[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: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
diff --git a/net/dccp/dccp.h b/net/dccp/dccp.h
index e282201..a602d92 100644
--- a/net/dccp/dccp.h
+++ b/net/dccp/dccp.h
@@ -92,6 +92,7 @@
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)
diff --git a/net/dccp/input.c b/net/dccp/input.c
index 86ad3ba..19d7e1d 100644
--- a/net/dccp/input.c
+++ b/net/dccp/input.c
@@ -122,6 +122,23 @@
(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 @@
: "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);
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 14ec1d2..604de8b 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -219,6 +219,7 @@
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;
diff --git a/net/dccp/sysctl.c b/net/dccp/sysctl.c
index 1260aab..9364b2f 100644
--- a/net/dccp/sysctl.c
+++ b/net/dccp/sysctl.c
@@ -18,6 +18,9 @@
#error This file should not be compiled without CONFIG_SYSCTL defined
#endif
+/* rate-limit for syncs in reply to sequence-invalid packets; RFC 4340, 7.5.4 */
+int sysctl_dccp_sync_ratelimit __read_mostly = HZ / 8;
+
static struct ctl_table dccp_default_table[] = {
{
.procname = "seq_window",
@@ -89,6 +92,13 @@
.mode = 0644,
.proc_handler = proc_dointvec,
},
+ {
+ .procname = "sync_ratelimit",
+ .data = &sysctl_dccp_sync_ratelimit,
+ .maxlen = sizeof(sysctl_dccp_sync_ratelimit),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_ms_jiffies,
+ },
{ .ctl_name = 0, }
};