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:
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.