CVE-2026-89704

Received Received - Intake

Writeback Error Handling Flaw in Linux Kernel NFSd

Publication date: 2026-09-11

Last updated on: 2026-09-14

Assigner: kernel.org

Description

In the Linux kernel, the following vulnerability has been resolved: nfsd: sample writeback error cursor before async COPY loop _nfsd_copy_file_range() samples dst->f_wb_err into "since" after the copy loop, then uses it to detect writeback errors via filemap_check_wb_err() once vfs_fsync_range() returns. Because the nfsd_file cache reuses a single struct file across requests targeting the same inode, a concurrent COMMIT or stable WRITE on dst advances dst->f_wb_err to the current mapping->wb_err via file_check_and_advance_wb_err() during its own vfs_fsync_range(). If that advancement lands between the writeback error appearing in mapping->wb_err and the COPY worker sampling "since", the worker captures the already-advanced cursor, errseq_check() sees cur == since and returns zero, and NFSD4_COPY_F_COMMITTED is set even though writeback failed. CB_OFFLOAD then encodes wr_stable_how = FILE_SYNC4, the client treats the copied data as durable, and the failure becomes silent data loss. Sample since once at the start of the function. The cursor then reflects state in effect before this COPY issues any writes, and filemap_check_wb_err() detects any error that occurs during the copy regardless of which thread first observes it. This matches the pattern used by nfsd_vfs_write() and nfsd4_clone_file_range().

CVSS Scores

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Meta Information

Published

2026-09-11

Last Modified

2026-09-14

Generated

2026-10-02

AI Q&A

2026-09-12

EPSS Evaluated

2026-09-30

NVD

CVE-2026-89704

EUVD

EUVD-2026-76616

Affected Vendors & Products

Vendor	Product	Version / Range
linux	linux_kernel	*

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-UNKNOWN

Attack-Flow Graph

Executive Summary

This vulnerability in the Linux kernel involves a race condition in the NFS server's file copy operation. The issue occurs when the kernel samples writeback error state after copying data but before checking for errors. If another process advances the error state during this window, the copy operation may incorrectly report success even if writeback failed, leading to silent data loss.

Detection Guidance

This vulnerability is specific to the Linux kernel's NFS server implementation and requires kernel-level inspection. Detection involves checking kernel logs for NFS-related errors or verifying if your kernel version includes the vulnerable code path. Use commands like 'dmesg | grep nfsd' or 'uname -a' to check kernel version and NFS-related messages.

Impact Analysis

This vulnerability could cause data loss if you rely on NFS for file operations. If writeback fails during a copy, the system may incorrectly report the operation as successful, leading to corrupted or lost data without warning. This affects systems using NFS for file sharing or storage.

Compliance Impact

This vulnerability could lead to silent data loss due to incorrect writeback error detection during file copy operations in the Linux kernel. For compliance standards like GDPR or HIPAA, which require data integrity and protection, this could result in unauthorized data loss or corruption, potentially violating requirements for secure data handling and auditability.

Mitigation Strategies

Apply the latest kernel patches from your Linux distribution to resolve this issue. If patches are unavailable, consider disabling NFS server functionality temporarily or restricting access to NFS shares until the fix is applied.

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