CVE-2025-21900 is a vulnerability identified in the Linux kernel's implementation of NFSv4 (Network File System version 4). The issue arises specifically during the recovery of state on files that have been 'sillyrenamed'—a technique used by NFS clients to rename a file temporarily when it is slated for deletion upon closing. The vulnerability occurs when a server reboot triggers an open reclaim operation that races with the application's close() system call. This race condition can cause the function put_nfs_open_context() to invoke a synchronous delegation return call (delegreturn) that is not marked as privileged, leading to a deadlock. The root cause is that the call to nfs4_inode_return_delegation_on_close() does not handle the delegation return asynchronously, which is necessary to avoid blocking operations that can cause deadlocks. The fix involves catching the delegation return and scheduling it asynchronously, preventing the deadlock scenario. This vulnerability affects Linux kernel versions identified by the given commit hashes, indicating it is present in specific kernel builds prior to the patch. Although no known exploits are reported in the wild, the vulnerability can cause denial of service conditions by freezing NFS operations, impacting systems relying on NFSv4 for file sharing and stateful file operations.

For European organizations, especially those operating data centers, cloud services, or enterprise environments that utilize Linux servers with NFSv4 for file sharing, this vulnerability poses a risk of service disruption. The deadlock can cause NFS clients or servers to hang during file operations, potentially leading to application stalls or system unavailability. This is particularly critical for industries relying on high availability and data integrity, such as financial services, telecommunications, healthcare, and government institutions. The inability to properly close files and reclaim state can degrade performance and availability, impacting business continuity. Since NFS is widely used in enterprise Linux environments across Europe, the vulnerability could affect a broad range of organizations if exploited or triggered inadvertently during server reboots or maintenance. Although exploitation does not appear to require user interaction or authentication, the impact is primarily on availability rather than confidentiality or integrity.

Organizations should promptly apply the Linux kernel patch that addresses this vulnerability by ensuring the asynchronous handling of delegation returns in NFSv4. System administrators should monitor Linux kernel updates and prioritize upgrading affected kernel versions to the fixed releases. In environments where immediate patching is not feasible, administrators can consider temporarily disabling NFSv4 delegation features or avoid using sillyrename operations if possible, though this may impact functionality. Additionally, implementing robust monitoring of NFS server and client logs for signs of deadlocks or hangs can help detect potential issues early. Testing kernel updates in staging environments before deployment is recommended to avoid unexpected disruptions. For critical systems, planning maintenance windows to reboot servers with the patched kernel can prevent the deadlock from occurring during state recovery. Network segmentation and limiting access to NFS servers can reduce the attack surface, although this vulnerability does not require external exploitation.