Re: SQL-standard function bodies and creating SECURITY DEFINER routines securely

Hello

I confirmed, that setting search_path is indeed sometimes needed in case of SECURITY DEFINER routines that have SQL-standard bodies. See an example at the end of the letter.

I suggest the following paragraph to the documentation:

Starting from PostgreSQL 14 SQL-standard bodies can be used in SQL-language functions. This form tracks dependencies between the function and objects used in the function body. However, there is still a possibility that such function calls other code that reacts to search path. Thus, as a best practice, SECURITY DEFINER functions with SQL-standard bodies should also override search_path.

Best regards
Erki Eessaar

*************************
/*Table in the schema public.*/
CREATE TABLE public.A(a INTEGER PRIMARY KEY);

/*Table in the schema pg_temp.*/
CREATE TABLE pg_temp.A(a INTEGER PRIMARY KEY);

/*SECURITY DEFINER function without SQL-standard function body.*/
CREATE OR REPLACE FUNCTION f1 () RETURNS VOID
AS $$
INSERT INTO A(a) VALUES (1);
$$ LANGUAGE sql SECURITY DEFINER;

/*SECURITY DEFINER function with SQL-standard function body that invokes
the function without SQL-standard function body. It does not explicitly set the search path.*/
CREATE OR REPLACE FUNCTION f2 () RETURNS VOID
LANGUAGE sql SECURITY DEFINER
BEGIN ATOMIC
SELECT f1();
END;

/*SECURITY DEFINER function with SQL-standard function body that invokes
the function without SQL-standard function body. It does explicitly set the search path.*/
CREATE OR REPLACE FUNCTION f2_secure () RETURNS VOID
LANGUAGE sql SECURITY DEFINER
SET search_path = public, pg_temp
BEGIN ATOMIC
SELECT f1();
END;

SELECT f2();

SELECT * FROM public.A;
/*Result 0 rows.*/

SELECT * FROM pg_temp.A;
/*Result 1 row.*/

DELETE FROM pg_temp.A;

SELECT f2_secure();

SELECT * FROM public.A;
/*Result 1 row. SET search_path in the invoking function had an effect to the invoked function*/

SELECT * FROM pg_temp.A;
/*Result 0 rows.*/

________________________________
From: Bruce Momjian <bruce(at)momjian(dot)us>
Sent: Wednesday, September 28, 2022 8:07 PM
To: Erki Eessaar <erki(dot)eessaar(at)taltech(dot)ee>
Cc: pgsql-docs(at)lists(dot)postgresql(dot)org <pgsql-docs(at)lists(dot)postgresql(dot)org>
Subject: Re: SQL-standard function bodies and creating SECURITY DEFINER routines securely

On Tue, Aug 16, 2022 at 03:32:36PM -0400, Bruce Momjian wrote:
> On Sat, Dec 25, 2021 at 02:36:27PM +0000, Erki Eessaar wrote:
> >
> > Hello
> >
> > PostgreSQL 14 added the feature: "Allow SQL-language functions and procedures
> > to use SQL-standard function bodies."
> >
> > If I understand correctly, then in this case the system will track
> > dependencies between tables and routines that use the tables. Thus, the
> > SECURITY DEFINER routines that use the new approach do not require the
> > following mitigation, i.e., SET search_path= is not needed. The following part
> > of documentation does not mention this.
> >
> > https://www.postgresql.org/docs/current/sql-createfunction.html#
> > SQL-CREATEFUNCTION-SECURITY
> >
> > [elephant] PostgreSQL: Documentation: 14: CREATE FUNCTION
> > Overloading. PostgreSQL allows function overloading; that is, the
> > same name can be used for several different functions so long as
> > they have distinct input argument types.Whether or not you use it,
> > this capability entails security precautions when calling functions
> > in databases where some users mistrust other users; see Section
> > 10.3.. Two functions are considered the same if they have the same
> > ...
> > www.postgresql.org<http://www.postgresql.org>
>
> I have written the attached patch to mention this issue about sql_body
> functions.

The doc patch was reverted based on feedback in this email thread:

https://www.postgresql.org/message-id/flat/AM9PR01MB8268BF5E74E119828251FD34FE409%40AM9PR01MB8268.eurprd01.prod.exchangelabs.com

If you think we should add new wording, please suggest it, thanks.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

Indecision is a decision. Inaction is an action. Mark Batterson