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Not quite a security hole: CREATE LANGUAGE for non-superusers

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Not quite a security hole: CREATE LANGUAGE for non-superusers
Date: 2012-05-30 16:02:06
Message-ID: 2293.1338393726@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
We allow non-superuser database owners to execute CREATE LANGUAGE for a
trusted language (one marked as tmpldbacreate in pg_pltemplate).
Currently, the C-language support functions for the language end up owned
by that non-superuser.  This is on the hairy edge of being a security
hole, since generally it's supposed that a function owner can redefine the
function.  Could the non-superuser alter the function into a state where
it can be used unsafely?  A non-superuser cannot directly execute CREATE
OR REPLACE FUNCTION with LANGUAGE set to C, but what can he do if he owns
a function that already has that setting?

One possible attack path is to use ALTER FUNCTION RENAME, which with a
C language function might be thought to change the target entry point in
the language's shared library, thus leading at least to a server crash and
possibly to undesirable execution of C-level code.  But actually that
won't happen, because the target is identified by pg_proc.prosrc which is
set up from the function name at CREATE time and isn't changed by RENAME.
Besides, the functions at issue here are created in the pg_catalog schema,
and non-superusers do not have permission to rename anything in
pg_catalog.

One thing the owner *can* do is use ALTER FUNCTION to change secondary
properties of the function, such as strictness, volatility, SECURITY
DEFINER, etc.  So far as I can see, none of these properties are examined
for a PL support function when it is used to call or validate a function
in the language, so this doesn't constitute a security hole either.
Still, it's not very hard to envision innocent-looking extensions to ALTER
FUNCTION that might result in live security holes here.

A different line of attack is to replace the function altogether with
CREATE OR REPLACE FUNCTION, using a non-C-language definition.  It would
no longer be a gateway to executing non-SQL code ... but it would still be
referenced by the PL.  In this way the function owner could insert
trojan-horse code that would be executed whenever somebody else tried to
define or use a function written in the PL.  It turns out that the
placement of the support functions in pg_catalog saves us from this too,
but that seems rather accidental to me; it's not immediately obvious that
a REPLACE operation on an existing object should require CREATE rights on
the containing schema.

In short, neither I nor anybody else on the PG security list have been
able to think of an exploitable security issue here, but we're not
entirely convinced that there isn't one.  Can anyone think of an attack
vector we missed?

As of 9.2, there is a new hazard of the same ilk, namely the constructor
functions for range types, which are language INTERNAL and are being
created as owned by the type's creator.  This seems potentially a worse
hole than the CREATE LANGUAGE case, in that any random SQL user can create
a range type, not only the database owner (who might be assumed to be at
least somewhat trustworthy).  Furthermore, because these functions aren't
created in pg_catalog but in the type's creation schema, the protections
afforded to functions in pg_catalog don't help us.  I still don't see
any exploitable security hole from ALTER FUNCTION, but it is definitely
possible for a range type's creator to replace a constructor function with
a trojan horse.  The significance of that is debatable though, since you
more or less have to trust a type's creator anyway if you use any of its
functions.  (An example is that a domain's creator can trivially insert
trojan horse functions into the domain's CHECK constraints.)

Whether or not there is a live security hole in existing releases,
it seems clear that we could easily create one by accident in future.
To forestall that, I suggest that we should modify CREATE LANGUAGE and
CREATE RANGE TYPE to mark the support functions as owned by the bootstrap
superuser, not the caller of the CREATE operation.  This would ensure that
non-superusers couldn't muck around with the function definitions.
(Dropping the language or type still works, since cascade deletions don't
pay attention to who owns an object that the delete cascades to.)
At present it seems sufficient to patch this in HEAD, along the lines of
the attached proposed patch.

If anyone can think of a workable attack against existing releases, we
will need to back-patch some form of this change, and also advise DBAs
to manually alter the ownership of existing language support functions.
That would be enough of a pain in the rear that I don't want to counsel
DBAs to do it unless there's a demonstrable need.

Another point here is that if we replace the current pg_pltemplate-based
method of creating trusted languages, we will need to be sure that the
created C functions always end up owned by a superuser, else the problem
comes back again.  That will be a matter to consider when we think about
how CREATE EXTENSION works for that case.

Comments?

			regards, tom lane


Attachment: support-function-ownership.patch
Description: text/x-patch (4.8 KB)

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