Fix priv checks for ALTER <object> DEPENDS ON EXTENSION

Marking an object as dependant on an extension did not have any
privilege check whatsoever; this allowed any user to mark objects as
droppable by anyone able to DROP EXTENSION, which could be used to cause
system-wide havoc.  Disallow by checking that the calling user owns the
mentioned object.

(No constraints are placed on the extension.)

Security: CVE-2020-1720
Reported-by: Tom Lane
Discussion: 31605.1566429043@sss.pgh.pa.us

src/backend/commands/alter.c

@@ -438,6 +438,17 @@ ExecAlterObjectDependsStmt(AlterObjectDependsStmt *stmt, ObjectAddress *refAddre
 		get_object_address_rv(stmt->objectType, stmt->relation, (List *) stmt->object,
 							  &rel, AccessExclusiveLock, false);
 
+	/*
+	 * Verify that the user is entitled to run the command.
+	 *
+	 * We don't check any privileges on the extension, because that's not
+	 * needed.  The object owner is stipulating, by running this command, that
+	 * the extension owner can drop the object whenever they feel like it,
+	 * which is not considered a problem.
+	 */
+	check_object_ownership(GetUserId(),
+						   stmt->objectType, address, stmt->object, rel);
+
 	/*
 	 * If a relation was involved, it would have been opened and locked. We
 	 * don't need the relation here, but we'll retain the lock until commit.