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dccp: fix the adjustments to AWL and SWL

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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>
This commit is contained in:
Gerrit Renker 2010-10-11 20:35:40 +02:00
parent 0ed8ddf404
commit 0b53d4604a
3 changed files with 35 additions and 33 deletions
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20
net/dccp/dccp.h
View file

@ -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;
}

18
net/dccp/input.c
View file

@ -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);

30
net/dccp/minisocks.c
View file

@ -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
* 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).
*/
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);
newdp->dccps_gss = newdp->dccps_iss = dreq->dreq_iss;
newdp->dccps_gar = newdp->dccps_iss;
newdp->dccps_gsr = newdp->dccps_isr = 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.
*/
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));
/*
* 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