Managing Roles and Privileges

Greenplum Database manages database access permissions using the concept of roles. The concept of roles subsumes the concepts of users and groups. A role can be a database user, a group, or both. Roles can own database objects (for example, tables) and can assign privileges on those objects to other roles to control access to the objects. Roles can be members of other roles, thus a member role can inherit the object privileges of its parent role.

Every Greenplum Database system contains a set of database roles (users and groups). Those roles are separate from the users and groups managed by the operating system on which the server runs. However, for convenience you may want to maintain a relationship between operating system user names and Greenplum Database role names, since many of the client applications use the current operating system user name as the default.

In Greenplum Database, users log in and connect through the master instance, which then verifies their role and access privileges. The master then issues out commands to the segment instances behind the scenes as the currently logged in role.

Roles are defined at the system level, meaning they are valid for all databases in the system.

In order to bootstrap the Greenplum Database system, a freshly initialized system always contains one predefined superuser role (also referred to as the system user). This role will have the same name as the operating system user that initialized the Greenplum Database system. Customarily, this role is named gpadmin. In order to create more roles you first have to connect as this initial role.

Security Best Practices for Roles and Privileges

1. Secure the gpadmin system user. 

Greenplum requires a UNIX user id to install and initialize the Greenplum Database system. This system user is referred to as gpadmin in the Greenplum documentation. This gpadmin user is the default database superuser in Greenplum Database, as well as the file system owner of the Greenplum installation and its underlying data files. This default administrator account is fundamental to the design of Greenplum Database. The system cannot run without it, and there is no way to limit the access of this gpadmin user id.

This gpadmin user can bypass all security features of Greenplum Database. Anyone who logs on to a Greenplum host as this user id can read, alter or delete any data; including system catalog data and database access rights. Therefore, it is very important to secure the gpadmin user id and only provide access to essential system administrators. 

Administrators should only log in to Greenplum as gpadmin when performing certain system maintenance tasks (such as upgrade or expansion). Database users should never log on as gpadmin, and ETL or production workloads should never run as gpadmin.

2. Assign a distinct role to each user that logs in. 

For logging and auditing purposes, each user that is allowed to log in to Greenplum Database should be given their own database role. For applications or web services, consider creating a distinct role for each application or service. See “Creating New Roles (Users)” on page 31.

3. Use groups to manage access privileges. 

4. Limit users who have the SUPERUSER role attribute. 

Roles that are superusers bypass all access privilege checks in Greenplum Database, as well as resource queuing. Only system administrators should be given superuser rights. 

Creating New Roles (Users)

A user-level role is considered to be a database role that can log in to the database and initiate a database session. Therefore, when you create a new user-level role using the CREATE ROLE command, you must specify the LOGIN privilege. For example:

=# CREATE ROLE jsmith WITH LOGIN;

A database role may have a number of attributes that define what sort of tasks that role can perform in the database. You can set these attributes when you create the role, or later using the ALTER ROLE command. 

Role Attributes

SUPERUSER | NOSUPERUSER  

Determines if the role is a superuser. A superuser always bypasses all access permission checks within the database and has full access to everything. Superuser status is dangerous and should be used only when really needed. You must yourself be a superuser to create a new superuser. NOSUPERUSER is the default.

CREATEDB | NOCREATEDB

Determines if the role is allowed to create databases. NOCREATEDB is the default.

CREATEROLE | NOCREATEROLE 

Determines if the role is allowed to create and manage other roles. NOCREATEROLE is the default.

INHERIT | NOINHERIT  

Determines whether a role inherits the privileges of roles it is a member of. A role with the INHERIT attribute can automatically use whatever database privileges have been granted to all roles it is directly or indirectly a member of. INHERIT is the default.

LOGIN | NOLOGIN

Determines whether a role is allowed to log in. A role having the LOGIN attribute can be thought of as a user. Roles without this attribute are useful for managing database privileges (groups). NOLOGIN is the default.

CONNECTION LIMIT connlimit

If role can log in, this specifies how many concurrent connections the role can make. -1 (the default) means no limit.

PASSWORD ‘password’

Sets the role’s password. If you do not plan to use password authentication you can omit this option. If no password is specified, the password will be set to null and password authentication will always fail for that user. A null password can optionally be written explicitly as PASSWORD NULL.

ENCRYPTED | UNENCRYPTED

Controls whether the password is stored encrypted in the system catalogs. The default behavior is determined by the configuration parameter password_encryption (currently set to MD5). If the presented password string is already in MD5-encrypted format, then it is stored encrypted as-is, regardless of whether ENCRYPTED or UNENCRYPTED is specified (since the system cannot decrypt the specified encrypted password string). This allows reloading of encrypted passwords during dump/restore.

