CVE-2021-47579 is a vulnerability identified in the Linux kernel's overlay filesystem (overlayfs) implementation. The issue arises from how overlayfs handles directory creation when interacting with certain filesystems, specifically the cgroup2 filesystem. The vulnerability was discovered when Syzbot, an automated kernel fuzzer, triggered a warning in the function ovl_workdir_create(), which calls ovl_create_real(). The root cause is that the cgroup2 filesystem's mkdir operation returns without instantiating the new dentry (directory entry), which is unexpected behavior. Overlayfs expects a valid dentry with an associated inode after mkdir, but in this case, the new dentry's inode pointer is null. This leads to a warning condition (WARN_ON) in the kernel, indicating an abnormal state. The vulnerability does not directly cause a crash or memory corruption but indicates a logic flaw in overlayfs's handling of unusual filesystems. The fix involves changing overlayfs to call ovl_mkdir_real() directly from ovl_workdir_create() and rejecting cases where the new dentry is not properly instantiated early in the process, preventing the warning and potential undefined behavior later during overlayfs setup. This patch improves the robustness of overlayfs against edge cases involving filesystems like cgroup2 that do not behave as typical filesystems during directory creation. No known exploits are currently reported in the wild, and the vulnerability appears to be more of a stability and correctness issue rather than a direct security compromise vector. However, because overlayfs is widely used in container environments and other Linux-based systems, improper handling of filesystem operations could potentially be leveraged in complex attack scenarios or cause denial of service due to kernel warnings or instability.

For European organizations, the impact of CVE-2021-47579 is primarily related to system stability and reliability rather than direct compromise or data breach. Overlayfs is commonly used in containerization technologies such as Docker and Kubernetes, which are widely deployed in enterprise environments across Europe. A failure or warning in overlayfs could lead to container startup failures or unexpected behavior in containerized applications, potentially disrupting business operations. Although there is no evidence of active exploitation, the vulnerability could be leveraged in targeted attacks aiming to cause denial of service or to exploit subsequent kernel bugs triggered by this unexpected state. Organizations relying heavily on Linux containers or cgroup2 features should be aware of this vulnerability as it affects the underlying kernel filesystem operations. The impact on confidentiality and integrity is low, but availability could be affected if the kernel warnings escalate to crashes or resource exhaustion. Given the critical role of Linux in European IT infrastructure, especially in cloud and hosting providers, this vulnerability warrants timely patching to maintain operational stability.

European organizations should apply the Linux kernel patch that addresses CVE-2021-47579 as soon as it becomes available in their distribution's kernel updates. Specifically, updating to a kernel version that includes the fix for overlayfs handling of cgroup2 mkdir operations is essential. For containerized environments, ensure that container runtimes and orchestration platforms are also updated to versions compatible with the patched kernel. Additionally, organizations should monitor kernel logs for WARN_ON messages related to overlayfs and cgroup2 to detect any attempts to trigger this condition. As a proactive measure, limit the use of overlayfs with unusual or experimental filesystems until patches are applied. Security teams should also review container and host filesystem configurations to minimize exposure to this edge case. Finally, maintain robust kernel update policies and test patches in staging environments to prevent operational disruptions.