CVE-2016-9921 in QEMU
Summary
by MITRE
Quick emulator (Qemu) built with the Cirrus CLGD 54xx VGA Emulator support is vulnerable to a divide by zero issue. It could occur while copying VGA data when cirrus graphics mode was set to be VGA. A privileged user inside guest could use this flaw to crash the Qemu process instance on the host, resulting in DoS.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-9921 affects the Quick Emulator Qemu when configured with Cirrus CLGD 54xx VGA Emulator support, representing a critical divide by zero error that can be exploited to cause denial of service conditions. This flaw exists within the graphics handling subsystem of the virtualization platform, specifically when the cirrus graphics mode is set to VGA mode, creating a scenario where malicious code within a guest operating system can manipulate the emulator's behavior to trigger arithmetic exceptions. The vulnerability stems from inadequate input validation and error handling within the VGA data copying operations, where division operations fail to account for zero divisor conditions that can occur during graphics memory management processes. Such conditions are particularly dangerous in virtualized environments where guest operating systems may have elevated privileges or access to hardware emulation layers that can be leveraged for exploitation.
The technical exploitation of this vulnerability requires a privileged user within the guest environment to execute code that triggers the specific graphics mode configuration leading to the division by zero error. When the cirrus graphics mode is set to VGA, the emulator's VGA data copying routine attempts to perform division operations on values that may become zero, causing the Qemu process to terminate abruptly. This behavior aligns with CWE-369, which describes the weakness of dividing by zero in software systems, and demonstrates how such fundamental mathematical errors can cascade into system-wide failures. The vulnerability is particularly concerning because it can be triggered through legitimate graphics operations that occur during normal system usage, making it difficult to distinguish between benign and malicious execution paths. Attackers can exploit this condition by manipulating graphics rendering operations to force the emulator into a state where division by zero becomes inevitable, effectively creating a reliable method for crashing the host process.
The operational impact of CVE-2016-9921 extends beyond simple denial of service, as it can compromise the stability and availability of virtualized environments where Qemu serves as the primary emulation platform. When the Qemu process crashes due to this divide by zero condition, the entire virtual machine instance becomes unavailable, potentially affecting multiple users or services that depend on that virtual environment. This vulnerability is particularly relevant in cloud computing environments where multiple virtual machines share the same host resources, as a single compromised guest could potentially affect the entire host system's performance and stability. The flaw also demonstrates the broader security implications of virtualization platforms, where guest operating systems can leverage emulation layer vulnerabilities to attack host systems, a concept that maps directly to ATT&CK technique T1059.001 for command and scripting interpreter, as attackers may use graphics operations to execute malicious code that ultimately triggers the arithmetic error. Organizations using Qemu in production environments face significant risk from this vulnerability, as it can be exploited by attackers with guest access privileges to cause service disruption.
Mitigation strategies for CVE-2016-9921 focus on both immediate patching and architectural considerations to prevent exploitation of the divide by zero condition. The most effective solution involves applying the official Qemu patches that address the specific error handling issues within the cirrus graphics emulation code, ensuring that division operations properly validate their operands before execution. System administrators should also consider implementing additional monitoring and alerting mechanisms around Qemu processes to detect abnormal termination patterns that may indicate exploitation attempts. Configuration management practices should include disabling unnecessary graphics emulation features when they are not required for specific virtual machine configurations, reducing the attack surface for potential exploitation. The vulnerability also highlights the importance of proper input validation and error handling in virtualization software, as similar issues may exist in other graphics emulation components. Organizations should conduct thorough security assessments of their virtualization environments to identify other potential divide by zero conditions or similar arithmetic errors that could lead to system instability, particularly in hardware emulation layers that handle user input or external data streams. Furthermore, implementing proper process isolation and resource limiting for Qemu instances can help contain the impact of any successful exploitation attempts, preventing cascading failures that could affect multiple virtual machines or host services.