VALID UNTIL ‘timestamp’

Sets a date and time after which the role’s password is no longer valid. If omitted the password will be valid for all time.

RESOURCE QUEUE queue_name

Assigns the role to the named resource queue for workload management. Any statement that role issues is then subject to the resource queue’s limits. Note that the RESOURCE QUEUE attribute is not inherited; it must be set on each user-level (LOGIN) role.

DENY {deny_interval | deny_point}

Restricts access during an interval, specified by day or day and time. 

You can set these attributes when you create the role, or later using the ALTER ROLE command. For example:

=# ALTER ROLE jsmith WITH PASSWORD 'passwd123';

=# ALTER ROLE admin VALID UNTIL 'infinity';

=# ALTER ROLE jsmith LOGIN;

=# ALTER ROLE jsmith RESOURCE QUEUE adhoc;

=# ALTER ROLE jsmith DENY DAY 'Sunday';

A role can also have role-specific defaults for many of the server configuration settings. For example, to set the default schema search path for a role:

=# ALTER ROLE admin SET search_path TO myschema, public;

Creating Groups (Role Membership)

It is frequently convenient to group users together to ease management of object privileges: that way, privileges can be granted to, or revoked from, a group as a whole. In Greenplum Database this is done by creating a role that represents the group, and then granting membership in the group role to individual user roles.

Use the CREATE ROLE SQL command to create a new group role. For example:

=# CREATE ROLE admin CREATEROLE CREATEDB;

Once the group role exists, you can add and remove members (user roles) using the GRANT and REVOKE commands. For example:

=# GRANT admin TO john, sally;

=# REVOKE admin FROM bob;

For managing object privileges, you would then grant the appropriate permissions to the group-level role only . The member user roles then inherit the object privileges of the group role. For example:

=# GRANT ALL ON TABLE mytable TO admin;

=# GRANT ALL ON SCHEMA myschema TO admin;

=# GRANT ALL ON DATABASE mydb TO admin;

The role attributes LOGIN, SUPERUSER, CREATEDB, and CREATEROLE are never inherited as ordinary privileges on database objects are. User members must actually SET ROLE to a specific role having one of these attributes in order to make use of the attribute. In the above example, we gave CREATEDB and CREATEROLE to the admin role. If sally is a member of admin, she could issue the following command to assume the role attributes of the parent role:

=> SET ROLE admin;

Managing Object Privileges

When an object (table, view, sequence, database, function, language, schema, or tablespace) is created, it is assigned an owner. The owner is normally the role that executed the creation statement. For most kinds of objects, the initial state is that only the owner (or a superuser) can do anything with the object. To allow other roles to use it, privileges must be granted. Greenplum Database supports the following privileges for each object type:

Tables, Views, Sequences :SELECT,INSERT,UPDATE,DELETE,RULE,ALL

External Tables:SELECT,RULE,ALL

Databases: CONNECT,CREATE,TEMPORARY | TEMP,ALL

Functions:EXECUTE

Procedural Languages:USAGE

Schemas: CREATE,USAGE,ALL

Note: Privileges must be granted for each object individually. For example, granting ALL on a database does not grant full access to the objects within that database. It only grants all of the database-level privileges (CONNECT, CREATE, TEMPORARY) to the database itself.

Use the GRANT SQL command to give a specified role privileges on an object. For example:

=# GRANT INSERT ON mytable TO jsmith;

To revoke privileges, use the REVOKE command. For example:

=# REVOKE ALL PRIVILEGES ON mytable FROM jsmith;

You can also use the DROP OWNED and REASSIGN OWNED commands for managing objects owned by deprecated roles (Note: only an object’s owner or a superuser can drop an object or reassign ownership). For example:

=# REASSIGN OWNED BY sally TO bob;

=# DROP OWNED BY visitor;

Simulating Row and Column Level Access Control

Greenplum Database access control corresponds roughly to the Orange Book ‘C2’ level of security, not the ‘B1’ level. Greenplum Database currently supports access privileges at the object level. Row-level or column-level access is not supported, nor is labeled security.

Row-level and column-level access can be simulated using views to restrict the columns and/or rows that are selected. Row-level labels can be simulated by adding an extra column to the table to store sensitivity information, and then using views to control row-level access based on this column. Roles can then be granted access to the views rather than the base table. While these workarounds do not provide the same as "B1" level security, they may still be a viable alternative for many organizations that require more granular access control.