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>

3 files changed, 35 insertions, 33 deletions

diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index 019d6ffee354..e051c774ef5c 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -414,6 +414,23 @@ static inline void dccp_update_gsr(struct sock *sk, u64 seq)
         dp->dccps_gsr = seq;
         /* Sequence validity window depends on remote Sequence Window (7.5.1) */
         dp->dccps_swl = SUB48(ADD48(dp->dccps_gsr, 1), dp->dccps_r_seq_win / 4);
+        /*
+         * Adjust SWL so that it is not below ISR. In contrast to RFC 4340,
+         * 7.5.1 we perform this check beyond the initial handshake: W/W' are
+         * always > 32, so for the first W/W' packets in the lifetime of a
+         * connection we always have to adjust SWL.
+         * A second reason why we are doing this is that the window depends on
+         * the feature-remote value of Sequence Window: nothing stops the peer
+         * from updating this value while we are busy adjusting SWL for the
+         * first W packets (we would have to count from scratch again then).
+         * Therefore it is safer to always make sure that the Sequence Window
+         * is not artificially extended by a peer who grows SWL downwards by
+         * continually updating the feature-remote Sequence-Window.
+         * If sequence numbers wrap it is bad luck. But that will take a while
+         * (48 bit), and this measure prevents Sequence-number attacks.
+         */
+        if (before48(dp->dccps_swl, dp->dccps_isr))
+                dp->dccps_swl = dp->dccps_isr;
         dp->dccps_swh = ADD48(dp->dccps_gsr, (3 * dp->dccps_r_seq_win) / 4);
 }
 
@@ -424,6 +441,9 @@ static inline void dccp_update_gss(struct sock *sk, u64 seq)
         dp->dccps_gss = seq;
         /* Ack validity window depends on local Sequence Window value (7.5.1) */
         dp->dccps_awl = SUB48(ADD48(dp->dccps_gss, 1), dp->dccps_l_seq_win);
+        /* Adjust AWL so that it is not below ISS - see comment above for SWL */
+        if (before48(dp->dccps_awl, dp->dccps_iss))
+                dp->dccps_awl = dp->dccps_iss;
         dp->dccps_awh = dp->dccps_gss;
 }
 
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);
 
diff --git a/net/dccp/minisocks.c b/net/dccp/minisocks.c
index 128b089d3aef..d7041a0963af 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