From a75facf4bb3e866103ddca4f47a6f3ca4b090d8b Mon Sep 17 00:00:00 2001 From: Hengyu Liang Date: Thu, 29 Oct 2026 23:26:17 -0400 Subject: [PATCH] l2tp: only return sessions of the given tunnel in l2tp_session_get() Commit 5f77c18ea556 ("l2tp: use IDR for all session lookups") made pppol2tp_connect() and the netlink session commands find sessions with l2tp_session_get(), passing the socket of the tunnel they name. L2TPv3 session IDs are unique per network namespace, except that sessions carried over UDP are only unique per UDP socket, the socket of their tunnel. However, l2tp_v3_session_get() only checks the socket when another session has the same session ID. Otherwise it returns the session, whichever tunnel it belongs to. As of now, a session command that names the wrong tunnel acts on a session of another tunnel, and a PPPoL2TP socket for a second L2TPv3 tunnel fails with EEXIST, because it finds session 0 of the first tunnel. The issue can be reproduced with: ip l2tp add tunnel tunnel_id 3 peer_tunnel_id 3 encap udp \ local 127.0.0.1 remote 127.0.0.1 udp_sport 5003 udp_dport 6003 ip l2tp add tunnel tunnel_id 4 peer_tunnel_id 4 encap udp \ local 127.0.0.1 remote 127.0.0.1 udp_sport 5004 udp_dport 6004 ip l2tp add session tunnel_id 3 session_id 7 peer_session_id 7 ip l2tp del session tunnel_id 4 session_id 7 ip l2tp show session Before that commit, the delete fails with "No such device", and session 7 of tunnel 3 is still listed. After that commit, the delete succeeds, and session 7 of tunnel 3 is gone. This patch will make l2tp_session_get() return NULL when the L2TPv3 session it finds does not belong to the given socket. The receive path calls l2tp_v3_session_get() directly and is not changed. Fixes: 5f77c18ea556 ("l2tp: use IDR for all session lookups") Cc: stable@vger.kernel.org Signed-off-by: Hengyu Liang Acked-by: Guillaume Nault Link: https://patch.msgid.link/20261030032617.4065353-1-hengyul@cs.unc.edu Signed-off-by: Paolo Abeni --- net/l2tp/l2tp_core.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index 7e042a3e648c..0cd3e38bf662 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -315,10 +315,19 @@ EXPORT_SYMBOL_GPL(l2tp_v2_session_get); struct l2tp_session *l2tp_session_get(const struct net *net, struct sock *sk, int pver, u32 tunnel_id, u32 session_id) { + struct l2tp_session *session; + if (pver == L2TP_HDR_VER_2) return l2tp_v2_session_get(net, tunnel_id, session_id); - else - return l2tp_v3_session_get(net, sk, session_id); + + /* l2tp_v3_session_get() doesn't check sk if the ID hasn't collided */ + session = l2tp_v3_session_get(net, sk, session_id); + if (session && session->tunnel->sock != sk) { + l2tp_session_put(session); + return NULL; + } + + return session; } EXPORT_SYMBOL_GPL(l2tp_session_get);