dccp: fix the adjustments to AWL and SWL

This fixes a problem and a potential loophole with regard to seqno/ackno
validity: currently the initial adjustments to AWL/SWL are only performed
once at the begin of the connection, during the handshake.

Since the Sequence Window feature is always greater than Wmin=32 (7.5.2),
it is however necessary to perform these adjustments at least for the first
W/W' (variables as per 7.5.1) packets in the lifetime of a connection.

This requirement is complicated by the fact that W/W' can change at any time
during the lifetime of a connection.

Therefore it is better to perform that safety check each time SWL/AWL are
updated, as implemented by the patch.

A second problem solved by this patch is that the remote/local Sequence Window
feature values (which set the bounds for AWL/SWL/SWH) are undefined until the
feature negotiation has completed.

During the initial handshake we have more stringent sequence number protection;
the changes added by this patch effect that {A,S}W{L,H} are within the correct
bounds at the instant that feature negotiation completes (since the SeqWin
feature activation handlers call dccp_update_gsr/gss()).

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 6 insertions, 12 deletions

diff --git a/net/dccp/input.c b/net/dccp/input.c
index 10c957a88f4f..aecc8c7443c2 100644
--- a/net/dccp/input.c
+++ b/net/dccp/input.c
@@ -441,20 +441,14 @@ static int dccp_rcv_request_sent_state_process(struct sock *sk,
                 kfree_skb(sk->sk_send_head);
                 sk->sk_send_head = NULL;
 
-                dp->dccps_isr = DCCP_SKB_CB(skb)->dccpd_seq;
-                dccp_update_gsr(sk, dp->dccps_isr);
                 /*
-                 * SWL and AWL are initially adjusted so that they are not less than
-                 * the initial Sequence Numbers received and sent, respectively:
-                 *      SWL := max(GSR + 1 - floor(W/4), ISR),
-                 *      AWL := max(GSS - W' + 1, ISS).
-                 * These adjustments MUST be applied only at the beginning of the
-                 * connection.
-                 *
-                 * AWL was adjusted in dccp_v4_connect -acme
+                 * Set ISR, GSR from packet. ISS was set in dccp_v{4,6}_connect
+                 * and GSS in dccp_transmit_skb(). Setting AWL/AWH and SWL/SWH
+                 * is done as part of activating the feature values below, since
+                 * these settings depend on the local/remote Sequence Window
+                 * features, which were undefined or not confirmed until now.
                  */
-                dccp_set_seqno(&dp->dccps_swl,
-                               max48(dp->dccps_swl, dp->dccps_isr));
+                dp->dccps_gsr = dp->dccps_isr = DCCP_SKB_CB(skb)->dccpd_seq;
 
                 dccp_sync_mss(sk, icsk->icsk_pmtu_cookie);