CVE-2026-64119
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk6th percentile - higher than 6% of all known CVEs
Summary
In the Linux kernel L2TP module, a vulnerability was found due to using list_del_init() instead of list_del_rcu() when removing a session from the list. An unprivileged local user can pin a host CPU indefinitely by concurrently issuing GET, CREATE, and DELETE operations on an L2TP session.
Risk Assessment
An attacker with CAP_NET_ADMIN in a network namespace can cause a CPU hang, leading to denial of service (DoS) for the entire system. The process cannot be killed even with SIGKILL, and RCU grace periods stall, blocking system progress.
Recommendation
Immediately update the Linux kernel to a version containing the fix that replaces list_del_init() with list_del_rcu() in l2tp_session_unhash(). If an update is not possible, consider disabling the l2tp_core module if not required.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: l2tp: use list_del_rcu in l2tp_session_unhash An unprivileged local user can pin a host CPU indefinitely in l2tp_session_get_by_ifname() by issuing L2TP_CMD_SESSION_GET on L2TP_ATTR_IFNAME concurrently with L2TP_CMD_SESSION_CREATE and L2TP_CMD_SESSION_DELETE on the same tunnel. All three commands take GENL_UNS_ADMIN_PERM, so CAP_NET_ADMIN in the netns user namespace suffices; on any host that has l2tp_core loaded the trigger is reachable from a standard `unshare -Urn` sandbox. l2tp_session_unhash() removes a session from tunnel->session_list with list_del_init(), but that list is walked by l2tp_session_get_by_ifname() with list_for_each_entry_rcu() under rcu_read_lock_bh(). list_del_init() leaves the deleted entry's next/prev self-pointing; a reader that has loaded the entry and then advances pos->list.next reads &session->list, container_of()s back to the same session, and list_for_each_entry_rcu() never reaches the list head. The CPU stays in strcmp() inside the walker, with BH and preemption disabled, so RCU grace periods on the host stall behind it and the wedged thread cannot be killed (SIGKILL is delivered on syscall return). Use list_del_rcu() to match the existing list_add_rcu() in l2tp_session_register(); the deleted session remains visible to in-flight walkers with consistent next/prev pointers until kfree_rcu() in l2tp_session_free() releases it. tunnel->session_list has exactly one list_del_init() call site; the list_del_init (&session->clist) at l2tp_core.c:533 operates on the per-collision list, which is not walked under RCU. list_empty(&session->list) is not used anywhere in net/l2tp/ after the unhash point, so dropping the post-delete self-init is safe; the fix has no userspace-visible behavior change.