CVE-2016-8576 in QEMU
Summary
by MITRE
The xhci_ring_fetch function in hw/usb/hcd-xhci.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (infinite loop and QEMU process crash) by leveraging failure to limit the number of link Transfer Request Blocks (TRB) to process.
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Analysis
by VulDB Data Team • 09/29/2022
The vulnerability identified as CVE-2016-8576 resides within the QEMU virtualization platform's implementation of the xhci_ring_fetch function located in hw/usb/hcd-xhci.c. This function serves as a critical component in handling USB 3.0 host controller communications within the virtualized environment. The flaw manifests when the function fails to properly constrain the processing of link Transfer Request Blocks, which are fundamental data structures used in USB 3.0 communication protocols to manage data transfer operations between host controllers and peripheral devices. The issue specifically affects local guest OS administrators who possess administrative privileges within the virtual machine, enabling them to exploit this weakness through carefully crafted USB traffic patterns.
The technical nature of this vulnerability stems from inadequate input validation and loop control mechanisms within the USB controller emulation layer. When a malicious guest administrator crafts USB traffic containing excessive or malformed link TRB entries, the xhci_ring_fetch function enters an infinite loop during processing. This occurs because the function does not implement proper bounds checking or iteration limits on the number of link TRBs it will process, allowing an attacker to potentially exhaust system resources and cause the QEMU process to crash entirely. The vulnerability represents a classic case of insufficient loop termination conditions and lacks proper resource management controls that would prevent such unbounded processing scenarios.
The operational impact of CVE-2016-8576 extends beyond simple denial of service, as it can be leveraged to disrupt virtualized environments and potentially compromise the stability of entire virtual machine infrastructures. In cloud computing and data center environments where QEMU serves as a foundational virtualization technology, this vulnerability could enable attackers to systematically crash virtual machines, leading to service disruption and potential data loss. The local privilege escalation aspect means that even users with limited access within a guest OS could potentially cause significant damage to the host system's virtualization layer, making this particularly concerning for multi-tenant environments. According to CWE classification, this vulnerability maps to CWE-835: Loop with Unreachable Exit Condition (Loop Interference), which specifically addresses issues where loop termination conditions are not properly enforced.
Mitigation strategies for this vulnerability require immediate patching of QEMU installations to implement proper bounds checking and iteration limits within the xhci_ring_fetch function. System administrators should also consider implementing additional monitoring and resource limiting mechanisms around QEMU processes to detect and prevent resource exhaustion attacks. The ATT&CK framework categorizes this type of vulnerability under T1499.004: Endpoint Denial of Service, where adversaries leverage resource exhaustion techniques to disrupt system operations. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of local privilege escalation within virtualized environments. Regular security assessments of virtualization platforms and maintaining updated virtualization software are essential practices to prevent exploitation of similar vulnerabilities in the USB controller emulation layers of virtualization software.