public, via oss-sec

                    Xen Security Advisory XSA-107

    Mishandling of uninitialised FIFO-based event channel control blocks

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

When using the FIFO-based event channels, there are no checks for the
existence of a control block when binding an event or moving it to a
different VCPU.  This is because events may be bound when the ABI is
in 2-level mode (e.g., by the toolstack before the domain is started).

The guest may trigger a Xen crash in evtchn_fifo_set_pending() if:

  a) the event is bound to a VCPU without a control block; or
  b) VCPU 0 does not have a control block.

In case (a), Xen will crash when looking up the current queue.  In
(b), Xen will crash when looking up the old queue (which defaults to a
queue on VCPU 0).

IMPACT
======

A buggy or malicious guest can crash the host.

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

Xen 4.4 and onward are vulnerable.

MITIGATION
==========

None.

CREDITS
=======

This issue was originally reported by Vitaly Kuznetsov at Red Hat and
diagnosed as a security issue by David Vrabel at Citrix.

NOTE REGARDING LACK OF EMBARGO
==============================

This bug was publicly reported on xen-devel, before it was appreciated
that there was a security problem.
that there was a security problem.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa107-unstable.patch        xen-unstable
xsa107-4.4.patch             Xen 4.4.x

$ sha256sum xsa107*.patch
b92ba8085b6684abbc8b012ae1a580b9e7ed7c8e67071a9e70381d4c1009638b  xsa107-4.4.patch
cd954a5bd742c751f8db884a3f31bd636a8c5850acddf5f1160dd6be1f706a09  xsa107-unstable.patch
$