tcp: implement RFC 5961 3.2
[ Upstream commit 282f23c6ee343126156dd41218b22ece96d747e3 ]
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@google.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 8af0d44..d589468 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -258,6 +258,7 @@
SNMP_MIB_ITEM("TCPReqQFullDrop", LINUX_MIB_TCPREQQFULLDROP),
SNMP_MIB_ITEM("TCPRetransFail", LINUX_MIB_TCPRETRANSFAIL),
SNMP_MIB_ITEM("TCPRcvCoalesce", LINUX_MIB_TCPRCVCOALESCE),
+ 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 7a7724d..bf7a604 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -590,6 +590,13 @@
.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 3acebbd..3905418 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -88,6 +88,9 @@
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;
@@ -5265,6 +5268,23 @@
}
#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.
*/
@@ -5301,7 +5321,16 @@
/* 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;
}