pgsql: Prevent access to external files/URLs via XML entity references.

Prevent access to external files/URLs via XML entity references.

xml_parse() would attempt to fetch external files or URLs as needed to
resolve DTD and entity references in an XML value, thus allowing
unprivileged database users to attempt to fetch data with the privileges
of the database server. While the external data wouldn't get returned
directly to the user, portions of it could be exposed in error messages
if the data didn't parse as valid XML; and in any case the mere ability
to check existence of a file might be useful to an attacker.

The ideal solution to this would still allow fetching of references that
are listed in the host system's XML catalogs, so that documents can be
validated according to installed DTDs. However, doing that with the
available libxml2 APIs appears complex and error-prone, so we're not going
to risk it in a security patch that necessarily hasn't gotten wide review.
So this patch merely shuts off all access, causing any external fetch to
silently expand to an empty string. A future patch may improve this.

In HEAD and 9.2, also suppress warnings about undefined entities, which
would otherwise occur as a result of not loading referenced DTDs. Previous
branches don't show such warnings anyway, due to different error handling
arrangements.

Credit to Noah Misch for first reporting the problem, and for much work
towards a solution, though this simplistic approach was not his preference.
Also thanks to Daniel Veillard for consultation.

Security: CVE-2012-3489

Branch
------
REL8_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/a4b0c0aaf093a015bebe83a24c183e10a66c8c39

Modified Files
--------------
src/backend/utils/adt/xml.c | 28 ++++++++++++++++++++++++++++
src/test/regress/expected/xml.out | 20 ++++++++++++++++++++
src/test/regress/expected/xml_1.out | 14 ++++++++++++++
src/test/regress/sql/xml.sql | 6 ++++++
4 files changed, 68 insertions(+), 0 deletions(-)