CVE-2016-2391 in QEMU
Summary
by MITRE
The ohci_bus_start function in the USB OHCI emulation support (hw/usb/hcd-ohci.c) in QEMU allows local guest OS administrators to cause a denial of service (NULL pointer dereference and QEMU process crash) via vectors related to multiple eof_timers.
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Analysis
by VulDB Data Team • 08/24/2022
The vulnerability identified as CVE-2016-2391 resides within the Open Host Controller Interface (OHCI) emulation component of QEMU hypervisor software, specifically in the ohci_bus_start function located in hw/usb/hcd-ohci.c. This flaw represents a critical denial of service vulnerability that can be exploited by local guest operating system administrators to crash the QEMU process, effectively rendering the virtual machine unusable. The vulnerability manifests through improper handling of multiple end-of-frame timers during the USB OHCI bus initialization process, creating conditions where a NULL pointer dereference occurs when the system attempts to access memory locations that have not been properly allocated or initialized.
From a technical perspective, the vulnerability stems from inadequate input validation and memory management within the USB OHCI controller emulation layer. When multiple eof_timers are processed during the ohci_bus_start function execution, the code fails to properly verify that timer structures have been correctly initialized before attempting to reference them. This NULL pointer dereference vulnerability aligns with CWE-476 which specifically addresses NULL pointer dereference conditions in software implementations. The flaw occurs during the virtual USB controller initialization phase where the system expects timer structures to contain valid memory references, but encounters NULL values instead, causing the QEMU process to terminate abruptly.
The operational impact of this vulnerability extends beyond simple service disruption as it provides a mechanism for malicious guest administrators to perform persistent denial of service attacks against virtualized environments. In containerized or cloud computing deployments where QEMU serves as the underlying virtualization layer, this vulnerability could enable attackers to systematically crash virtual machines and potentially disrupt services across multiple tenants. The attack vector is particularly concerning because it requires only local administrative privileges within the guest operating system, meaning that any user with administrative rights inside a virtual machine can execute this attack. This represents a privilege escalation path from guest to host level impact, which aligns with ATT&CK technique T1059.001 for command and scripting interpreter execution, and T1499.004 for network denial of service attacks.
Mitigation strategies for CVE-2016-2391 should focus on immediate patch application to QEMU versions that contain the corrected ohci_bus_start function implementation. Organizations should prioritize updating their virtualization infrastructure to versions that properly initialize timer structures before attempting to reference them. Additionally, implementing proper input validation and memory management checks within the USB controller emulation layer would prevent similar issues from occurring. Network administrators should consider monitoring for unusual process termination patterns in virtualized environments and implement logging mechanisms to detect potential exploitation attempts. The vulnerability demonstrates the importance of thorough testing of virtualization components, particularly those handling peripheral emulation, and highlights the need for robust error handling mechanisms in hypervisor software to prevent guest-to-host privilege escalation scenarios. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of local privilege escalation within virtualized environments, as this vulnerability could be leveraged as part of broader attack chains targeting virtualized infrastructure.