CVE-2016-6490 in QEMU
Summary
by MITRE
The virtqueue_map_desc function in hw/virtio/virtio.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (infinite loop and QEMU process crash) via a zero length for the descriptor buffer.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability identified as CVE-2016-6490 represents a critical flaw in the QEMU virtualization platform that affects the virtio subsystem's handling of descriptor buffers within virtualized environments. This issue stems from the virtqueue_map_desc function located in the hw/virtio/virtio.c source file, which governs how virtual I/O queues process data descriptors. The flaw manifests when a guest operating system administrator deliberately manipulates the descriptor buffer length to zero, creating a condition that causes the QEMU process to enter an infinite loop before ultimately crashing. This vulnerability specifically targets the virtualization layer where guest operating systems communicate with virtualized hardware through virtio drivers, making it particularly dangerous in multi-tenant cloud environments where guest administrators might attempt to exploit such weaknesses to disrupt other virtual machines or the host system itself.
The technical implementation of this vulnerability falls under CWE-835, which describes the weakness of an infinite loop in software systems. The flaw occurs because the virtqueue_map_desc function fails to properly validate the length parameter of descriptor buffers before attempting to process them. When a zero-length buffer is encountered, the function's loop logic becomes trapped in an infinite iteration pattern, consuming excessive CPU resources and eventually causing the QEMU process to terminate. This behavior represents a classic denial of service attack vector that can be exploited by malicious actors within the guest environment to disrupt the virtualized system's operation. The vulnerability demonstrates poor input validation practices in the virtualization stack, where the system does not adequately check for edge cases such as zero-length memory regions that could cause control flow anomalies.
The operational impact of CVE-2016-6490 extends beyond simple service disruption to encompass broader security implications within virtualized infrastructures. Local guest administrators with access to virtual machine environments can leverage this vulnerability to perform persistent denial of service attacks against QEMU processes, potentially affecting multiple virtual machines running on the same host system. This weakness is particularly concerning in cloud computing environments where tenant isolation is paramount, as it could allow one tenant to compromise the resources of other tenants or the underlying host infrastructure. The vulnerability also aligns with ATT&CK technique T1499.001, which covers network denial of service attacks, and T1070.006, which involves indicator removal on host systems, as the infinite loop behavior can obscure legitimate system issues and make troubleshooting difficult. The crash condition can result in data loss, service interruption, and potential compromise of the virtualization platform's stability.
Mitigation strategies for CVE-2016-6490 require immediate patching of affected QEMU versions, with the vulnerability being addressed through proper validation of descriptor buffer lengths before processing. System administrators should implement monitoring solutions to detect unusual CPU consumption patterns that might indicate exploitation attempts, while also maintaining strict access controls to prevent unauthorized guest administrators from manipulating virtualized hardware interfaces. The fix typically involves adding a length validation check in the virtqueue_map_desc function to ensure that zero-length buffers are properly handled rather than processed in a loop. Organizations should also consider implementing virtualization-specific security controls that limit guest access to potentially dangerous I/O operations and maintain regular vulnerability assessments of their virtualization environments. This vulnerability underscores the importance of comprehensive input validation in virtualization platforms and demonstrates how seemingly benign edge cases can lead to critical system failures in complex virtualized environments.