In the Linux kernel, the following vulnerability has been resolved: smb: client: avoid leaking refcount in cifs_queue_oplock_break() cifs_

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.