CVE-2016-4037 in QEMU
Summary
by MITRE
The ehci_advance_state function in hw/usb/hcd-ehci.c in QEMU allows local guest OS administrators to cause a denial of service (infinite loop and CPU consumption) via a circular split isochronous transfer descriptor (siTD) list, a related issue to CVE-2015-8558.
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Analysis
by VulDB Data Team • 08/22/2022
The vulnerability identified as CVE-2016-4037 resides within the QEMU virtual machine monitor's USB emulation subsystem, specifically in the ehci_advance_state function located in hw/usb/hcd-ehci.c. This flaw represents a critical security issue that affects virtualized environments where guest operating systems can exploit a design weakness in the USB EHCI (Enhanced Host Controller Interface) controller implementation. The vulnerability stems from inadequate input validation and state management within the USB transfer descriptor processing logic, creating a scenario where malicious or compromised guest administrators can manipulate the USB controller's operation.
The technical flaw manifests when a circular split isochronous transfer descriptor (siTD) list is constructed within the guest operating system. An siTD represents a specific type of USB transfer descriptor used for isochronous data transfers, which are time-critical and require guaranteed bandwidth allocation. When the guest OS creates a circular reference within the siTD list structure, the ehci_advance_state function fails to properly detect this loop condition during its state advancement algorithm. This failure causes the function to enter an infinite loop where it continuously processes the same set of descriptors without making forward progress, leading to complete CPU consumption on the host system.
The operational impact of this vulnerability extends beyond simple denial of service, as it can severely compromise the availability and performance of the entire virtualized environment. Since the issue affects the USB controller emulation layer, any guest OS administrator with local access can trigger this condition, making it particularly dangerous in multi-tenant virtualization environments or cloud computing platforms where guest isolation is paramount. The infinite loop consumes 100% of a CPU core, preventing legitimate virtual machine operations from proceeding normally and potentially causing cascading failures across multiple virtual machines sharing the same host resources. This vulnerability directly relates to CVE-2015-8558, indicating a pattern of similar flaws in USB controller implementations within virtualization platforms.
From a cybersecurity perspective, this vulnerability aligns with CWE-838, which covers insufficient input validation, and represents a classic example of a resource exhaustion attack. The ATT&CK framework categorizes this under T1499.004, specifically "Evasion: Virtualization/Sandbox Evasion," as it exploits the virtualization environment itself to create a denial of service condition. The flaw demonstrates poor defensive programming practices in handling linked data structures and insufficient loop detection mechanisms within the USB controller's state machine. Organizations using QEMU-based virtualization platforms must implement immediate mitigations including updating to patched versions, implementing resource limits on virtual machines, and monitoring for unusual CPU consumption patterns. The vulnerability also highlights the importance of proper input validation in virtualization components and underscores the need for comprehensive testing of edge cases in emulated hardware controllers.