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Date: Tue, 17 Jul 2012 10:13:05 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Miller <davem@...emloft.net>
Cc: netdev <netdev@...r.kernel.org>,
Kiran Kumar Kella <kkiran@...adcom.com>
Subject: [PATCH net-next v2] tcp: implement RFC 5961 3.2
From: Eric Dumazet <edumazet@...gle.com>
Implement the RFC 5691 mitigation against Blind
Reset attack using RST bit.
Idea is to validate incoming RST sequence,
to match RCV.NXT value, instead of previouly accepted
window : (RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND)
If sequence is in window but not an exact match, send
a "challenge ACK", so that the other part can resend an
RST with the appropriate sequence.
Add a new sysctl, tcp_challenge_ack_limit, to limit
number of challenge ACK sent per second.
Add a new SNMP counter to count number of challenge acks sent.
(netstat -s | grep TCPChallengeACK)
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Kiran Kumar Kella <kkiran@...adcom.com>
---
v2: respin to net-next, better changelog
tcp_challengeack_limit -> tcp_challenge_ack_limit
tcp_time_stamp -> jiffies
Documentation/networking/ip-sysctl.txt | 5 +++
include/linux/snmp.h | 1
include/net/tcp.h | 1
net/ipv4/proc.c | 1
net/ipv4/sysctl_net_ipv4.c | 7 +++++
net/ipv4/tcp_input.c | 31 ++++++++++++++++++++++-
6 files changed, 45 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index e20c17a..e1e0215 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -565,6 +565,11 @@ tcp_limit_output_bytes - INTEGER
reduce the size of individual GSO packet (64KB being the max)
Default: 131072
+tcp_challenge_ack_limit - INTEGER
+ Limits number of Challenge ACK sent per second, as recommended
+ in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
+ Default: 100
+
UDP variables:
udp_mem - vector of 3 INTEGERs: min, pressure, max
diff --git a/include/linux/snmp.h b/include/linux/snmp.h
index 6e4c511..673e0e9 100644
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -237,6 +237,7 @@ enum
LINUX_MIB_TCPOFOQUEUE, /* TCPOFOQueue */
LINUX_MIB_TCPOFODROP, /* TCPOFODrop */
LINUX_MIB_TCPOFOMERGE, /* TCPOFOMerge */
+ LINUX_MIB_TCPCHALLENGEACK, /* TCPChallengeACK */
__LINUX_MIB_MAX
};
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 439984b..85c5090 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -254,6 +254,7 @@ extern int sysctl_tcp_thin_linear_timeouts;
extern int sysctl_tcp_thin_dupack;
extern int sysctl_tcp_early_retrans;
extern int sysctl_tcp_limit_output_bytes;
+extern int sysctl_tcp_challenge_ack_limit;
extern atomic_long_t tcp_memory_allocated;
extern struct percpu_counter tcp_sockets_allocated;
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index dae25e76..3e8e78f 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -261,6 +261,7 @@ static const struct snmp_mib snmp4_net_list[] = {
SNMP_MIB_ITEM("TCPOFOQueue", LINUX_MIB_TCPOFOQUEUE),
SNMP_MIB_ITEM("TCPOFODrop", LINUX_MIB_TCPOFODROP),
SNMP_MIB_ITEM("TCPOFOMerge", LINUX_MIB_TCPOFOMERGE),
+ SNMP_MIB_ITEM("TCPChallengeACK", LINUX_MIB_TCPCHALLENGEACK),
SNMP_MIB_SENTINEL
};
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 70730f7..3f6a1e7 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -605,6 +605,13 @@ static struct ctl_table ipv4_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec
},
+ {
+ .procname = "tcp_challenge_ack_limit",
+ .data = &sysctl_tcp_challenge_ack_limit,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec
+ },
#ifdef CONFIG_NET_DMA
{
.procname = "tcp_dma_copybreak",
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cc4e12f..c841a89 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -88,6 +88,9 @@ int sysctl_tcp_app_win __read_mostly = 31;
int sysctl_tcp_adv_win_scale __read_mostly = 1;
EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
+/* rfc5961 challenge ack rate limiting */
+int sysctl_tcp_challenge_ack_limit = 100;
+
int sysctl_tcp_stdurg __read_mostly;
int sysctl_tcp_rfc1337 __read_mostly;
int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
@@ -5247,6 +5250,23 @@ out:
}
#endif /* CONFIG_NET_DMA */
+static void tcp_send_challenge_ack(struct sock *sk)
+{
+ /* unprotected vars, we dont care of overwrites */
+ static u32 challenge_timestamp;
+ static unsigned int challenge_count;
+ u32 now = jiffies / HZ;
+
+ if (now != challenge_timestamp) {
+ challenge_timestamp = now;
+ challenge_count = 0;
+ }
+ if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
+ NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
+ tcp_send_ack(sk);
+ }
+}
+
/* Does PAWS and seqno based validation of an incoming segment, flags will
* play significant role here.
*/
@@ -5283,7 +5303,16 @@ static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
/* Step 2: check RST bit */
if (th->rst) {
- tcp_reset(sk);
+ /* RFC 5961 3.2 :
+ * If sequence number exactly matches RCV.NXT, then
+ * RESET the connection
+ * else
+ * Send a challenge ACK
+ */
+ if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt)
+ tcp_reset(sk);
+ else
+ tcp_send_challenge_ack(sk);
goto discard;
}
--
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