CVE-2026-98870 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: remove stale channels from all sessions on teardown
ksmbd_sessions_deregister() removes a connection's channels from other sessions' channel lists only while conn->binding is still set:
if (conn->binding) {
hash_for_each_safe(sessions_table, ...) ksmbd_chann_del(conn, sess); }
conn->binding is a transient flag: it is cleared once a binding SESSION_SETUP completes, and also by a subsequent non-binding SESSION_SETUP on the same connection (a reauthentication on a bound channel, or a new SessionId==0 setup). A connection that has bound a channel into another session's ksmbd_chann_list and then clears conn->binding leaves that channel behind when it disconnects: the channel, whose chann->conn points at the now freed struct ksmbd_conn, stays on the owner session's list.
When the owning connection later tears down, the second loop dereferences the stale channel:
xa_for_each(&sess->ksmbd_chann_list, chann_id, chann) if (chann->conn != conn) ksmbd_conn_set_exiting(chann->conn); /* freed */
which is a use-after-free write into the freed ksmbd_conn (the same stale channel is also walked by show_proc_session() through /proc). The session is leaked as well, because its channel list never empties.
Remove the conn->binding gate so a connection always removes its channels from every session on teardown.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability in question resides within the ksmbd subsystem of the Linux kernel, specifically affecting how network connections and their associated channels are managed during session teardown operations. The core technical flaw is rooted in an incorrect conditional check that governs the cleanup process for stale channel references. When a connection establishes a binding with another session, it registers its channel into that session's channel list via ksmbd_chann_add. However, upon disconnection or teardown, the function ksmbd_sessions_deregister was designed to remove these channels from other sessions' lists only if the conn->binding flag remained set. This flag is transient and is cleared once a binding SESSION_SETUP completes successfully or when a subsequent non-binding SESSION_SETUP occurs on the same connection, such as during reauthentication or a new setup with SessionId zero. Consequently, if a channel has been bound to another session but the conn->binding flag is subsequently cleared before teardown, the cleanup logic fails to execute for that specific scenario.
This logical oversight results in stale channels remaining attached to their owner sessions even after the connection they reference has been freed. The primary operational impact arises when the owning connection eventually tears down and triggers a second loop within ksmbd_sessions_deregister. This loop iterates through all channels associated with each session, including those that are now stale because they point to memory structures of already-freed connections. Specifically, the code attempts to call ksmbd_conn_set_exiting on chann->conn for any channel not belonging to the current connection being torn down. Since chann->conn points to a freed struct ksmbd_conn object due to the earlier failure to remove it from the other session's list, this operation constitutes a use-after-free vulnerability. This memory corruption can lead to kernel instability, crashes, or potentially be exploited for arbitrary code execution depending on how the freed memory is reused by subsequent allocations.
Furthermore, these stale channels are not only problematic during internal teardown processes but also expose themselves through user-space interfaces. The show_proc_session function iterates over session channel lists via /proc filesystem entries to display status information. When this function encounters a stale channel pointing to freed memory, it triggers the same use-after-free condition, allowing local users with access to procfs to potentially crash the system or extract sensitive kernel data from the corrupted state. Additionally, because the channel list of the affected session is never properly emptied due to these lingering references, the associated session structure itself becomes leaked in memory. Over time, repeated occurrences of this flaw can lead to significant resource exhaustion and denial of service conditions as kernel memory is consumed by unreleased session objects.
The resolution involves removing the conditional gate that checks for conn->binding during the teardown process. By ensuring that a connection always removes its channels from every session's list regardless of the binding state, the system guarantees that no stale references persist after disconnection. This fix aligns with best practices for memory safety in kernel development by preventing dangling pointers and ensuring complete cleanup of dynamic resources. From a classification perspective, this vulnerability is categorized under CWE-416: Use After Free, as it involves accessing memory after it has been freed, leading to undefined behavior. In terms of the MITRE ATT&CK framework, while primarily a stability issue, such vulnerabilities can be leveraged in exploitation chains for privilege escalation or denial of service attacks against Linux-based systems running ksmbd services. System administrators should apply kernel updates that include this patch immediately and monitor system logs for signs of instability related to network file sharing services.