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            Xen Security Advisory CVE-2015-2756 / XSA-126
                              version 4

             Unmediated PCI command register access in qemu

UPDATES IN VERSION 4
====================

Normalize version tags.

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

HVM guests are currently permitted to modify the memory and I/O decode
bits in the PCI command register of devices passed through to them.
Unless the device is an SR-IOV virtual function, after disabling one or
both of these bits subsequent accesses to the MMIO or I/O port ranges
would - on PCI Express devices - lead to Unsupported Request responses.
The treatment of such errors is platform specific.

Furthermore (at least) devices under control of the Linux pciback
driver in the host are handed to guests with the aforementioned bits
turned off.  This means that such accesses can similarly lead to
Unsupported Request responses until these flags are set as needed by
the guest.

IMPACT
======

In the event that the platform surfaces aforementioned UR responses as
Non-Maskable Interrupts, and either the OS is configured to treat NMIs
as fatal or (e.g. via ACPI's APEI) the platform tells the OS to treat
these errors as fatal, the host would crash, leading to a Denial of
Service.

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

Xen versions 3.3 and onwards are vulnerable due to supporting PCI
pass-through.

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

Only HVM guests with their device model run in Dom0 can take advantage
of this vulnerability.

Any domain which is given access to a non-SR-IOV virtual function PCI
Express device can take advantage of this vulnerability.

MITIGATION
==========

This issue can be avoided by not assigning PCI Express devices other
than SR-IOV virtual functions to untrusted HVM guests.  This issue can
also be avoided by only using PV guests or HVM guests with their
device model run in a separate (stub) domain.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa126-qemuu.patch      qemu-xen master, 4.5.x, 4.4.x
xsa126-qemuu-4.3.patch  qemu-xen 4.3.x
xsa126-qemut.patch      qemu-xen-traditional 4.5.x - 4.2.x

For those already having the original patch in place, applying the
appropriate attached incremental patch addresses the regression.

xsa126-qemuu-incr.patch      qemu-xen master, 4.5.x, 4.4.x
xsa126-qemuu-4.3-incr.patch  qemu-xen 4.3.x
xsa126-qemut-incr.patch      qemu-xen-traditional 4.5.x - 4.2.x

$ sha256sum xsa126*.patch
bd69a0d18127793a9aa2097062ecaef76df6e6b8f729406d7d52cf66519e3b0d  xsa126-qemut-incr.patch
2a9b8f73b2a4f0cfb6b724c9a0a72dbf08cae87cd382f61f563218c32d1036a7  xsa126-qemut.patch
658bc483d1110e4e04de2d70fba1cdb20c5cecdc2f419db2d82bddc3ae1690b6  xsa126-qemuu-4.3-incr.patch
090d9262a9e9d24f0f4eca35cb0d56831d5cec6a6ba38b4c7e276d767de660c1  xsa126-qemuu-4.3.patch
3f7b6737c08ff7e119bec16c8c3b3cb832429f1410e687edf622fab57a22842e  xsa126-qemuu-incr.patch
eb5b93600267639b2cda1c5e2f937ddbecbf6c8cbd19dbb355224c39c2e40d3e  xsa126-qemuu.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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