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PostgreSQL Extension Whitelist

This extension implements extension whitelisting, and will actively prevent users from installing extensions not in the provided list. Also, this extension implements a form of sudo facility in that the whitelisted extensions will get installed as if superuser. Privileges are dropped before handing the control back to the user.

The operations CREATE EXTENSION, DROP EXTENSION, ALTER EXTENSION ... UPDATE, and COMMENT ON EXTENSION are run by superuser. The ALTER EXTENSION ... ADD|DROP command is intentionally not supported so as not to allow users to modify an already installed extension. That means that it's not currently possible to CREATE EXTENSION ... FROM 'unpackaged';.

Note that the extension script is running as if run by a stored procedure owned by your bootstrap superuser and with SECURITY DEFINER, meaning that the extension and all its objects are owned by this superuser.

PostgreSQL versions 10 and later are supported.

Licence

The pgextwlist PostgreSQL extension is released under The PostgreSQL Licence, a liberal Open Source license, similar to the BSD or MIT licenses.

Install

  1. Install the server development packages (on Ubuntu, this would look like apt-get install postgresql-server-dev-all)

  2. then:

    make
sudo make install

This will generate a pgextwlist.so shared library that you will have to install in

`pg_config --pkglibdir`/plugins

so that your backend loads it automatically.

Setup

You need to define the list of extensions that are whitelisted, the user that performs the extension installing, and the error behavior.

  • local_preload_libraries

    Add pgextwlist to the local_preload_libraries setting. Don't forget to add the module in the $plugin directory.

  • extwlist.extensions

    List of extensions allowed for installation.

    To allow only certain users to use the whitelist, use ALTER ROLE instead of setting this parameter globally:

    ALTER ROLE adminuser SET extwlist.extensions = 'pg_stat_statements, postgis';

  • extwlist.custom_path

    Filesystem path where to look for custom scripts.

Usage

That's quite simple:

$ edit postgresql.conf to add local_preload_libraries, custom_variable_classes and extwlist.extensions

dim=# show extwlist.extensions;
show extwlist.extensions;
 extwlist.extensions
---------------------
 hstore,cube
(1 row)

dim=# create extension foo;
create extension foo;
ERROR:  extension "foo" is not whitelisted
DETAIL:  Installing the extension "foo" failed, because it is not on the whitelist of user-installable extensions.
HINT:  Your system administrator has allowed users to install certain extensions. See: SHOW extwlist.extensions;

dim=# create extension hstore;
create extension hstore;
WARNING:  => is deprecated as an operator name
DETAIL:  This name may be disallowed altogether in future versions of PostgreSQL.
CREATE EXTENSION

dim=# \dx
\dx
                           List of installed extensions
  Name   | Version |   Schema   |                   Description
---------+---------+------------+--------------------------------------------------
 hstore  | 1.0     | public     | data type for storing sets of (key, value) pairs
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)

Even if you're not superuser:

dim=> select rolsuper from pg_roles where rolname = current_user;
select rolsuper from pg_roles where rolname = current_user;
 rolsuper
----------
 f
(1 row)

dim=> create extension hstore;
create extension hstore;
WARNING:  => is deprecated as an operator name
DETAIL:  This name may be disallowed altogether in future versions of PostgreSQL.
CREATE EXTENSION

dim=> create extension earthdistance;
create extension earthdistance;
ERROR:  extension "earthdistance" is not whitelisted
DETAIL:  Installing the extension "earthdistance" failed, because it is not on the whitelist of user-installable extensions.
HINT:  Your system administrator has allowed users to install certain extensions. SHOW extwlist.extensions;

dim=> \dx
\dx
                           List of installed extensions
  Name   | Version |   Schema   |                   Description
---------+---------+------------+--------------------------------------------------
 hstore  | 1.0     | public     | data type for storing sets of (key, value) pairs
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)

dim=> drop extension hstore;
drop extension hstore;
DROP EXTENSION

Custom Scripts

Some extensions are installing objects that only the superuser can make use of by default, it's then a good idea to tweak permissions and grant usage to the current_user or even the database owner, depending.

The custom scripts feature allows to do that by providing scripts to be run around the execution of the extension's script itself.

create extension custom scripts

For the creation of extension extname version 1.0 the following scripts will be used when they do exist, as shown here:

  • ${extwlist.custom_path}/extname/before--1.0.sql

  • ${extwlist.custom_path}/extname/before-create.sql, only when the previous one, specific to the version being installed, does not exists.

  • The CREATE EXTENSION command now runs normally

  • ${extwlist.custom_path}/extname/after--1.0.sql

  • ${extwlist.custom_path}/extname/after-create.sql, only when the specific one does not exist

alter extension update custom scripts

For the update of extension extname from version 1.0 to version 1.1 the following scripts will be used when they do exist, as shown here:

  • ${extwlist.custom_path}/extname/before--1.0--1.1.sql

  • ${extwlist.custom_path}/extname/before-update.sql, only when the specific one does not exists.

  • The ALTER EXTENSION UPDATE command now runs normally

  • ${extwlist.custom_path}/extname/after--1.0--1.1.sql

  • ${extwlist.custom_path}/extname/after-update.sql only when the specific one does not exists.

comment on extension and drop extension scripts

Similarly:

  • ${extwlist.custom_path}/extname/before-comment.sql

  • ${extwlist.custom_path}/extname/after-comment.sql

  • ${extwlist.custom_path}/extname/before-drop.sql

  • ${extwlist.custom_path}/extname/after-drop.sql

Version-specific hook files are not supported here.

custom scripts templating

Before executing them, the extwlist extension applies the following substitutions to the custom scripts:

  • any line that begins with \echo is removed,

  • the literal @extschema@ is unconditionally replaced by the current schema being used to create the extension objects,

  • the literal @current_user@ is replaced by the name of the current user,

  • the literal @database_owner@ is replaced by the name of the current database owner.

Tip: remember that you can execute DO blocks if you need dynamic SQL.

Internals

The whitelisting works by overloading the ProcessUtility_hook and gaining control each time a utility statement is issued. When this statement is a CREATE EXTENSION, the extension's name is extracted from the parsetree and checked against the whitelist. Superuser is obtained as in the usual SECURITY DEFINER case, except hard coded to target the bootstrap user.