CVE-2025-38567: 
Linux Ubuntu vulnerability analysis and mitigation

A reference leak vulnerability was discovered in the Linux kernel's NFS server implementation, specifically in the nfsd_open_local_fh() function. The vulnerability was assigned CVE-2025-38567 and was publicly disclosed on August 19, 2025 (NVD).

The vulnerability occurs when two calls to nfsd_open_local_fh() race and both successfully call nfsd_file_acquire_local(). In this scenario, both calls obtain an extra reference to the net to accompany the file reference stored in *pnf. When one of them fails to store the file reference in *pnf using xchg(), it drops that reference but fails to drop the accompanying reference to the net. This creates a reference leak that prevents proper shutdown of the NFS server (NVD).

When exploited, this vulnerability causes the NFS server to hang during shutdown in nfsd_shutdown_net(), specifically while waiting for &nn->nfsd_net_free_done. This prevents proper system shutdown and could lead to resource exhaustion (NVD).

The vulnerability has been patched by adding the missing nfsd_net_put() call to properly release the reference. The fix has been implemented in the Linux kernel codebase (NVD).