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Commit a94f0f97 authored by Gerrit Renker's avatar Gerrit Renker Committed by David S. Miller
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[DCCP]: Rate-limit DCCP-Syncs 



This implements a SHOULD from RFC 4340, 7.5.4:
 "To protect against denial-of-service attacks, DCCP implementations SHOULD
  impose a rate limit on DCCP-Syncs sent in response to sequence-invalid packets,
  such as not more than eight DCCP-Syncs per second."

The rate-limit is maintained on a per-socket basis. This is a more stringent
policy than enforcing the rate-limit on a per-source-address basis and
protects against attacks with forged source addresses.

Moreover, the mechanism is deliberately kept simple. In contrast to
xrlim_allow(), bursts of Sync packets in reply to sequence-invalid packets
are not supported.  This foils such attacks where the receipt of a Sync
triggers further sequence-invalid packets. (I have tested this mechanism against
xrlim_allow algorithm for Syncs, permitting bursts just increases the problems.)

In order to keep flexibility, the timeout parameter can be set via sysctl; and
the whole mechanism can even be disabled (which is however not recommended).

The algorithm in this patch has been improved with regard to wrapping issues
thanks to a suggestion by Arnaldo.

Commiter note: Rate limited the step 6 DCCP_WARN too, as it says we're
               sending a sync.

Signed-off-by: default avatarGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: default avatarIan McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@ghostprotocols.net>
parent ee1a1592

Documentation/networking/dccp.txt

+5 −0
Original line number Diff line number Diff line
@@ -112,6 +112,11 @@ tx_qlen = 5 
	The size of the transmit buffer in packets. A value of 0 corresponds

	to an unbounded transmit buffer.



sync_ratelimit = 125 ms

	The timeout between subsequent DCCP-Sync packets sent in response to

	sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit

	of this parameter is milliseconds; a value of 0 disables rate-limiting.



Notes

=====

include/linux/dccp.h

+2 −0
Original line number Diff line number Diff line
@@ -470,6 +470,7 @@ struct dccp_ackvec; 
 * @dccps_pcrlen - receiver partial checksum coverage (via sockopt)

 * @dccps_ndp_count - number of Non Data Packets since last data packet

 * @dccps_mss_cache - current value of MSS (path MTU minus header sizes)

 * @dccps_rate_last - timestamp for rate-limiting DCCP-Sync (RFC 4340, 7.5.4)

 * @dccps_minisock - associated minisock (accessed via dccp_msk)

 * @dccps_hc_rx_ackvec - rx half connection ack vector

 * @dccps_hc_rx_ccid - CCID used for the receiver (or receiving half-connection)
@@ -505,6 +506,7 @@ struct dccp_sock { 
	__u16				dccps_pcrlen;

	unsigned long			dccps_ndp_count;

	__u32				dccps_mss_cache;

	unsigned long			dccps_rate_last;

	struct dccp_minisock		dccps_minisock;

	struct dccp_ackvec		*dccps_hc_rx_ackvec;

	struct ccid			*dccps_hc_rx_ccid;

net/dccp/dccp.h

+1 −0
Original line number Diff line number Diff line
@@ -92,6 +92,7 @@ extern int sysctl_dccp_feat_ack_ratio; 
extern int  sysctl_dccp_feat_send_ack_vector;

extern int  sysctl_dccp_feat_send_ndp_count;

extern int  sysctl_dccp_tx_qlen;

extern int  sysctl_dccp_sync_ratelimit;



/*

 *	48-bit sequence number arithmetic (signed and unsigned)

net/dccp/input.c

+20 −9
Original line number Diff line number Diff line
@@ -122,6 +122,23 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb) 
		    (ackno != DCCP_PKT_WITHOUT_ACK_SEQ))

			dp->dccps_gar = ackno;

	} else {

		unsigned long now = jiffies;

		/*

		 *   Step 6: Check sequence numbers

		 *      Otherwise,

		 *         If P.type == Reset,

		 *            Send Sync packet acknowledging S.GSR

		 *         Otherwise,

		 *            Send Sync packet acknowledging P.seqno

		 *      Drop packet and return

		 *

		 *   These Syncs are rate-limited as per RFC 4340, 7.5.4:

		 *   at most 1 / (dccp_sync_rate_limit * HZ) Syncs per second.

		 */

		if (time_before(now, (dp->dccps_rate_last +

				      sysctl_dccp_sync_ratelimit)))

			return 0;



		DCCP_WARN("DCCP: Step 6 failed for %s packet, "

			  "(LSWL(%llu) <= P.seqno(%llu) <= S.SWH(%llu)) and "

			  "(P.ackno %s or LAWL(%llu) <= P.ackno(%llu) <= S.AWH(%llu), "
@@ -132,15 +149,9 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb) 
							      : "exists",

			  (unsigned long long) lawl, (unsigned long long) ackno,

			  (unsigned long long) dp->dccps_awh);

		/*

		 *   Step 6: Check sequence numbers

		 *      Otherwise,

		 *         If P.type == Reset,

		 *            Send Sync packet acknowledging S.GSR

		 *         Otherwise,

		 *            Send Sync packet acknowledging P.seqno

		 *      Drop packet and return

		 */



		dp->dccps_rate_last = now;



		if (dh->dccph_type == DCCP_PKT_RESET)

			seqno = dp->dccps_gsr;

		dccp_send_sync(sk, seqno, DCCP_PKT_SYNC);

net/dccp/proto.c

+1 −0
Original line number Diff line number Diff line
@@ -219,6 +219,7 @@ int dccp_init_sock(struct sock *sk, const __u8 ctl_sock_initialized) 
	sk->sk_write_space	= dccp_write_space;

	icsk->icsk_sync_mss	= dccp_sync_mss;

	dp->dccps_mss_cache	= 536;

	dp->dccps_rate_last	= jiffies;

	dp->dccps_role		= DCCP_ROLE_UNDEFINED;

	dp->dccps_service	= DCCP_SERVICE_CODE_IS_ABSENT;

	dp->dccps_l_ack_ratio	= dp->dccps_r_ack_ratio = 1;