CVE-2016-7909 in QEMU
Summary
by MITRE
The pcnet_rdra_addr function in hw/net/pcnet.c in QEMU (aka Quick Emulator) allows local guest OS administrators to cause a denial of service (infinite loop and QEMU process crash) by setting the (1) receive or (2) transmit descriptor ring length to 0.
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Analysis
by VulDB Data Team • 09/22/2022
The vulnerability identified as CVE-2016-7909 resides within the QEMU hypervisor implementation, specifically in the pcnet_rdra_addr function located in the hw/net/pcnet.c file. This flaw represents a critical security issue that affects virtualized environments where QEMU serves as the underlying virtualization platform. The vulnerability manifests when local guest operating system administrators manipulate the network interface controller parameters, particularly targeting the receive or transmit descriptor ring length settings. The affected component implements the PCNet network adapter emulation which is commonly used in virtual machine configurations to provide network connectivity. This vulnerability operates at the hypervisor level, making it particularly dangerous as it can be exploited by malicious actors who have gained administrative access within a guest operating system, potentially leading to complete system compromise through denial of service attacks.
The technical flaw stems from inadequate input validation within the pcnet_rdra_addr function, which fails to properly handle edge cases when descriptor ring lengths are set to zero. When an attacker configures either the receive or transmit descriptor ring to a length of zero, the function enters an infinite loop during processing. This occurs because the implementation does not account for the scenario where ring lengths equal zero, causing the loop conditions to never terminate properly. The absence of proper bounds checking and validation mechanisms allows malicious input to trigger the problematic code path. According to CWE classification, this vulnerability maps to CWE-129, which describes improper validation of array index or buffer length, and CWE-691, which covers insufficient control of a resource through a long-lived or permanent control mechanism. The flaw specifically exploits the lack of input sanitization in the network adapter's descriptor ring management system, where the code assumes ring lengths will always be positive integers greater than zero.
The operational impact of this vulnerability extends beyond simple denial of service, creating significant risks for virtualized environments and cloud computing infrastructures that rely on QEMU for virtual machine management. When the infinite loop occurs, it consumes excessive CPU resources and can cause the QEMU process to crash entirely, resulting in complete service disruption for all virtual machines hosted on that hypervisor. This vulnerability is particularly concerning because it requires only local administrative privileges within a guest operating system, meaning that any attacker with access to a virtual machine can potentially compromise the entire host system. The attack vector is relatively simple to execute, requiring minimal technical expertise to exploit, making it a preferred target for attackers seeking to disrupt services. From an ATT&CK framework perspective, this vulnerability aligns with T1499.004, which covers network denial of service attacks, and T1548.001, which covers abuse of privilege through local system manipulation.
Mitigation strategies for CVE-2016-7909 should focus on both immediate patching and architectural improvements to prevent similar vulnerabilities. The primary solution involves applying the official QEMU patch that introduces proper validation of descriptor ring lengths before processing, ensuring that zero-length rings are handled gracefully rather than causing infinite loops. Organizations should implement network segmentation and access controls to limit guest OS administrative privileges, reducing the attack surface available to potential exploiters. Additionally, monitoring systems should be deployed to detect unusual CPU utilization patterns that may indicate the occurrence of such denial of service attacks. The vulnerability highlights the importance of robust input validation in hypervisor components, particularly those handling hardware emulation parameters. Regular security assessments of virtualization environments should include testing for similar edge case vulnerabilities in network adapter emulations and other hardware components. System administrators should also consider implementing automated failover mechanisms and redundant virtualization infrastructure to minimize the impact of such attacks on overall service availability.