[kernel-xen] Xen Security Advisory 104 - Race condition in HVMOP_track_dirty_vram

[Steven Haigh]

                    Xen Security Advisory XSA-104
                              version 2

               Race condition in HVMOP_track_dirty_vram

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

Public Release.

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

The routine controlling the setup of dirty video RAM tracking latches
the value of a pointer before taking the respective guarding lock, thus
making it possible for a stale pointer to be used by the time the lock
got acquired and the pointer gets dereferenced.

The hypercall providing access to the affected function is available to
the domain controlling HVM guests.

IMPACT
======

Malicious or buggy stub domain kernels or tool stacks otherwise living
outside of Domain0 can mount a denial of service attack which, if
successful, can affect the whole system.

Only domains controlling HVM guests can exploit this vulnerability.
(This includes domains providing hardware emulation services to HVM
guests.)

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

Xen versions from 4.0.0 onwards are vulnerable.

This vulnerability is only applicable to Xen systems using stub
domains or other forms of disaggregation of control domains for HVM
guests.

MITIGATION
==========

There is no mitigation available for this issue.

(The security of a Xen system using stub domains is still better than
with a qemu-dm running as an unrestricted dom0 process.  Therefore
users with these configurations should not switch to an unrestricted
dom0 qemu-dm.)

CREDITS
=======

This issue was discovered by Andrew Cooper at Citrix.

RESOLUTION
==========

Fixed in xen-4.2.5-2

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