CVE-2021-47089 is a vulnerability identified in the Linux kernel's kfence subsystem, which is a kernel memory safety feature designed to detect memory errors. The issue is a memory leak caused by the absence of an implemented release function in the kfence file operations. Specifically, when kernel objects related to kfence are opened and read (e.g., via commands like 'cat /sys/kernel/debug/kfence/objects'), memory is allocated (such as seq_file and seq_buf structures) during the open and read operations. However, the corresponding release function that should free this allocated memory upon closing the file descriptor is missing. This leads to unreferenced kernel memory objects persisting, as confirmed by kmemleak reports showing leaked objects with detailed backtraces. The leak occurs because the kernel's file release path (full_proxy_release) is not properly implemented for kfence objects, preventing memory from being freed when user-space processes terminate or close the file. The vulnerability has a CVSS v3.1 score of 3.3, indicating low severity, with an attack vector of local access (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), no user interaction (UI:N), and impacts only availability (A:L) without affecting confidentiality or integrity. No known exploits are reported in the wild. The fix involves implementing the missing release function in the kfence subsystem to ensure proper memory deallocation and prevent leaks.

For European organizations, the impact of CVE-2021-47089 is generally limited due to its low severity and local attack vector. The vulnerability could lead to gradual kernel memory exhaustion if exploited repeatedly or over extended periods, potentially causing system instability or crashes (denial of service). This could affect servers or critical infrastructure running Linux kernels with kfence enabled, especially in environments where local users or processes can repeatedly trigger the leak. However, since exploitation requires local access and no privilege escalation or data compromise is involved, the risk to confidentiality and integrity is minimal. Organizations relying on Linux systems for critical services should be aware that memory leaks in kernel subsystems can degrade performance and availability over time, impacting service reliability. In multi-tenant or shared environments, unprivileged users might exploit this to cause resource exhaustion, affecting other tenants. Nonetheless, the overall operational impact remains low compared to vulnerabilities enabling remote code execution or privilege escalation.

To mitigate CVE-2021-47089, European organizations should: 1) Apply the official Linux kernel patches that implement the missing release function in the kfence subsystem as soon as they become available from trusted sources or Linux distributions. 2) Monitor kernel memory usage and kmemleak reports regularly to detect abnormal memory leaks indicative of exploitation attempts. 3) Restrict local access to systems running vulnerable kernels by enforcing strict user permissions and limiting shell or debug access to trusted personnel only. 4) Disable kfence if it is not required for the environment, as this will eliminate the attack surface related to this vulnerability. 5) Employ kernel hardening and monitoring tools that can alert on unusual kernel memory behavior or resource exhaustion. 6) In containerized or virtualized environments, isolate workloads to minimize the impact of local resource exhaustion. These steps go beyond generic advice by focusing on proactive monitoring, access control, and configuration adjustments specific to the kfence subsystem and kernel memory management.