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>

2 files changed, 16 insertions, 18 deletions

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 3e8e78f12a38..2a5240b2ea61 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -232,7 +232,6 @@ static const struct snmp_mib snmp4_net_list[] = {
         SNMP_MIB_ITEM("TCPDSACKOfoSent", LINUX_MIB_TCPDSACKOFOSENT),
         SNMP_MIB_ITEM("TCPDSACKRecv", LINUX_MIB_TCPDSACKRECV),
         SNMP_MIB_ITEM("TCPDSACKOfoRecv", LINUX_MIB_TCPDSACKOFORECV),
-        SNMP_MIB_ITEM("TCPAbortOnSyn", LINUX_MIB_TCPABORTONSYN),
         SNMP_MIB_ITEM("TCPAbortOnData", LINUX_MIB_TCPABORTONDATA),
         SNMP_MIB_ITEM("TCPAbortOnClose", LINUX_MIB_TCPABORTONCLOSE),
         SNMP_MIB_ITEM("TCPAbortOnMemory", LINUX_MIB_TCPABORTONMEMORY),
@@ -262,6 +261,7 @@ static const struct snmp_mib snmp4_net_list[] = {
         SNMP_MIB_ITEM("TCPOFODrop", LINUX_MIB_TCPOFODROP),
         SNMP_MIB_ITEM("TCPOFOMerge", LINUX_MIB_TCPOFOMERGE),
         SNMP_MIB_ITEM("TCPChallengeACK", LINUX_MIB_TCPCHALLENGEACK),
+        SNMP_MIB_ITEM("TCPSYNChallenge", LINUX_MIB_TCPSYNCHALLENGE),
         SNMP_MIB_SENTINEL
 };
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c841a8990377..8aaec5536111 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5270,8 +5270,8 @@ static void tcp_send_challenge_ack(struct sock *sk)
 /* 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 @@ static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 
         /* 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 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
                         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 @@ slow_path:
          *      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 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
         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 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                 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) {