CVE-2016-9913 in QEMU
Summary
by MITRE
Memory leak in the v9fs_device_unrealize_common function in hw/9pfs/9p.c in QEMU (aka Quick Emulator) allows local privileged guest OS users to cause a denial of service (host memory consumption and possibly QEMU process crash) via vectors involving the order of resource cleanup.
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Analysis
by VulDB Data Team • 10/27/2022
The vulnerability identified as CVE-2016-9913 represents a critical memory management flaw within QEMU's 9p filesystem implementation that enables local privilege escalation attacks leading to denial of service conditions. This issue specifically affects the v9fs_device_unrealize_common function located in the hw/9pfs/9p.c source file, which is responsible for cleaning up resources when virtualized 9p filesystem devices are removed from guest operating systems. The flaw manifests when the cleanup sequence does not properly handle resource deallocation, creating persistent memory references that prevent garbage collection and lead to progressive memory consumption on the host system.
The technical implementation of this vulnerability stems from improper resource management during the device unrealization process in QEMU's 9p filesystem subsystem. When a guest operating system attempts to remove or destroy a 9p filesystem device, the v9fs_device_unrealize_common function fails to correctly release all allocated memory structures and references associated with the device. This memory leak occurs due to the incorrect ordering of cleanup operations, where dependent resources are not properly freed before their references are destroyed, leaving memory segments in an inconsistent state. The vulnerability is particularly dangerous because it operates within the guest OS context, allowing privilege-escalated users to exploit it without requiring external network access or elevated host privileges.
From an operational impact perspective, this vulnerability enables attackers with local access to a guest operating system to systematically consume host memory resources through repeated exploitation attempts. The memory leak can progressively degrade host system performance until the QEMU process either consumes all available host memory or crashes entirely, resulting in complete denial of service for all virtual machines running on that host. This creates significant security implications for multi-tenant cloud environments where guest users could potentially disrupt other virtual machines sharing the same host infrastructure. The vulnerability aligns with CWE-401, which categorizes memory leaks as a fundamental weakness in resource management, and can be mapped to ATT&CK technique T1499.001, which describes resource exhaustion attacks targeting system availability.
Mitigation strategies for CVE-2016-9913 require immediate patching of affected QEMU versions through official security updates that correct the resource cleanup ordering in the v9fs_device_unrealize_common function. System administrators should also implement monitoring solutions to detect unusual memory consumption patterns in virtualized environments, particularly when multiple guests are accessing 9p filesystems. Additional protective measures include restricting guest OS privileges for 9p filesystem access, implementing memory limits on QEMU processes, and regularly auditing virtual machine configurations to prevent unnecessary 9p filesystem exposure. Organizations should also consider implementing network segmentation and access controls to limit potential attack vectors and reduce the impact of such vulnerabilities in their virtualized infrastructures. The fix addresses the root cause by ensuring proper resource cleanup ordering and implementing robust reference counting mechanisms to prevent memory leaks during device removal operations.