From: Paolo Abeni <pabeni(a)redhat.com>
Date: Fri,  3 Jul 2020 18:06:04 +0200

> The RFC 8684 mandates that no-data DATA FIN packets should carry
> a DSS with 0 sequence number and data len equal to 1. Currently,
> on FIN retransmission we re-use the existing mapping; if the previous
> fin transmission was part of a partially acked data packet, we could
> end-up writing in the egress packet a non-compliant DSS.
> 
> The above will be detected by a "Bad mapping" warning on the receiver
> side.
> 
> This change addresses the issue explicitly checking for 0 len packet
> when adding the DATA_FIN option.
> 
> Fixes: 6d0060f600ad ("mptcp: Write MPTCP DSS headers to outgoing data packets")
> Reported-by: syzbot+42a07faa5923cfaeb9c9(a)syzkaller.appspotmail.com
> Tested-by: Christoph Paasch <cpaasch(a)apple.com>
> Reviewed-by: Christoph Paasch <cpaasch(a)apple.com>
> Reviewed-by: Mat Martineau <mathew.j.martineau(a)linux.intel.com>
> Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>

Applied and queued up for -stable, thank you.