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 <hengyul@cs.unc.edu>
Acked-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20261030032617.4065353-1-hengyul@cs.unc.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

0 files changed, 0 insertions, 0 deletions