CVE-2025-38338

Summary

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/14f5549ad163be2c018abc1bb38370fff617a243
• https://git.kernel.org/stable/c/5bf0b9eeb0174686f22c2e5b8fb9f47ad25da6f5
• https://git.kernel.org/stable/c/1e93b61d3eaa14bfebcc2716ac09d43f3845d420
• https://git.kernel.org/stable/c/4c10fa44bc5f700e2ea21de2fbae520ba21f19d9

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