Bug 1397043 (CVE-2016-9637, xsa199)

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

The code in qemu which implements ioport read/write looks up the
specified ioport address in a dispatch table.  The argument to the
dispatch function is a uint32_t, and is used without a range check,
even though the table has entries for only 2^16 ioports.

When qemu is used as a standalone emulator, ioport accesses are
generated only from cpu instructions emulated by qemu, and are
therefore necessarily 16-bit, so there is no vulnerability.

When qemu is used as a device model within Xen, io requests are
generated by the hypervisor and read by qemu from a shared ring.  The
entries in this ring use a common structure, including a 64-bit
address field, for various accesses, including ioport addresses.

Xen will write only 16-bit address ioport accesses.  However,
depending on the Xen and qemu version, the ring may be writeable by
the guest.  If so, the guest can generate out-of-range ioport
accesses, resulting in wild pointer accesses within qemu.


IMPACT
======

A malicious guest administrator can escalate their privilege to that
of the qemu process.


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

PV guests cannot exploit the vulnerability.

ARM systems are not vulnerable.

HVM domains run with QEMU stub domains cannot exploit the
vulnerability.  (A QEMU stub domain is used if xl's domain
configuration file contains "device_model_stubdomain_override=1".)

Guests using the modern "qemu-xen" device model, with a qemu version
of at least 1.6.0 (for example, as provided by the Xen Project in its
Xen 4.4.0 and later releases), cannot exploit the vulnerability.

x86 HVM guests, not configured with qemu stub domains, using a version
of qemu older than qemu upstream 1.6.0, can exploit the vulnerability.

x86 HVM guests using the traditional "qemu-xen-traditional", not
configured with qemu stub domains, can therefore exploit the
vulnerability.

In tabular form:

Guest      Xen       QEMU    QEMU "traditional"            Status
type       version   stub      and/or qemu version

ARM        any       n/a     n/a         any               OK
x86 PV     any       n/a     n/a         any               OK

x86 HVM    any       yes     qemu-xen-traditional          OK

x86 HVM    any       no      qemu-xen*   >= 1.6.0          OK
x86 HVM    >= 4.4    no      qemu-xen*   Xen supplied      OK

x86 HVM    any       no      qemu-xen*   < 1.6.0           Vulnerable
x86 HVM    <= 4.3    no      qemu-xen*   Xen supplied      Vulnerable

x86 HVM    any       no      qemu-xen-traditional          Vulnerable

[*] qemu-xen is the default when qemu stub domains are not in
use, since Xen 4.3.


MITIGATION
==========

Enabling stubdomains will mitigate this issue, by reducing the
escalation to only those privileges accorded to the service domain.
In a usual configuration, a service domain has only the privilege of
the guest, so this eliminates the vulnerability.

Running HVM guests with the default upstream device model, in Xen 4.4
and later, will also avoid this vulnerability.


External References:

http://xenbits.xen.org/xsa/advisory-199.html

Acknowledgements:

Name: the Xen project