CVE-2026-53393 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: reset write verifier on deferred writeback errors
nfsd_vfs_write() and nfsd_commit() both call filemap_check_wb_err() to detect deferred writeback errors, but neither rotates the server's write verifier (nn->writeverf) when this check fails. Every other durable-storage-failure path in these functions calls commit_reset_write_verifier() before returning an error.
The missing rotation means clients holding UNSTABLE write data under the current verifier will COMMIT, receive the unchanged verifier back, and conclude their data is durable — silently dropping data that failed writeback. This violates the UNSTABLE+COMMIT durability contract (RFC 1813 §3.3.7, RFC 8881 §18.32).
Add commit_reset_write_verifier() calls at both filemap_check_wb_err() error sites, matching the pattern used by adjacent error paths in the same functions. The helper already filters -EAGAIN and -ESTALE internally, so the calls are unconditionally safe.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides within the nfsd implementation of the linux kernel where a critical flaw exists in how write verifier management is handled during deferred writeback operations. This issue affects the Network File System version 3 and 4 implementations that rely on durable storage semantics for data consistency. The core problem manifests when nfsd_vfs_write() and nfsd_commit() functions process file operations that encounter deferred writeback errors, specifically through the filemap_check_wb_err() function call.
The technical flaw stems from an inconsistent error handling pattern where two distinct code paths fail to rotate the server's write verifier nn->writeverf when filemap_check_wb_err() returns an error condition. While all other durable-storage-failure scenarios within these functions properly invoke commit_reset_write_verifier() before propagating errors, this specific error detection path omits the necessary verifier rotation. This creates a dangerous state where clients can incorrectly assume their UNSTABLE write data has been successfully committed to durable storage.
The operational impact of this vulnerability is severe and directly violates fundamental NFS durability contracts defined in RFC 1813 section 3.3.7 and RFC 8881 section 18.32. When deferred writeback errors occur, the server maintains the same write verifier for subsequent COMMIT operations, allowing clients to believe their data has been properly stored despite actual write failures. This silent data loss scenario completely undermines the UNSTABLE+COMMIT durability contract that NFS relies upon for ensuring data consistency between client and server operations.
The vulnerability represents a clear violation of CWE-1107, which addresses improper handling of error conditions in storage systems, and aligns with ATT&CK technique T1490 for data destruction through storage corruption. The missing verifier rotation creates a persistent state where clients maintain false confidence in their data's durability status, leading to potential data loss that may not be detected until much later. This flaw affects any system running kernel versions containing the vulnerable nfsd implementation and particularly impacts environments relying on NFS for critical data storage operations.
The remediation approach involves adding explicit commit_reset_write_verifier() calls at both error sites where filemap_check_wb_err() fails, ensuring these calls match the established error handling pattern used by other durable-storage failure paths within the same functions. The helper function already incorporates internal filtering for -EAGAIN and -ESTALE error conditions, making these additions unconditionally safe without introducing new failure modes or performance impacts. This targeted fix restores proper verifier management and ensures that all storage error conditions properly invalidate the current write verifier, maintaining the integrity of NFS durability semantics and preventing silent data loss scenarios.