packet: do not enqueue outgoing packets after sending SSH2_MSG_NEWKEYS

When we decide we need to rekey, we enqueue any further packets until we've sent
our SSH2_MSG_NEWKEYS message, after which we dequeue these packets and send them
to the other side. This enqueueing is done based on ssh_packet_in_rekey checking
the session flags and whether DH handshake state is marked as finished.

However, the handshake state is not reset to DH_STATE_FINISHED until the other
side has sent us their new keys. This leaves a gap between sending our new keys
and receiving the other side's new keys where we would still decide to enqueue a
packet.

These enqueued packets will not be dequeued as we've already sent our new keys.
Once we've received the other side's new keys, we'll go back to a finished
handshake and we will send out our caller's new data, skipping however much data
we've enqueued.

Fix this by changing ssh_packet_in_rekey to return false once we've sent our new
keys.

Signed-off-by: Carlos Martín Nieto <carlosmn@github.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
(cherry picked from commit 346e6db318)

src/packet.c

@@ -1755,10 +1755,12 @@ static bool
 ssh_packet_in_rekey(ssh_session session)
 {
     /* We know we are rekeying if we are authenticated and the DH
-     * status is not finished
+     * status is not finished, but we only queue packets until we've
+     * sent our NEWKEYS.
      */
     return (session->flags & SSH_SESSION_FLAG_AUTHENTICATED) &&
-           (session->dh_handshake_state != DH_STATE_FINISHED);
+           (session->dh_handshake_state != DH_STATE_FINISHED) &&
+           (session->dh_handshake_state != DH_STATE_NEWKEYS_SENT);
 }
 
 int ssh_packet_send(ssh_session session)