CVE-2015-8953 in Linux
Summary
by MITRE
fs/overlayfs/copy_up.c in the Linux kernel before 4.2.6 uses an incorrect cleanup code path, which allows local users to cause a denial of service (dentry reference leak) via filesystem operations on a large file in a lower overlayfs layer.
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Analysis
by VulDB Data Team • 09/26/2022
The vulnerability described in CVE-2015-8953 resides within the overlay filesystem implementation of the Linux kernel, specifically in the copy_up.c file that handles operations between overlay layers. This flaw affects kernel versions prior to 4.2.6 and represents a critical issue that can lead to system instability through resource exhaustion. The overlay filesystem is designed to present multiple filesystems or directories as a single directory structure, commonly used in containerization technologies and system imaging where upper and lower layers need to be managed dynamically.
The technical root cause of this vulnerability stems from an incorrect cleanup code path in the overlayfs implementation that fails to properly release dentry references when processing large files in the lower layer of an overlay filesystem. When a filesystem operation occurs on a large file located in the lower overlay layer, the kernel's copy_up mechanism attempts to move the file to the upper layer while maintaining proper reference counting. However, due to flawed cleanup logic, dentry references are not properly deallocated, creating a memory leak that accumulates over time. This reference leak specifically impacts the dentry cache which maintains metadata about filesystem objects and their relationships within the kernel's virtual filesystem layer.
The operational impact of this vulnerability manifests as a denial of service condition that gradually consumes system memory resources. As users perform filesystem operations on large files within the lower overlay layer, the accumulation of unreleased dentry references causes the kernel's dentry cache to fill up progressively. Eventually, this leads to memory exhaustion that can cause the system to become unresponsive or crash entirely. The vulnerability is particularly concerning in containerized environments where overlay filesystems are extensively used, as it can affect multiple containers running on the same host system. The denial of service occurs without requiring special privileges, making it exploitable by unprivileged local users who can simply perform routine filesystem operations to trigger the memory leak.
This vulnerability aligns with CWE-401 which specifically addresses improper release of memory resources and can be categorized under the ATT&CK technique T1499.1 for Resource Hijacking, where adversaries consume system resources to cause denial of service. The flaw demonstrates how seemingly routine filesystem operations can be weaponized to exhaust system resources, particularly affecting systems running older kernel versions that lack the patched cleanup logic. Organizations using overlay filesystems for containerization, virtualization, or system imaging should prioritize kernel updates to address this vulnerability, as the impact can be severe in production environments where resource management is critical.
Mitigation strategies for this vulnerability primarily involve upgrading the Linux kernel to version 4.2.6 or later where the cleanup code path has been corrected. System administrators should also implement monitoring for unusual memory consumption patterns that might indicate the presence of this vulnerability. Additional defensive measures include limiting the use of overlay filesystems in environments where memory resources are constrained and implementing proper resource limits for containerized applications. The vulnerability underscores the importance of kernel security updates and proper resource management in preventing memory exhaustion attacks that can compromise system availability.