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Xen Patches Two QEMU Vulnerabilities

The Xen Project has released patches to address two vulnerabilities affecting the Xen virtualization software.

The Xen Project has released patches to address two vulnerabilities affecting the Xen virtualization software.

An advisory published by the Xen Project on Monday shows that one of the security holes is a leak of uninitialized heap memory related to the RTL8139 network card (CVE-2015-5165).

The vulnerability, caused by the lack of proper input validation in the C+ mode offload emulation, can lead to uninitialized memory from the QEMU machine emulator process’s heap getting leaked to the domain and the network.

The flaw allows a guest to read host-level data relating to itself from the QEMU process, including information on the devices backing the emulated devices, and passwords that the host admin might not want to share with the administrator of the guest system.

“All Xen systems running x86 HVM guests without stubdomains which have been configured with an emulated RTL8139 driver model (which is the default) are vulnerable,” the Xen Project explained in its advisory.

The Xen Project security team has noted that both the traditional (qemu-xen-traditional) and upstream-based (qemu-xen) QEMU device models are potentially vulnerable, but the patches from the QEMU Project are only for the latest version of the software and they cannot be applied directly to the traditional fork. Since it doesn’t have the resources to backport the patches, the Xen Project encourages users who are able to backport the patches to share them so that they can be distributed to others.

As a workaround, Xen customers can avoid the use of emulated network devices, or just avoid the use of the affected RTL8139 network card.

The second vulnerability has been described as a use-after-free in the QEMU/Xen block unplug protocol (CVE-2015-5166).

“When unplugging an emulated block device the device was not fully unplugged, meaning a second unplug attempt would attempt to unplug the device a second time using a previously freed pointer,” the Xen Project explained in a separate advisory.

The vulnerability allows an HVM (hardware virtual machine) guest with access to an emulated IDE disk device to take over the QEMU process and elevate privileges to the ones of the QEMU process.

The security hole affects all Xen systems running x86 HVM guests using the upstream based qemu-xen.

The Xen Project has credited Donghai Zhu of Alibaba for finding the vulnerabilities. Affected users should install the patches, or apply mitigations where possible.

On July 27, the Xen Project released patches for a VM escape vulnerability that allowed a privileged user in a guest operating system with a CD-ROM drive enabled to execute arbitrary code on the host.

One of the most serious virtualization vulnerabilities disclosed over the past period has been dubbed “VENOM.” The security bug exists in QEMU’s virtual floppy drive code — used by platforms such as Xen, KVM and VirtualBox — and it can be exploited to escape the guest VM and execute arbitrary code on the host system.

Written By

Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.

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