CVE-2018-20124 in QEMU
Summary
by MITRE
hw/rdma/rdma_backend.c in QEMU allows guest OS users to trigger out-of-bounds access via a PvrdmaSqWqe ring element with a large num_sge value.
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Analysis
by VulDB Data Team • 10/26/2024
The vulnerability identified as CVE-2018-20124 resides within the QEMU virtualization platform's RDMA (Remote Direct Memory Access) backend implementation, specifically in the hw/rdma/rdma_backend.c file. This flaw represents a critical security issue that affects virtualized environments where RDMA hardware acceleration is enabled. The vulnerability manifests when a guest operating system user manipulates the PvrdmaSqWqe (PVRDMA Send Work Queue Element) ring element by setting an excessively large num_sge value, which directly corresponds to the number of scatter-gather elements in the RDMA operation. This condition creates a scenario where the virtual machine's guest can bypass normal memory access controls and potentially execute unauthorized memory operations.
The technical root cause of this vulnerability stems from insufficient input validation within the RDMA backend code. When processing RDMA work queue elements, the system fails to properly validate the num_sge parameter against expected bounds, allowing malicious input to cause array indexing beyond allocated memory regions. This out-of-bounds access vulnerability falls under CWE-129, which specifically addresses insufficient validation of length of inputs, and can be categorized as a buffer overflow condition. The flaw enables an attacker to manipulate memory layout by providing an oversized scatter-gather element count, potentially leading to memory corruption that could be exploited for privilege escalation or denial of service attacks. The vulnerability is particularly concerning in virtualized environments where multiple guest operating systems share the same physical hardware resources.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates potential pathways for privilege escalation within virtualized environments. An attacker with access to a guest operating system could leverage this vulnerability to gain elevated privileges within the hypervisor, potentially compromising the entire virtualization infrastructure. The attack surface is significant since RDMA is commonly used in high-performance computing environments, data centers, and cloud platforms where QEMU virtual machines are prevalent. This vulnerability directly impacts the security isolation guarantees that virtualization platforms are designed to provide, allowing a compromised guest to potentially access memory regions belonging to other virtual machines or even the host operating system. The ATT&CK framework categorizes this as a privilege escalation technique under T1068, where an attacker exploits weaknesses in system controls to gain higher-level permissions.
Mitigation strategies for CVE-2018-20124 primarily involve applying the vendor-provided patches that implement proper input validation for the num_sge parameter in RDMA work queue elements. Organizations should prioritize updating their QEMU installations to versions that include the fix, which typically involves adding bounds checking and sanitization routines to validate the scatter-gather element count before processing. Additionally, implementing network segmentation and access controls can limit the attack surface by restricting guest OS capabilities within virtualized environments. System administrators should also consider disabling RDMA functionality in QEMU when it is not required for specific workloads, as this eliminates the attack vector entirely. Monitoring for anomalous RDMA operations and implementing intrusion detection systems can help identify potential exploitation attempts. The vulnerability highlights the importance of input validation in virtualization components and underscores the need for comprehensive security testing of hypervisor subsystems, particularly those handling hardware acceleration features that operate at privileged levels within the virtualization stack.