that
apparmor is also functioning.
Once mod_apparmor is loaded within Apache, all requests to Apache will
cause mod_apparmor to attempt to change into a hat named by the URI
(e.g. /app/some.cgi). If no such hat is found, it will fall back to
attempting to use the hat DEFAULT_URI; if that also does not exist,
it will fall back to using the global Apache profile. Most static web
pages can simply make use of the DEFAULT_URI hat.
Additionally, before any requests come in to Apache, mod_apparmor
will attempt to change hat into the HANDLING_UNTRUSTED_INPUT hat.
mod_apparmor will attempt to use this hat while Apache is doing the
initial parsing of a given http request, before its given to a specific
handler (like mod_php) for processing.
Because defining hats for every URI/URL often becomes tedious, mod_apparmor
provides the AAHatName and AADefaultHatName Apache configuration options.
AAHatName allows you to specify a hat to be used for a given Apache
<Directory>, <DirectoryMatch>, <Location> or
<LocationMatch> directive (see the Apache documenation for more
details). Note that mod_apparmor behavior can become confused if
<Directory*> and <Location*> directives are intermingled
and it is recommended to use one type of directive. If the hat specified by
AAHatName does not exist in the Apache profile, then it falls back to the
behavior described above.
AADefaultHatName allows you to specify a default hat to be used for
virtual hosts and other Apache server directives, so that you can have
different defaults for different virtual hosts. This can be overridden by
the AAHatName directive and is checked for only if there isn't a matching
AAHatName or hat named by the URI. If the AADefaultHatName hat does not
exist, it falls back to the DEFAULT_URI hat if it exists (as described
URI REQUEST SUMMARY
When profiling with mod_apparmor, it is helpful to keep the following order
of operations in mind:
On each URI request, mod_apparmor will first aa_change_hat(2) into
^HANDLING_UNTRUSTED_INPUT, if it exists.
Then, after performing the initial parsing of the request, mod_apparmor
1. try to aa_change_hat(2) into a matching AAHatName hat if it exists and
applies, otherwise it will
2. try to aa_change_hat(2) into the URI itself, otherwise it will
3. try to aa_change_hat(2) into an AADefaultHatName hat if it has been defined
for the server/vhost, otherwise it will
4. try to aa_change_hat(2) into the DEFAULT_URI hat, if it exists, otherwise it
5. fall back to the global Apache policy
mod_apparmor() currently only supports apache2, and has only been tested
with the prefork MPM configuration --- threaded configurations of Apache
may not work correctly.