CVE-2025-21654 is a vulnerability identified in the Linux kernel's overlay filesystem (overlayfs) implementation, specifically related to the encoding of file handles (fid) from inodes that have no aliases. The issue arises when userspace applications invoke the inotify_show_fdinfo() function on an overlayfs watched inode whose dentry aliases have been discarded due to cache drops (drop_caches). This triggers a WARN_ON() assertion in the kernel, indicating an unexpected condition. The root cause is that the function ovl_encode_fh(), responsible for encoding file handles, fails if it cannot find an alias for the inode. This failure impacts the reporting of file identifiers (fid) in fanotify events, particularly FAN_DELETE_SELF, which notifies when a watched file is deleted. The vulnerability does not cause a direct crash or memory corruption but leads to failure in encoding the file handle, which can disrupt the correct functioning of file monitoring and security tools relying on fanotify and inotify mechanisms. The fix involves deferring the search for inode aliases until absolutely necessary, allowing ovl_encode_fh() to succeed in common cases where aliasing is not required, thus preventing the assertion failure and ensuring proper fid reporting. No known exploits are reported in the wild, and the vulnerability was publicly disclosed in January 2025 without an assigned CVSS score.

For European organizations, this vulnerability primarily affects systems running Linux kernels with overlayfs enabled, which is common in containerized environments, cloud infrastructure, and modern Linux distributions widely used across enterprises. The failure to correctly encode file handles in fanotify events can impair security monitoring, auditing, and file integrity checking tools that rely on these kernel notifications. This could lead to blind spots in detecting file deletions or modifications, potentially delaying the identification of malicious activities such as unauthorized file tampering or ransomware behavior. While the vulnerability does not directly allow privilege escalation or remote code execution, the degradation of security monitoring capabilities could be exploited by attackers to maintain persistence or evade detection. Organizations with critical infrastructure, cloud services, or container orchestration platforms are particularly at risk of operational impact due to reduced visibility into filesystem events. Additionally, compliance with data protection regulations (e.g., GDPR) could be indirectly affected if security monitoring gaps lead to undetected data breaches.

1. Apply the official Linux kernel patches that address CVE-2025-21654 as soon as they become available from trusted Linux distribution vendors or kernel maintainers. 2. For organizations using container platforms (e.g., Docker, Kubernetes), ensure that the underlying host kernels are updated to patched versions to maintain the integrity of overlayfs operations. 3. Enhance monitoring by supplementing fanotify/inotify-based tools with additional file integrity monitoring solutions that do not solely rely on overlayfs file handle encoding. 4. Implement strict access controls and minimize the use of drop_caches operations in production environments to reduce the likelihood of triggering the vulnerable code path. 5. Conduct thorough testing of security monitoring tools after kernel updates to verify that file event notifications are correctly reported. 6. Maintain robust incident response procedures to quickly detect and respond to suspicious filesystem activities that might bypass fanotify/inotify alerts due to this vulnerability.