CVE-2017-6505 in QEMU
Summary
by MITRE
The ohci_service_ed_list function in hw/usb/hcd-ohci.c in QEMU (aka Quick Emulator) allows local guest OS users to cause a denial of service (infinite loop) via vectors involving the number of link endpoint list descriptors.
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Analysis
by VulDB Data Team • 10/15/2019
The vulnerability identified as CVE-2017-6505 resides within the Open Host Controller Interface implementation of QEMU, specifically in the ohci_service_ed_list function located in hw/usb/hcd-ohci.c. This flaw represents a classic denial of service condition that can be exploited by local users within a guest operating system environment. The vulnerability manifests when processing USB endpoint list descriptors, creating a scenario where the function enters an infinite loop that consumes excessive system resources and renders the virtual machine unresponsive. The issue stems from inadequate validation of descriptor counts and link pointer handling within the USB host controller emulation layer, allowing malicious guest code to manipulate the endpoint descriptor list in a manner that causes the host controller service routine to continuously iterate without termination.
From a technical perspective, this vulnerability operates at the intersection of virtualization security and USB protocol implementation. The ohci_service_ed_list function is responsible for servicing USB endpoint descriptors within the Open Host Controller Interface specification, which governs how USB devices communicate with host controllers. When the function processes endpoint descriptors with malformed or maliciously crafted link pointers and descriptor counts, it fails to properly validate the loop termination conditions. This creates a path where the function continuously follows link pointers without encountering proper termination conditions, leading to the infinite loop behavior. The vulnerability specifically affects the USB host controller emulation layer in QEMU, which implements the OHCI standard for USB 1.1 host controller functionality, making it particularly relevant to virtual environments where USB device passthrough or emulation is utilized.
The operational impact of CVE-2017-6505 extends beyond simple service disruption, as it represents a potential vector for resource exhaustion attacks within virtualized environments. Local guest users can leverage this vulnerability to consume CPU cycles indefinitely, potentially causing the host system to become unresponsive or leading to denial of service conditions that affect other virtual machines running on the same host. This vulnerability is particularly concerning in multi-tenant virtualization environments where guest isolation is paramount, as it allows a compromised guest to affect the stability of the entire host system. The infinite loop behavior can also prevent proper system shutdown procedures and may cause cascading failures when combined with other resource exhaustion vulnerabilities. From an attack perspective, this vulnerability aligns with the attack pattern described in the ATT&CK framework under the T1499.004 technique for Network Denial of Service, as it can be used to exhaust system resources and render services unavailable.
Mitigation strategies for CVE-2017-6505 should focus on both immediate patching and architectural defenses within virtualization environments. The primary remediation involves updating QEMU to versions that include proper input validation and loop termination checks within the ohci_service_ed_list function, ensuring that descriptor counts and link pointers are properly validated before processing. Organizations should implement strict USB device access controls and limit guest OS capabilities to reduce the attack surface. The vulnerability demonstrates the importance of proper input validation in virtualization components, aligning with CWE-129 which addresses issues related to insufficient validation of length fields. Additionally, monitoring systems should be configured to detect anomalous CPU usage patterns that might indicate the exploitation of this vulnerability. Security teams should also consider implementing virtual machine isolation measures and resource quotas to prevent a single compromised guest from affecting host system stability. The vulnerability underscores the critical need for robust validation of all input data within virtualization components and highlights the importance of following secure coding practices as outlined in industry standards such as the OWASP Secure Coding Practices and NIST guidelines for virtualization security.