CVE-2026-97557: smb: client: avoid leaking refcount in cifs_queue_oplock_break()

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-25 · updated 2026-10-30

Description

In the Linux kernel, the following vulnerability has been resolved: smb: client: avoid leaking refcount in cifs_queue_oplock_break() cifs_queue_oplock_break() unconditionally takes a reference on the target file before queueing cifs_oplock_break(). Only that work item decreases the reference counter again. If another oplock break arrives while that work is still queued, queue_work() will return false and not queue this second work item. As a result, we will never reach the point to drop the file reference again and are leaking this reference. This can be triggered when interacting with a slow-responding server. As a result, later unmount operations for this file system will fail with BUG: Dentry ... still in use (1) [unmount of cifs cifs] VFS: Busy inodes after unmount of cifs (cifs) kernel BUG at fs/super.c:777! Fix this by only incrementing the reference count if the work has been queued successfully. Taking it after queue_work() is safe because all three callers hold tcon->open_file_lock across the call and _cifsFileInfo_put() decrements under that same lock, so a worker that starts the handler in the window cannot drop the reference before it has been taken.

Affected products

Linux: from 3.16.72 before 3.17; from 4.9.171 before 4.10; from 4.14.114 before 4.15; from 4.19.37 before 4.20; from 5.0.10 before 5.1 — fixed in 3.17, 4.10, 4.15, 4.20, 5.1

How to fix

Upgrade to a fixed version: Linux 3.17, 4.10, 4.15, 4.20. Check the vendor advisory for mitigations.

References

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