CVE-2017-9330 in QEMU
Summary
by MITRE
QEMU (aka Quick Emulator), when built with the USB OHCI Emulation support, allows local guest OS users to cause a denial of service (infinite loop) by leveraging an incorrect return value.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-9330 resides within QEMU, a widely-used open-source emulator that enables users to run multiple operating systems on a single machine through virtualization. This particular flaw manifests when QEMU is compiled with USB OHCI (Open Host Controller Interface) emulation support, which is essential for emulating USB controllers in virtualized environments. The vulnerability specifically affects local guest operating system users who can exploit this weakness to trigger a denial of service condition characterized by an infinite loop within the emulator's USB handling code.
The technical root cause of this vulnerability stems from an incorrect return value handling within the USB OHCI emulation code. When processing certain USB transfer requests, the emulator fails to properly validate or handle the return values from underlying USB operations, leading to a scenario where the emulator enters an infinite loop during USB data transfer processing. This condition occurs because the code does not adequately check for error states or properly handle edge cases in USB transaction completion, causing the emulator to repeatedly execute the same code path without progressing to the next state in the USB transfer sequence.
From an operational perspective, this vulnerability poses significant risks to virtualized environments where multiple operating systems may be running simultaneously. Local guest users who have access to the virtual machine can exploit this flaw to cause the entire QEMU emulator to hang, effectively rendering the virtual machine unresponsive and potentially impacting other virtual machines running on the same host system. The infinite loop condition consumes CPU resources continuously, which can lead to system performance degradation or complete system lockup, particularly in resource-constrained virtualization environments.
The impact of this vulnerability extends beyond simple denial of service as it can be leveraged by malicious actors within the guest operating system to disrupt services or cause system instability. This weakness is particularly concerning in multi-tenant virtualization environments where guest users might attempt to compromise the stability of other virtual machines or the host system itself. The vulnerability aligns with CWE-691, which addresses inadequate control flow in software systems, and can be categorized under ATT&CK technique T1499.001 for network denial of service attacks, as the infinite loop effectively creates a service disruption that impacts system availability.
Mitigation strategies for this vulnerability include applying the latest QEMU patches and updates from the maintainers, which typically involve correcting the return value handling logic in the USB OHCI emulation code. Organizations should also implement proper virtualization security measures including guest user isolation, monitoring for unusual CPU consumption patterns, and maintaining updated virtualization platforms. Additionally, administrators should consider disabling USB OHCI emulation when not required for specific virtual machine configurations, and implementing network segmentation to limit potential attack vectors. The vulnerability demonstrates the importance of thorough input validation and error handling in virtualization software, particularly in components that interface with hardware emulation layers that can be directly manipulated by guest operating systems.