CVE-2025-38393 in Linux
Summary
by MITRE • 07/25/2025
In the Linux kernel, the following vulnerability has been resolved:
NFSv4/pNFS: Fix a race to wake on NFS_LAYOUT_DRAIN
We found a few different systems hung up in writeback waiting on the same page lock, and one task waiting on the NFS_LAYOUT_DRAIN bit in pnfs_update_layout(), however the pnfs_layout_hdr's plh_outstanding count was zero.
It seems most likely that this is another race between the waiter and waker similar to commit ed0172af5d6f ("SUNRPC: Fix a race to wake a sync task"). Fix it up by applying the advised barrier.
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Analysis
by VulDB Data Team • 08/01/2026
The vulnerability CVE-2025-38393 represents a critical race condition within the Linux kernel's Network File System version 4 implementation, specifically affecting parallel NFS (pNFS) functionality. This issue manifests as system hangs during writeback operations when multiple processes attempt to access the same page lock while waiting on the NFS_LAYOUT_DRAIN bit within the pnfs_update_layout() function. The affected systems demonstrate a peculiar behavior where the pnfs_layout_hdr's plh_outstanding count shows zero despite active waiters, indicating a fundamental synchronization problem between waiting and waking processes.
The technical flaw stems from a race condition between the waiter and waker threads in the pNFS subsystem, directly mirroring a previously identified issue documented in commit ed0172af5d6f which addressed similar synchronization problems in the SUNRPC framework. The vulnerability occurs when the NFS_LAYOUT_DRAIN bit transitions from set to clear, but the waiting process fails to properly synchronize with the waking process due to missing memory barriers or proper synchronization primitives. This race condition allows processes to remain indefinitely blocked even when the underlying resource should be available, creating a deadlock scenario that can bring entire systems to a halt during concurrent file write operations.
The operational impact of this vulnerability extends beyond simple system hangs to encompass complete service disruption in environments heavily reliant on NFSv4 and pNFS implementations. Organizations running file servers, storage clusters, or distributed applications that depend on NFS for data access face significant risk of extended downtime and performance degradation. The vulnerability affects systems where multiple clients simultaneously access the same files or where layout updates occur frequently during write operations, making it particularly dangerous in high-throughput storage environments such as media servers, database clusters, or scientific computing farms. The intermittent nature of the race condition makes it challenging to reproduce and debug, often leading to subtle performance degradation before complete system lockups occur.
Mitigation strategies for CVE-2025-38393 require immediate kernel updates from vendors that include the fix based on the barrier approach recommended in the commit referenced in the description. System administrators should prioritize patching affected NFS servers and clients, particularly in production environments where concurrent file access patterns are common. Additional monitoring should be implemented to detect stalled NFS operations and page lock contention, as early warning systems can help identify when the race condition is occurring before complete system hangs occur. The fix addresses the underlying synchronization issue by ensuring proper memory barriers are in place during the transition of the NFS_LAYOUT_DRAIN bit, following established patterns from similar fixes in the SUNRPC subsystem. Organizations should also consider implementing circuit breaker patterns or timeout mechanisms in applications that rely heavily on NFSv4 to prevent cascading failures when such race conditions occur.
This vulnerability aligns with CWE-362, which describes a race condition in concurrent programming where two or more threads access shared data concurrently, and the outcome depends on the timing of their execution. From an ATT&CK perspective, this issue falls under the T1499.004 technique category related to Network Denial of Service, as it can lead to complete service unavailability. The fix demonstrates the importance of proper synchronization primitives in kernel-level code and highlights the ongoing challenges in ensuring correct behavior in complex distributed systems where multiple threads must coordinate access to shared resources while maintaining system responsiveness and availability.