CVE-2020-25625 in QEMU
Summary
by MITRE
hw/usb/hcd-ohci.c in QEMU 5.0.0 has an infinite loop when a TD list has a loop.
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Analysis
by VulDB Data Team • 10/26/2024
The vulnerability identified as CVE-2020-25625 affects QEMU version 5.0.0 and resides within the ohci (Open Host Controller Interface) USB host controller emulation code located in hw/usb/hcd-ohci.c. This flaw represents a critical denial of service vulnerability that can be exploited through malformed USB transfer descriptors within the OHCI controller implementation. The issue manifests when the USB host controller encounters a Transfer Descriptor (TD) list that contains a loop structure, causing the emulation to enter an infinite loop state that prevents normal operation of the virtual machine.
The technical root cause stems from inadequate validation of the TD list structure during USB data transfer processing. When the OHCI controller processes a TD list containing circular references or loops, the processing logic fails to detect these malformed structures and continues iterating through the loop indefinitely. This behavior violates standard USB protocol implementations and demonstrates a fundamental flaw in the input validation mechanisms within the QEMU USB controller emulation. The vulnerability specifically impacts the ohci controller implementation which is used to emulate USB host controllers in virtualized environments, making it particularly dangerous in cloud computing and virtualization scenarios where multiple virtual machines share host resources.
The operational impact of this vulnerability extends beyond simple denial of service as it can lead to complete system hangs and resource exhaustion within virtualized environments. Attackers can exploit this weakness by crafting malicious USB devices or transferring data through USB interfaces that contain malformed TD lists, causing the QEMU hypervisor to consume excessive CPU resources or become completely unresponsive. This vulnerability affects all systems running QEMU 5.0.0 with USB controller emulation enabled, including cloud providers, development environments, and virtualization platforms that rely on QEMU for USB device passthrough functionality. The infinite loop condition can persist until manual intervention occurs, potentially causing cascading failures in multi-tenant virtualization environments where one compromised VM could affect others on the same host.
Security mitigations for this vulnerability include immediate patching of QEMU to versions that contain the fix for the TD list loop detection logic. Organizations should also implement monitoring systems to detect unusual CPU usage patterns that might indicate exploitation attempts. Network segmentation and USB device access controls should be enforced to limit potential attack vectors, while regular security assessments of virtualization environments should be conducted to identify similar vulnerabilities in other emulated hardware components. This vulnerability aligns with CWE-835, which describes the weakness of an infinite loop or infinite recursion in software implementations, and corresponds to ATT&CK technique T1489, which covers denial of service through resource exhaustion. The fix typically involves implementing proper loop detection mechanisms in the TD list traversal logic and adding bounds checking to prevent unbounded iteration through potentially malformed USB transfer descriptor structures.