Re: [GENERAL] [ANNOUNCE] Advisory on possibly insecure security definer functions

> > It has come to the attention of the core team of the PostgreSQL project
> > that insecure programming practice is widespread in SECURITY DEFINER
> > functions. Many of these functions are exploitable in that they allow
> > users that have the privilege to execute such a function to execute
> > arbitrary code with the privileges of the owner of the function.
> >
> > The SECURITY DEFINER property of functions is a special non-default
> > property that causes such functions to be executed with the privileges
> > of their owner rather than with the privileges of the user invoking the
> > function (the default mode, SECURITY INVOKER). Thus, this mechanism is
> > very similar to the "setuid" mechanism in Unix operating systems.
> >
> > Because SQL object references in function code are resolved at run time,
> > any references to SQL objects that are not schema qualified are
> > resolved using the schema search path of the session at run time, which
> > is under the control of the calling user. By installing functions or
> > operators with appropriate signatures in other schemas, users can then
> > redirect any function or operator call in the function code to
> > implementations of their choice, which, in case of SECURITY DEFINER
> > functions, will still be executed with the function owner privileges.
> > Note that even seemingly innocent invocations of arithmetic operators
> > are affected by this issue, so it is likely that a large fraction of
> > all existing functions are exploitable.
> >
> > The proper fix for this problem is to insert explicit SET search_path
> > commands into each affected function to produce a known safe schema
> > search path. Note that using the default search path, which includes a
> > reference to the "$user" schema, is not safe when unqualified
> > references are intended to be found in the "public" schema and "$user"
> > schemas exist or can be created by other users. It is also not
> > recommended to rely on rigorously schema-qualifying all function and
> > operator invocations in function source texts, as such measures are
> > likely to induce mistakes and will furthermore make the source code
> > harder to read and maintain.
>
> But if we insert a set schema search_path command in an SQL function,
> the caller will be affected by it. Doing reset search_path before
> returning to caller might solve some of problems, but it will not
> recover caller's special search_path. How do you solve the problem?

I looked into this more and I think I'm afraid the proposed solution

> The proper fix for this problem is to insert explicit SET search_path
> commands into each affected function to produce a known safe schema
> search path.

actually does not work for SQL functions. For example,

CREATE OR REPLACE FUNCTION foo(INTEGER, INTEGER) RETURNS INTEGER AS $$
SET search_path To pg_catalog,public;
SELECT mod($1,$2);
$$ LANGUAGE sql SECURITY DEFINER;

If an attacker creates public.mod() to do something bad and override
his search_path to public,pg_catalog before executing foo(), his
attack will succeed since calling to mod() is resolved in the plan
time thus it will be resolved to public.mod, rather than
pg_catalog.mod.

Am I missing something?
--
Tatsuo Ishii
SRA OSS, Inc. Japan