|Public release ||2016-12-06 12:00|
|Updated ||2016-12-06 12:11|
|Title ||qemu ioport array overflow|
Filesadvisory-199.txt (signed advisory file)
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Xen Security Advisory CVE-2016-9637 / XSA-199
qemu ioport array overflow
UPDATES IN VERSION 3
Clarify the IMPACT description, by escalating privilege to that of the
qemu process, not necesserily the host.
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.
A malicious guest administrator can escalate their privilege to that
of the qemu process.
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
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.
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.
This issue was discovered by email@example.com of the Huawei
Security Test Team.
Applying the attached patch resolves this issue.
xsa199-trad.patch qemu-xen-traditional, all versions
$ sha256sum xsa199*
DEPLOYMENT DURING EMBARGO
Deployment of the patch described above (or others which are
substantially similar) is permitted during the embargo, even on
public-facing systems with untrusted guest users and administrators.
However deployment of the mitigations described above is NOT permitted
(except where all the affected systems and VMs are administered and
used only by organisations which are members of the Xen Project
Security Issues Predisclosure List). Specifically, deployment on
public cloud systems is NOT permitted.
This is because in all cases the configuration change may be visible
to the guest which could lead to the rediscovery of the vulnerability.
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
(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:
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Xenproject.org Security Team