CVE-2026-72170
Received Received - Intake

Linux kernel nlink race condition in 9p filesystem

Publication date: 2026-08-15

Last updated on: 2026-08-15

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: 9p: skip nlink update in cacheless mode to fix WARN_ON v9fs_dec_count() unconditionally calls drop_nlink() on regular files, even when the inode's nlink is already zero. In cacheless mode the client refetches inode metadata from the server (the source of truth) on every operation, so by the time v9fs_remove() returns, the locally cached nlink may already reflect the post-unlink value: 1. Client initiates unlink, server processes it and sets nlink to 0 2. Client refetches inode metadata (nlink=0) before unlink returns 3. Client's v9fs_remove() completes successfully 4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0 This race is easily triggered under heavy unlink workloads, such as stress-ng's unlink stressor, producing the following warning: WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8 Call trace: drop_nlink+0x4c/0xc8 v9fs_remove+0x1e0/0x250 [9p] v9fs_vfs_unlink+0x20/0x38 [9p] vfs_unlink+0x13c/0x258 ... In cacheless mode the server is authoritative and the inode is on its way out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count() entirely when neither CACHE_META nor CACHE_LOOSE is set, which both avoids the warning and removes a class of nlink races (two concurrent unlinkers observing nlink > 0 and both calling drop_nlink()) that an nlink == 0 guard alone would only narrow rather than close.

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Meta Information

Published
2026-08-15
Last Modified
2026-08-15
Generated
2026-08-15
AI Q&A
2026-08-15
EPSS Evaluated
N/A
NVD
CVE-2026-72170
EUVD
EUVD-2026-58928

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel *

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Exploitability

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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This is a Linux kernel vulnerability in the 9p filesystem (v9fs) where v9fs_dec_count() incorrectly calls drop_nlink() on regular files even when their link count (nlink) is already zero. In cacheless mode, the client frequently refreshes inode metadata from the server, leading to a race condition during unlink operations. This causes a kernel warning (WARN_ON) when drop_nlink() is called on an inode with nlink=0.

Detection Guidance

This vulnerability is specific to the Linux kernel's 9p filesystem in cacheless mode and may trigger a WARN_ON warning during heavy unlink workloads. Detection involves monitoring kernel logs for the warning message: 'WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8'. Check logs with: dmesg | grep -i 'drop_nlink' or journalctl -k | grep -i 'drop_nlink'.

Impact Analysis

Under heavy unlink workloads, this vulnerability can trigger kernel warnings and potentially cause filesystem inconsistencies. While it does not directly corrupt data, it may lead to performance issues or system instability during high I/O operations involving file deletions.

Mitigation Strategies

Apply the kernel patch that resolves this issue by skipping nlink updates in cacheless mode. Update your Linux kernel to a version containing the fix. If using a distribution kernel, check for updates from your vendor. Monitor kernel logs after applying the patch to confirm the warning no longer appears.

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