Subject: Re: Extension security improvement: Add support for extensions with an owned schema

From: Marco Slot <[email-masked]>
Date: Wed, 5 Jun 2024 16:09:08 +0200

On Sun, Jun 2, 2024, 02:08 Jelte Fennema-Nio <[email-masked]> wrote:
> This patch adds a new "owned_schema" option to the extension control
> file that can be set to true to indicate that this extension wants to
> own the schema in which it is installed.

Huge +1

Many managed PostgreSQL services block superuser access, but provide a
way for users to trigger a create/alter extension as superuser. There
have been various extensions whose SQL scripts can be tricked into
calling a function that was pre-created in the extension schema. This
is usually done by finding an unqualified call to a pg_catalog
function/operator, and overloading it with one whose arguments types
are a closer match for the provided values, which then takes
precedence regardless of search_path order. The custom function can
then do something like "alter user foo superuser".

The sequence of steps assumes the user already has some kind of admin
role and is gaining superuser access to their own database server.
However, the superuser implicitly has shell access, so it gives
attackers an additional set of tools to poke around in the managed
service. For instance, they can change the way the machine responds to
control plane requests, which can sometimes trigger further
escalations. In addition, many applications use the relatively
privileged default user, which means SQL injection issues can also
escalate into superuser access and beyond.

There are some static analysis tools like
https://github.com/timescale/pgspot that address this issue, though it
seems like a totally unnecessary hole. Using schema = pg_catalog,
relocatable = false, and doing an explicit create schema (without "if
not exists") plugs the hole by effectively disabling extension
schemas. For extensions I'm involved in, I consider this to be a hard
requirement.

I think Jelte's solution is preferable going forward, because it
preserves the flexibility that extension schemas were meant to
provide, and makes the potential hazards of reusing a schema more
explicit.

cheers,
Marco