[kernel-xen] Xen Security Advisory 61 - libxl partially sets up HVM passthrough even with disabled iommu

[Steven Haigh]

                    Xen Security Advisory XSA-61

     libxl partially sets up HVM passthrough even with disabled iommu

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

With HVM domains, libxl's setup of PCI passthrough devices does the
IOMMU setup after giving (via the device model) the guest access to
the hardware and advertising it to the guest.

If the IOMMU is disabled the overall setup fails, but after the device
has been made available to the guest; subsequent DMA instructions from
the guest to the device will cause wild DMA.

IMPACT
======

A HVM domain, given access to a device which bus mastering capable in
the absence of a functioning IOMMU, can mount a privilege escalation
or denial of service attack affecting the whole system.

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

1. Only systems which pass busmastering-capable PCI devices through to
   untrusted guests are vulnerable.  (Most PCI devices are
   busmastering-capable.)

2. Only systems which use libxl as part of the toolstack are
   vulnerable.

   The major consumer of libxl functionality is the xl toolstack which
   became the default in Xen 4.2.

   In addition to this libvirt can optionally make use of libxl. This
   can be queried with
           # virsh version
   which will report "xenlight" if libxl is in use.  libvirt currently
   prefers the xend backend if xend is running.

   The xend and xapi toolstacks do not currently use libxl.

3. Only Xen versions 4.0.x through 4.2.x are vulnerable.

4. Only HVM domains can take advantage of this vulnerability.

5. Systems which have a functioning IOMMU are NOT vulnerable.

MITIGATION
==========

This issue can be avoided by not assigning PCI devices to HVM guests
when there is no functioning IOMMU.

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

This issue was disclosed publicly on xen-devel; the person reporting
it did not appreciate that it was a security issue.  Additionally the
patch to fix the issue was already applied to the respective branches
(in particular resulting in Xen 4.3 not being vulnerable).  Under the
circumstances the Xen.org security team do not consider that this
advisory should be embargoed.

Also, we apologise for the delay to this advisory message, which was
due to an oversight by us.

CREDITS
=======

George Dunlap found the issue as a bug, which on examination by the
Xenproject.org Security Team turned out to be a security problem.

RESOLUTION
==========

xen-4.2.3-2 is currently being built and will be distributed to the
mirrors shortly.

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