Problem statement: 1) both paths (primary path1 and alternate
path2) are up after the association has been established i.e.,
HB packets are normally exchanged, 2) path2 gets inactive after
path_max_retrans * max_rto timed out (i.e. path2 is down completely),
3) now, if a transmission times out on the only surviving/active
path1 (any ~1sec network service impact could cause this like
a channel bonding failover), then the retransmitted packets are
sent over the inactive path2; this happens with partial failover
and without it.

Besides not being optimal in the above scenario, a small failure
or timeout in the only existing path has the potential to cause
long delays in the retransmission (depending on RTO_MAX) until
the still active path is reselected. Further, when the T3-timeout
occurs, we have active_patch == retrans_path, and even though the
timeout occurred on the initial transmission of data, not a
retransmit, we end up updating retransmit path.

RFC4960, section 6.4. "Multi-Homed SCTP Endpoints" states under
6.4.1. "Failover from an Inactive Destination Address" the
following:

  Some of the transport addresses of a multi-homed SCTP endpoint
  may become inactive due to either the occurrence of certain
  error conditions (see Section 8.2) or adjustments from the
  SCTP user.

  When there is outbound data to send and the primary path
  becomes inactive (e.g., due to failures), or where the SCTP
  user explicitly requests to send data to an inactive
  destination transport address, before reporting an error to
  its ULP, the SCTP endpoint should try to send the data to an
  alternate __active__ destination transport address if one
  exists.

  When retransmitting data that timed out, if the endpoint is
  multihomed, it should consider each source-destination address
  pair in its retransmission selection policy. When retransmitting
  timed-out data, the endpoint should attempt to pick the most
  divergent source-destination pair from the original
  source-destination pair to which the packet was transmitted.

  Note: Rules for picking the most divergent source-destination
  pair are an implementation decision and are not specified
  within this document.

So, we should first reconsider to take the current active
retransmission transport if we cannot find an alternative
active one. If all of that fails, we can still round robin
through unkown, partial failover, and inactive ones in the
hope to find something still suitable.

Commit 4141ddc0 ("sctp: retran_path update bug fix") broke
that behaviour by selecting the next inactive transport when
no other active transport was found besides the current assoc's
peer.retran_path. Before commit 4141ddc0, we would have
traversed through the list until we reach our peer.retran_path
again, and in case that is still in state SCTP_ACTIVE, we would
take it and return. Only if that is not the case either, we
take the next inactive transport.

Besides all that, another issue is that transports in state
SCTP_UNKNOWN could be preferred over transports in state
SCTP_ACTIVE in case a SCTP_ACTIVE transport appears after
SCTP_UNKNOWN in the transport list yielding a weaker transport
state to be used in retransmission.

This patch mostly reverts 4141ddc0, but also rewrites
this function to introduce more clarity and strictness into
the code. A strict priority of transport states is enforced
in this patch, hence selection is active > unkown > partial
failover > inactive.

Fixes: 4141ddc0

 ("sctp: retran_path update bug fix")
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Acked-by: Vlad Yasevich <yasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>