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, 9 insertions, 21 deletions

diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
index 128b089..d7041a0 100644
--- a/net/dccp/minisocks.c
+++ b/net/dccp/minisocks.c
@@ -121,30 +121,18 @@ struct sock *dccp_create_openreq_child(struct sock *sk,
 		 *
 		 *    Choose S.ISS (initial seqno) or set from Init Cookies
 		 *    Initialize S.GAR := S.ISS
-		 *    Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookies
-		 */
-		newdp->dccps_gar = newdp->dccps_iss = dreq->dreq_iss;
-		dccp_update_gss(newsk, dreq->dreq_iss);
-
-		newdp->dccps_isr = dreq->dreq_isr;
-		dccp_update_gsr(newsk, dreq->dreq_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.
+		 *    Set S.ISR, S.GSR from packet (or Init Cookies)
+		 *
+		 *    Setting AWL/AWH and SWL/SWH happens as part of the feature
+		 *    activation below, as these windows all depend on the local
+		 *    and remote Sequence Window feature values (7.5.2).
 		 */
-		dccp_set_seqno(&newdp->dccps_swl,
-			       max48(newdp->dccps_swl, newdp->dccps_isr));
-		dccp_set_seqno(&newdp->dccps_awl,
-			       max48(newdp->dccps_awl, newdp->dccps_iss));
+		newdp->dccps_gss = newdp->dccps_iss = dreq->dreq_iss;
+		newdp->dccps_gar = newdp->dccps_iss;
+		newdp->dccps_gsr = newdp->dccps_isr = dreq->dreq_isr;
 
 		/*
-		 * Activate features after initialising the sequence numbers,
-		 * since CCID initialisation may depend on GSS, ISR, ISS etc.
+		 * Activate features: initialise CCIDs, sequence windows etc.
 		 */
 		if (dccp_feat_activate_values(newsk, &dreq->dreq_featneg)) {
 			/* It is still raw copy of parent, so invalidate