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Commit 0b53d460 authored by Gerrit Renker's avatar Gerrit Renker
Browse files

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: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
parent 0ed8ddf4

net/dccp/dccp.h

+20 −0
Original line number Diff line number Diff line
@@ -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;

}

net/dccp/input.c

+6 −12
Original line number Diff line number Diff line
@@ -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);

net/dccp/minisocks.c

+9 −21
Original line number Diff line number Diff line
@@ -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