CVE-2026-97557 smb: client: avoid leaking refcount in cifs_queue_oplock_break()
A refcount leak in the Linux kernel CIFS client causes kernel panics during unmount, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-97557?
CVE-2026-97557 is a high-severity denial of service vulnerability in Linux, published on 25 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-97557?
How do I fix CVE-2026-97557?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-97557 being exploited?
There is no known exploitation of CVE-2026-97557 in the wild and no public exploit code.
Sources
Also known as AZL-104543, BELL-CVE-2026-97557, DEBIAN-CVE-2026-97557, GHSA-v47h-9mpw-4682, UBUNTU-CVE-2026-97557.