Make contrib modules' installation scripts more secure.

Hostile objects located within the installation-time search_path could
capture references in an extension's installation or upgrade script.
If the extension is being installed with superuser privileges, this
opens the door to privilege escalation.  While such hazards have existed
all along, their urgency increases with the v13 "trusted extensions"
feature, because that lets a non-superuser control the installation path
for a superuser-privileged script.  Therefore, make a number of changes
to make such situations more secure:

* Tweak the construction of the installation-time search_path to ensure
that references to objects in pg_catalog can't be subverted; and
explicitly add pg_temp to the end of the path to prevent attacks using
temporary objects.

* Disable check_function_bodies within installation/upgrade scripts,
so that any security gaps in SQL-language or PL-language function bodies
cannot create a risk of unwanted installation-time code execution.

* Adjust lookup of type input/receive functions and join estimator
functions to complain if there are multiple candidate functions.  This
prevents capture of references to functions whose signature is not the
first one checked; and it's arguably more user-friendly anyway.

* Modify various contrib upgrade scripts to ensure that catalog
modification queries are executed with secure search paths.  (These
are in-place modifications with no extension version changes, since
it is the update process itself that is at issue, not the end result.)

Extensions that depend on other extensions cannot be made fully secure
by these methods alone; therefore, revert the "trusted" marking that
commit eb67623c9 applied to earthdistance and hstore_plperl, pending
some better solution to that set of issues.

Also add documentation around these issues, to help extension authors
write secure installation scripts.

Patch by me, following an observation by Andres Freund; thanks
to Noah Misch for review.

Security: CVE-2020-14350

1 files changed, 17 insertions, 4 deletions

diff --git a/src/backend/commands/extension.c b/src/backend/commands/extension.c
index 39c9411367a..a4ab97cf5c4 100644
--- a/src/backend/commands/extension.c
+++ b/src/backend/commands/extension.c
@@ -821,9 +821,21 @@ execute_extension_script(Oid extensionOid, ExtensionControlFile *control,
 								 GUC_ACTION_SAVE, true, 0, false);
 
 	/*
-	 * Set up the search path to contain the target schema, then the schemas
-	 * of any prerequisite extensions, and nothing else.  In particular this
-	 * makes the target schema be the default creation target namespace.
+	 * Similarly disable check_function_bodies, to ensure that SQL functions
+	 * won't be parsed during creation.
+	 */
+	if (check_function_bodies)
+		(void) set_config_option("check_function_bodies", "off",
+								 PGC_USERSET, PGC_S_SESSION,
+								 GUC_ACTION_SAVE, true, 0, false);
+
+	/*
+	 * Set up the search path to have the target schema first, making it be
+	 * the default creation target namespace.  Then add the schemas of any
+	 * prerequisite extensions, unless they are in pg_catalog which would be
+	 * searched anyway.  (Listing pg_catalog explicitly in a non-first
+	 * position would be bad for security.)  Finally add pg_temp to ensure
+	 * that temp objects can't take precedence over others.
 	 *
 	 * Note: it might look tempting to use PushOverrideSearchPath for this,
 	 * but we cannot do that.  We have to actually set the search_path GUC in
@@ -837,9 +849,10 @@ execute_extension_script(Oid extensionOid, ExtensionControlFile *control,
 		Oid			reqschema = lfirst_oid(lc);
 		char	   *reqname = get_namespace_name(reqschema);
 
-		if (reqname)
+		if (reqname && strcmp(reqname, "pg_catalog") != 0)
 			appendStringInfo(&pathbuf, ", %s", quote_identifier(reqname));
 	}
+	appendStringInfoString(&pathbuf, ", pg_temp");
 
 	(void) set_config_option("search_path", pathbuf.data,
 							 PGC_USERSET, PGC_S_SESSION,