Pam apparmor example

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Example pam_apparmor configuration

AppArmor allows you great flexibility in setting up policies with pam_apparmor. This example provides a basic pam_apparmor configuration using the change_hat methodolgy (the current implementation) that can be extended later for your site requirements. In this example, we confine the 'su' binary such that when a user uses su to run commands as another user, AppArmor will apply access controls on the changed to user. This can later be extended to other binaries such as 'sshd' and 'login'. This example is known to work on Ubuntu 10.04 LTS with AppArmor 2.5, but should work fine with AppArmor 2.3 (it is based on techniques from RBAC_2_3).

It is recommended while configuring pam_apparmor that you login to another terminal as root so that in case something goes wrong with your configuration, you can use this terminal to make any necessary changes. Eg:

 $ sudo -i

or:

 $ su -

Overview

The idea behind pam_apparmor is simple: when a someone uses a confined binary, that binary will transition to an AppArmor role via PAM. So if 'su' is configured for use with pam_apparmor, when a user invokes 'su', PAM is consulted. When the PAM session is started, pam_apparmor will change_hat() to either a hat that matches the username, a hat that matches the primary group, or the DEFAULT hat (which depends on the order specified in the pam configuration). This hat (typically) provides a rudimentary policy and declares a transition to a role profile when the user's shell is started. So with our su example:

  1. The user does 'su - gray'
  2. su then performs a change_hat() (via pam_apparmor) to ^gray
  3. When gray's shell is started, the user transitions to the confined_user profile

Put simply, when the user does 'su - gray', the user changes to the 'confined_user' role.

Initial configuration

Make sure you have pam_apparmor available on your system. For example, on Ubuntu:

 $ sudo apt-get install libpam-apparmor

Next, integrate pam_apparmor with su by adjusting /etc/pam.d/su to have:

 # omit 'debug' when in production
 session optional     pam_apparmor.so order=user,group,default debug

pam_apparmor policy files

In this example, we will break out the pam_apparmor policy into various files to help illustrate the various parts and how they work together. It also serves as a method to make maintaining your policy easier. AppArmor doesn't care if the files are broken apart or in one monolithic file, but we will use the following files in this example:

  • /etc/apparmor.d/pam_binaries: policy for binaries with profiles (eg 'su')
  • /etc/apparmor.d/pam_roles: policy for hats referenced in pam/mappings (ie, our 'roles')
  • /etc/apparmor.d/pam/mappings: hats referenced in pam_binaries

In other words, /etc/apparmor.d/pam_binaries contains policy for the binaries that are integrated with pam (in this case 'su'). /etc/apparmor.d/pam_roles contains the policy for the different roles at your site, and /etc/apparmor.d/pam/mappings maps login names to an AppArmor role.

/etc/apparmor.d/pam_binaries

Create /etc/apparmor.d/pam_binaries to have:

 #
 # This file contains the policy for the confined binaries that use
 # libpam-apparmor.
 #

 #include <tunables/global>

 /bin/su {
    #include <abstractions/authentication>
    #include <abstractions/base>
    #include <abstractions/nameservice>

    # Include the file with all of our username/group to role mappings
    #include <pam/mappings>

    capability chown,
    capability setgid,
    capability setuid,

    owner /etc/environment r,
    owner /etc/shells r,
    owner /etc/default/locale r,
    owner @{HOMEDIRS}/*/.Xauthority rw,
    owner @{HOMEDIRS}/*/.Xauthority-c w,
    owner @{HOMEDIRS}/*/.Xauthority-l w,
    @{HOME}/.xauth* rw,
    owner @{PROC}/sys/kernel/ngroups_max r,
    /usr/bin/xauth rix,
    owner /var/run/utmp rwk,
 }

/etc/apparmor.d/pam_roles

Now create different roles in /etc/apparmor.d/pam_roles:

 #
 # This file contains the roles as referenced by pam/mappings
 #

 #include <tunables/global>

 # By default, allow users to read, lock and link to their own files anywhere,
 # but only write to files in their home directory. Only allow limited execution
 # of files.
 profile default_user {
    #include <abstractions/base>
    #include <abstractions/bash>
    #include <abstractions/consoles>
    #include <abstractions/nameservice>

    deny capability sys_ptrace,

    owner /** rkl,
    @{PROC}/** r,

    /bin/**  Pixmr,
    /usr/bin/** Pixmr,
    owner @{HOMEDIRS}/ w,
    owner @{HOMEDIRS}/** w,
 }

