From 979c017803c40829b03acd9e5236e354b7622360 Mon Sep 17 00:00:00 2001 From: Michael Bommarito Date: Mon, 18 May 2026 14:34:47 -0400 Subject: [PATCH] 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. Fixes: 89b768ec2dfef ("l2tp: use rcu list add/del when updating lists") Cc: stable@vger.kernel.org # 6.11+ Signed-off-by: Michael Bommarito Link: https://patch.msgid.link/20260518183447.64078-1-michael.bommarito@gmail.com Signed-off-by: Jakub Kicinski --- net/l2tp/l2tp_core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index 157fc23ce4e1..1455f67e01dd 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -1360,7 +1360,7 @@ static void l2tp_session_unhash(struct l2tp_session *session) spin_lock_bh(&pn->l2tp_session_idr_lock); /* Remove from the per-tunnel list */ - list_del_init(&session->list); + list_del_rcu(&session->list); /* Remove from per-net IDR */ if (tunnel->version == L2TP_HDR_VER_3) {