CVE-2016-9923 in QEMU
Summary
by MITRE
Quick Emulator (Qemu) built with the 'chardev' backend support is vulnerable to a use after free issue. It could occur while hotplug and unplugging the device in the guest. A guest user/process could use this flaw to crash a Qemu process on the host resulting in DoS.
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Analysis
by VulDB Data Team • 07/24/2019
The vulnerability identified as CVE-2016-9923 affects Quick Emulator (QEMU) when built with chardev backend support, representing a critical use after free flaw that can be exploited through device hotplug and unplug operations within guest environments. This issue resides in the virtualization layer where QEMU manages character device backends, creating a scenario where improper memory management during device lifecycle operations can lead to severe consequences. The vulnerability specifically manifests when guest users or processes interact with hotplugged devices, particularly during the unplugging phase, where memory allocated to device structures may be freed but subsequently accessed, creating a dangerous state that can be leveraged for exploitation.
The technical flaw stems from inadequate memory management within QEMU's chardev backend implementation, where device structures are not properly synchronized during concurrent hotplug and unplug operations. When a device is removed from a guest system while still being referenced by the host QEMU process, the memory management subsystem fails to prevent access to freed memory blocks, resulting in undefined behavior that can manifest as memory corruption. This use after free condition occurs because the QEMU code does not adequately track device references or implement proper synchronization mechanisms between guest device operations and host memory management routines, creating a race condition that allows malicious guest processes to trigger the vulnerability.
The operational impact of this vulnerability extends beyond simple denial of service, as it provides a potential vector for more sophisticated attacks within virtualized environments. A guest user or process with access to device hotplug functionality can deliberately exploit this flaw to cause QEMU processes to crash, effectively creating a denial of service condition that can disrupt virtual machine operations and potentially impact other virtual machines sharing the same host. The vulnerability is particularly concerning in multi-tenant cloud environments where guest isolation is paramount, as it could allow privilege escalation or compromise of the host system through careful exploitation of the memory corruption. According to CWE classification, this vulnerability maps to CWE-416 Use After Free, which is categorized under the broader class of memory safety issues that have been consistently identified as high-risk in virtualization environments.
Mitigation strategies for CVE-2016-9923 should focus on both immediate patching and operational hardening measures. The primary solution involves applying the official QEMU patches that address the memory management issues in chardev backend operations, ensuring proper reference counting and synchronization mechanisms are implemented. Organizations should also implement strict device access controls within guest environments, limiting the ability of untrusted users to perform hotplug operations that could trigger the vulnerability. Additionally, monitoring systems should be configured to detect unusual QEMU process crashes or memory access patterns that might indicate exploitation attempts. From an ATT&CK perspective, this vulnerability aligns with techniques involving privilege escalation and denial of service, and defensive measures should include process isolation, memory protection mechanisms, and regular security assessments of virtualization environments to prevent exploitation through device management interfaces.