CVE-2016-4020 in QEMU
Summary
by MITRE
The patch_instruction function in hw/i386/kvmvapic.c in QEMU does not initialize the imm32 variable, which allows local guest OS administrators to obtain sensitive information from host stack memory by accessing the Task Priorty Register (TPR).
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Analysis
by VulDB Data Team • 08/22/2022
The vulnerability identified as CVE-2016-4020 resides within the QEMU virtualization platform's handling of Intel KVM virtual APIC functionality, specifically in the hw/i386/kvmvapic.c component. This issue represents a classic case of uninitialized variable usage that creates a potential information disclosure channel between guest and host systems. The patch_instruction function fails to properly initialize the imm32 variable before its use, creating a scenario where guest operating system administrators can exploit this oversight to access sensitive data from the host's stack memory.
The technical flaw manifests through the improper handling of the Task Priority Register (TPR) within the virtual APIC implementation. When the patch_instruction function processes certain instruction patches, it relies on the imm32 variable without proper initialization, leaving its contents undefined and potentially containing remnants of previous stack operations. This uninitialized variable can contain sensitive information such as cryptographic keys, memory addresses, or other confidential data that was previously stored in the same memory location. Attackers with administrative privileges within the guest operating system can leverage this vulnerability by manipulating the TPR access patterns to indirectly read this uninitialized memory content.
The operational impact of CVE-2016-4020 extends beyond simple information disclosure, as it creates a potential vector for privilege escalation and data leakage attacks. A local guest administrator can use this vulnerability to extract sensitive host memory contents, which may include kernel memory structures, cryptographic keys, or other confidential information that could be used for further exploitation. This vulnerability particularly affects virtualized environments where guest operating systems have administrative access, as it allows attackers to bypass the typical isolation boundaries between guest and host systems. The flaw operates at the hypervisor level, making it particularly dangerous since it can be exploited even when guest operating systems are properly secured.
From a cybersecurity framework perspective, this vulnerability maps directly to CWE-457: Use of Uninitialized Variable, which is classified under the broader category of software weaknesses that can lead to information disclosure and privilege escalation. The ATT&CK framework would categorize this under T1059.001: Command and Scripting Interpreter - PowerShell and T1566.001: Phishing - Spearphishing Attachment, as attackers might use this vulnerability to gather intelligence before launching more sophisticated attacks. The vulnerability also relates to T1003.003: OS Credential Dumping - LSASS Memory, as the leaked memory could contain credential-related information. Mitigation strategies should focus on proper code review processes, static analysis tools, and ensuring all variables are properly initialized before use. The most effective remediation involves patching the QEMU implementation to properly initialize the imm32 variable, as well as implementing runtime checks to prevent uninitialized variable usage in critical code paths. Organizations should also consider implementing network segmentation and access controls to limit guest administrative privileges, reducing the attack surface available to potential exploiters of this vulnerability.