Revert "dccp: don't free ccid2_hc_tx_sock struct in dccp_disconnect()"

This reverts commit 2677d20.

This fixes an issue that after disconnect, dccps_hc_tx_ccid will still be
kept, allowing the socket to be reused as a listener socket, and the cloned
socket will free its dccps_hc_tx_ccid, leading to a later use after free,
when the listener socket is closed.

This addresses CVE-2020-16119.

Fixes: 2677d20 (dccp: don't free ccid2_hc_tx_sock struct in dccp_disconnect())
Reported-by: Hadar Manor
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/dccp/proto.c

@@ -279,7 +279,9 @@ int dccp_disconnect(struct sock *sk, int flags)
 
 	dccp_clear_xmit_timers(sk);
 	ccid_hc_rx_delete(dp->dccps_hc_rx_ccid, sk);
+	ccid_hc_tx_delete(dp->dccps_hc_tx_ccid, sk);
 	dp->dccps_hc_rx_ccid = NULL;
+	dp->dccps_hc_tx_ccid = NULL;
 
 	__skb_queue_purge(&sk->sk_receive_queue);
 	__skb_queue_purge(&sk->sk_write_queue);