CVE-2025-38338 fs/nfs/read: fix double-unlock bug in nfs_return_empty_folio()
A race condition in the Linux kernel NFS client can cause a deadlock, leading to a denial of service when files are read during truncation.
- 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-2025-38338?
CVE-2025-38338 is a high-severity vulnerability in Linux kernel, published on 10 Jul 2025. 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:
fs/nfs/read: fix double-unlock bug in nfs_return_empty_folio()
Sometimes, when a file was read while it was being truncated by another NFS client, the kernel could deadlock because folio_unlock() was called twice, and the second call would XOR back the `PG_locked` flag.
Most of the time (depending on the timing of the truncation), nobody notices the problem because folio_unlock() gets called three times, which flips `PG_locked` back off:
1. vfs_read, nfs_read_folio, ... nfs_read_add_folio, nfs_return_empty_folio 2. vfs_read, nfs_read_folio, ... netfs_read_collection, netfs_unlock_abandoned_read_pages 3. vfs_read, ... nfs_do_read_folio, nfs_read_add_folio, nfs_return_empty_folio
The problem is that nfs_read_add_folio() is not supposed to unlock the folio if fscache is enabled, and a nfs_netfs_folio_unlock() check is missing in nfs_return_empty_folio().
Rarely this leads to a warning in netfs_read_collection():
------------[ cut here ]------------ R=0000031c: folio 10 is not locked WARNING: CPU: 0 PID: 29 at fs/netfs/read_collect.c:133 netfs_read_collection+0x7c0/0xf00 [...] Workqueue: events_unbound netfs_read_collection_worker RIP: 0010:netfs_read_collection+0x7c0/0xf00 [...] Call Trace: <TASK> netfs_read_collection_worker+0x67/0x80 process_one_work+0x12e/0x2c0 worker_thread+0x295/0x3a0
Most of the time, however, processes just get stuck forever in folio_wait_bit_common(), waiting for `PG_locked` to disappear, which never happens because nobody is really holding the folio lock.
Weakness: CWE-415
Which versions are affected by CVE-2025-38338?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-38338?
Apply the vendor's patch.
Is CVE-2025-38338 being exploited?
There is no known exploitation of CVE-2025-38338 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-64911, BELL-CVE-2025-38338, DEBIAN-CVE-2025-38338, GHSA-3696-vcmq-4g83, ROOT-OS-UBUNTU-2404-CVE-2025-38338, UBUNTU-CVE-2025-38338.