tcp: implement RFC 5961 4.2
Implement the RFC 5691 mitigation against Blind
Reset attack using SYN bit.
Section 4.2 of RFC 5961 advises to send a Challenge ACK and drop
incoming packet, instead of resetting the session.
Add a new SNMP counter to count number of challenge acks sent
in response to SYN packets.
(netstat -s | grep TCPSYNChallenge)
Remove obsolete TCPAbortOnSyn, since we no longer abort a TCP session
because of a SYN flag.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c841a89..8aaec55 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5270,8 +5270,8 @@
/* Does PAWS and seqno based validation of an incoming segment, flags will
* play significant role here.
*/
-static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
- const struct tcphdr *th, int syn_inerr)
+static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
+ const struct tcphdr *th, int syn_inerr)
{
const u8 *hash_location;
struct tcp_sock *tp = tcp_sk(sk);
@@ -5323,20 +5323,22 @@
/* step 3: check security and precedence [ignored] */
- /* step 4: Check for a SYN in window. */
- if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
+ /* step 4: Check for a SYN
+ * RFC 5691 4.2 : Send a challenge ack
+ */
+ if (th->syn) {
if (syn_inerr)
TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
- NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN);
- tcp_reset(sk);
- return -1;
+ NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
+ tcp_send_challenge_ack(sk);
+ goto discard;
}
- return 1;
+ return true;
discard:
__kfree_skb(skb);
- return 0;
+ return false;
}
/*
@@ -5366,7 +5368,6 @@
const struct tcphdr *th, unsigned int len)
{
struct tcp_sock *tp = tcp_sk(sk);
- int res;
if (sk->sk_rx_dst) {
struct dst_entry *dst = sk->sk_rx_dst;
@@ -5555,9 +5556,8 @@
* Standard slow path.
*/
- res = tcp_validate_incoming(sk, skb, th, 1);
- if (res <= 0)
- return -res;
+ if (!tcp_validate_incoming(sk, skb, th, 1))
+ return 0;
step5:
if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
@@ -5877,7 +5877,6 @@
struct tcp_sock *tp = tcp_sk(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
int queued = 0;
- int res;
tp->rx_opt.saw_tstamp = 0;
@@ -5932,9 +5931,8 @@
return 0;
}
- res = tcp_validate_incoming(sk, skb, th, 0);
- if (res <= 0)
- return -res;
+ if (!tcp_validate_incoming(sk, skb, th, 0))
+ return 0;
/* step 5: check the ACK field */
if (th->ack) {