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SQL Migration scripts

General structure

This directory contains the component sql scripts which are merged together to form the extension install and upgrade script. The three components are bootstrap, incremental, and idempotent.

The final sql script is constructed by concatenating the following individual pieces, in this order:

  • bootstrap
  • auto-generated idempotent pgx SQL
  • incremental
  • idempotent

Writing SQL migrations

When adding new SQL, you need only consider the incremental and idempotent directories. The incremental directory contains sql scripts which should each be applied to the database only one time, in deterministic order. The idempotent directory contains sql scripts which can or should be applied repeatedly.

As a general rule of thumb, new table/type/function/view additions all belong in incremental. A further general rule is that existing incremental files should not be modified.

The following belongs in idempotent:

  • redefinition of an existing function/view
  • redefinition of an existing operator (as long as failed creation is caught)

In principle, the idempotent folder provides an overview of the current implementation of all functions, and a mechanism to simply change the implementation. When adding a new function in an incremental script, it is recommended to add the same function to a script in idempotent.

Security

As this extension is marked as trusted, it is imperative that we follow the Postgres guidelines on Security Considerations for Extensions 1. In general, there are two classes of vulnerability which we need to account for:

a) Unsafe object creation b) Unsafe search_path

In the next couple of paragraphs we will (briefly) explain the mechanisms behind these vulnerabilities. This description is not intended to be fully comprehensive.

Unsafe object creation is an attack which can be applied when the extension uses CREATE OR REPLACE or CREATE ... IF NOT EXISTS. In this attack, the attacker pre-creates an object which will later be CREATE OR REPLACEd by the extension during installation. As the attacker is the owner of this object, they can later modify the object and get malicious code executed.

Unsafe search_path is an attack which can be applied when the extension does not sufficiently schema-qualify objects. In this attack, the attacker creates an object (in a schema which they own), and tricks the extension into using (or executing) that object instead of the intended object. This is possible because objects with better-matching signatures are chosen over objects which take more general arguments.

Note: The fact that the Promscale extension is marked as trusted means that all SQL in the install script, and all SECURITY DEFINER functions are executed as the bootstrap superuser.

The Promscale extension's approach to securing its SQL consists of the following:

  1. CREATE all schemas which will contain objects (without IF NOT EXISTS)
  2. Use CREATE OR REPLACE or CREATE ... IF NOT EXISTS in those schemas
  3. Use SET search_path = pg_catalog, pg_temp on functions and procedures where possible
  4. Explicitly schema-qualify all objects and operators in functions without SET search_path = pg_catalog, pg_temp
  5. Use SET LOCAL search_path = pg_catalog, pg_temp; on procedures which perform transaction control
  6. Explicitly REVOKE ALL ... FROM PUBLIC for SECURITY DEFINER functions and procedures
  7. Use SET LOCAL search_path = pg_catalog, pg_temp; at the beginning of the installation script

Step 1. ensures that the bootstrap superuser (on Postgres 13 and 14) or the installing superuser (on postgres 12) is the owner of all schemas. If a schema already exists before the extension is created, then the creation will fail. This is by design. If schemas were allowed to be owned by another user, then that user could hijack ownership of SECURITY DEFINER functions and escalate privileges to the boostrap superuser.

Step 1. allows step 2. to be safe. With it, we know that only superusers can own the extension's schemas, meaning that it is safe to use CREATE OR REPLACE or CREATE ... IF NOT EXISTS, as only a superuser could pre-create an object in the extension's schema, and superusers already have superuser privileges.

Step 3. ensures that the bodies of our SQL and PLPGSQL functions are not vulnerable to the Unsafe search_path attack. Note: There are situations in which we cannot or do not want to use SET search_path, for instance in procedures performing transaction control, or for functions which we desire to be inlined.

Step 4. must be applied in situations in which Step 3. cannot be applied.

Step 5. provides additional security in procedures which perform transaction control. Note: This approach is not possible in functions, and not necessary for procedures which do not perform transaction control, which is why we do not use it for them.

Step 6. is necessary, as by default functions are executable by PUBLIC, which is undesirable for SECURITY DEFINER.

Step 7. ensures that top-level SQL which is executed during the extension install is also fully schema-qualified.