When a peer decides to close one subflow in the middle of a connection
having multiple subflows, the receiver of the first FIN should accept
that, and close the subflow on its side as well. If not, the subflow
will stay half closed, and would even continue to be used until the end
of the MPTCP connection or a reset from the network.

The issue has not been seen before, probably because the in-kernel
path-manager always sends a RM_ADDR before closing the subflow. Upon the
reception of this RM_ADDR, the other peer will initiate the closure on
its side as well. On the other hand, if the RM_ADDR is lost, or if the
path-manager of the other peer only closes the subflow without sending a
RM_ADDR, the subflow would switch to TCP_CLOSE_WAIT, but that's it,
leaving the subflow half-closed.

So now, when the subflow switches to the TCP_CLOSE_WAIT state, and if
the MPTCP connection has not been closed before with a DATA_FIN, the
kernel owning the subflow schedules its worker to initiate the closure
on its side as well.

This issue can be easily reproduced with packetdrill, as visible in [1],
by creating an additional subflow, injecting a FIN+ACK before sending
the DATA_FIN, and expecting a FIN+ACK in return.

Fixes: 40947e13 ("mptcp: schedule worker when subflow is closed")
Cc: stable@vger.kernel.org
Link: https://github.com/multipath-tcp/packetdrill/pull/154 [1]
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240826-net-mptcp-close-extra-sf-fin-v1-1-905199fe1172@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>