CVE-2026-89635 ksmbd: only rebind the reopened file's own oplock on durable reconnect
An authenticated attacker can trigger a use-after-free in the Linux kernel's ksmbd module by reconnecting durable handles, potentially leading to code execution or system crash.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89635?
CVE-2026-89635 is a critical use after free vulnerability in Linux, published on 11 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:
ksmbd: only rebind the reopened file's own oplock on durable reconnect
ksmbd_reopen_durable_fd() walks the inode's m_op_list and rebinds every detached oplock to the reconnecting session:
list_for_each_entry_rcu(op, &ci->m_op_list, op_entry, lockdep_is_held(&ci->m_lock)) { if (op->conn) continue; op->conn = ksmbd_conn_get(fp->conn); op->sess = work->sess; }
The only key is op->conn == NULL, which every detached durable handle on that inode matches, not just the one owned by fp. When two sessions hold durable handles on the same file and both disconnect, reconnecting one of them adopts the other session's oplock: op->sess is overwritten with the reconnecting session without taking a reference on it, while op->conn pins the connection.
The sibling teardown path, session_fd_check(), keys on the identity of the connection being torn down (op->conn == conn) rather than on shared state, and so does not have this problem.
Once the adopting session is destroyed, ksmbd_session_destroy() frees it while the foreign oplock still points at it. The reader in ksmbd_close_fd_app_instance_id() validates only opinfo->conn, which is still live thanks to the reference taken above, and then dereferences the stale session:
if (!opinfo->conn) { up_read(&fp->f_ci->m_lock); goto out; }
ft = &opinfo->sess->file_table; write_lock(&ft->lock);
BUG: KASAN: slab-use-after-free in _raw_write_lock+0x74/0xd0 Write of size 4 at addr ffff88810a970528 by task kworker/0:0/9 Workqueue: ksmbd-io handle_ksmbd_work Call Trace: _raw_write_lock+0x74/0xd0 ksmbd_close_fd_app_instance_id+0x183/0x410 smb2_open+0x1346/0x4430 handle_ksmbd_work+0x2bb/0x7b0
Reached from an authenticated session against a share with the default durable-handle and oplock configuration: two sessions open the same file with a durable-v2 handle and an RH lease under distinct AppInstanceIds, both log off, one reconnects with DH2C, and a later durable-v2 create carrying the other AppInstanceId walks into the freed session.
Constrain the loop to the oplock owned by the file being reopened.
Which versions are affected by CVE-2026-89635?
How do I fix CVE-2026-89635?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-89635 being exploited?
There is no known exploitation of CVE-2026-89635 in the wild and no public exploit code.
Sources
Also known as GHSA-rg7m-2q29-w52v, UBUNTU-CVE-2026-89635.