CVE-2026-72170 9p: skip nlink update in cacheless mode to fix WARN_ON
A race condition in the Linux kernel 9p filesystem causes a kernel warning and potential instability during heavy unlink operations in cacheless mode.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-72170?
CVE-2026-72170 is a high-severity vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-72170?
How do I fix CVE-2026-72170?
Apply the vendor's patch.
Is CVE-2026-72170 being exploited?
There is no known exploitation of CVE-2026-72170 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-72170, DEBIAN-CVE-2026-72170, GHSA-g6rc-qhh9-wvwv, UBUNTU-CVE-2026-72170.