 # Allow confined_users to read, write, lock and link to their own files
 # anywhere, and execute from some places.
 profile confined_user {
    #include <abstractions/base>
    #include <abstractions/bash>
    #include <abstractions/consoles>
    #include <abstractions/nameservice>

    deny capability sys_ptrace,

    owner /** rwkl,
    @{PROC}/** r,

    /bin/**  Pixmr,
    /usr/bin/** Pixmr,
    owner @{HOMEDIRS}/bin/** ixmr,
 }

/etc/apparmor.d/pam/mappings

Now create /etc/apparmor.d/pam/mappings to map the usernames/groups to roles:

 #
 # This file contains the mappings from users to roles for the binaries
 # confined with AppArmor and configured for use with libpam-apparmor. Users
 # without a mapping will not be able to login.
 #

 # The default hat is a confined user. The hat contains only the permissions
 # necessary to transition to the user's login shell. All other permissions have
 # been moved into the default_user profile.
 ^DEFAULT {
   #include <abstractions/authentication>
   #include <abstractions/nameservice>

   capability dac_override,
   capability setgid,
   capability setuid,

   /etc/default/su r,
   /etc/environment r,
   @{HOMEDIRS}/.xauth* w,

   /bin/{,b,d,rb}ash Px -> default_user,
   /bin/{c,k,tc}sh Px -> default_user,
 }

 # gray is a confined user. The hat contains only the permissions necessary
 # to transition to gray's login shell. All other permissions have been
 # moved into the confined_user profile.
 ^gray {
   #include <abstractions/authentication>
   #include <abstractions/nameservice>

   capability dac_override,
   capability setgid,
   capability setuid,

   /etc/default/su r,
   /etc/environment r,
   @{HOMEDIRS}/.xauth* w,

   /bin/{,b,d,rb}ash Px -> confined_user,
   /bin/{c,k,tc}sh Px -> confined_user,
 }

 # Don't confine members whose primary group is 'admin' who are not specifically
 # confined. Systems without this special primary group may want to define an
 # unconfined 'root' hat in this manner (depending on site policy). 
 ^admin {
   #include <abstractions/authentication>
   #include <abstractions/nameservice>

   capability dac_override,
   capability setgid,
   capability setuid,

   /etc/default/su r,
   /etc/environment r,
   @{HOMEDIRS}/.xauth* w,

   /bin/{,b,d,rb}ash Ux,
   /bin/{c,k,tc}sh Ux,
 }

Applying the policy

After adjusting policy, you must reload the profiles and roles (the mappings are pulled in automatically):

 $ sudo apparmor_parser -r -T -W /etc/apparmor.d/pam_binaries /etc/apparmor.d/pam_roles

See if they were loaded:

 $ sudo aa-status
 ...
 17 profiles are in enforce mode.
    /bin/su
    /bin/su//admin
    /bin/su//gray
 ...
    confined_user
 ...

You may of course update /etc/apparmor.d/pam_binaries and /etc/apparmor.d/pam_roles individually. Changes to /etc/apparmor.d/pam/mappings require you to reload /etc/apparmor.d/pam_binaries (because it has the mappings file as an #include).

Extending

Once you are comfortable with your 'su' configuration, you can extend this to other binaries like sshd and login by:

  • adding a 'session optional pam_apparmor.so ...' entry to the corresponding pam configuration
  • adding policy for the binary to /etc/apparmor.d/pam_binaries

To add a new user or role:

  • adjust /etc/apparmor.d/pam_roles for the new role
  • adjust /etc/apparmor.d/pam/mappings to map the login name to the AppArmor role

Caveats

pam_apparmor does not consult secondary groups, so using role groups is currently limited to primary groups only.

Final thoughts

Much of /etc/apparmor.d/pam/mappings is boiler plate and can be abstracted out and then added via an #include. Also remember that there is nothing special about the pam_binaries, pam_roles and pam/mappings files. You can use other names or include all of the policy in one file. For ease of understanding and maintenance on larger sites, breaking the policy up into smaller chunks in this way can help a lot.

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