CVE-2026-64119

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.