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            Xen Security Advisory CVE-2023-34319 / XSA-432
                               version 2

        Linux: buffer overrun in netback due to unusual packet

UPDATES IN VERSION 2
====================

Public release.

ISSUE DESCRIPTION
=================

The fix for XSA-423 added logic to Linux'es netback driver to deal with
a frontend splitting a packet in a way such that not all of the headers
would come in one piece.  Unfortunately the logic introduced there
didn't account for the extreme case of the entire packet being split
into as many pieces as permitted by the protocol, yet still being
smaller than the area that's specially dealt with to keep all (possible)
headers together.  Such an unusual packet would therefore trigger a
buffer overrun in the driver.

IMPACT
======

An unprivileged guest can cause Denial of Service (DoS) of the host by
sending network packets to the backend, causing the backend to crash.

Data corruption or privilege escalation seem unlikely but have not been
ruled out.

VULNERABLE SYSTEMS
==================

All systems using a Linux based network backend with kernel 3.19 and
newer are vulnerable, on the assumption that the fix for XSA-423 was
taken.  Systems using other network backends are not known to be
vulnerable.

MITIGATION
==========

Using another PV network backend (e.g. the qemu based "qnic" backend)
will mitigate the problem.

Using a dedicated network driver domain per guest will mitigate the
problem.

CREDITS
=======

This issue was discovered by Ross Lagerwall of Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa432-linux.patch           Linux 6.3 - 6.5-rc

$ sha256sum xsa432*
bf7acd23be1d185c40aca8b4f7700e25afd482d9ac8671ae22b021380b059091  xsa432-linux.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.

(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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