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/include/linux/snmp.h b/include/linux/snmp.h
index 673e0e928b2bbaaf35a04af3f148cb6703a1e921..e5fcbd079e4a17dfc60ff8c38151ddda96edb26e 100644 (file)
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -208,7 +208,6 @@ enum
        LINUX_MIB_TCPDSACKOFOSENT,              /* TCPDSACKOfoSent */
        LINUX_MIB_TCPDSACKRECV,                 /* TCPDSACKRecv */
        LINUX_MIB_TCPDSACKOFORECV,              /* TCPDSACKOfoRecv */
-       LINUX_MIB_TCPABORTONSYN,                /* TCPAbortOnSyn */
        LINUX_MIB_TCPABORTONDATA,               /* TCPAbortOnData */
        LINUX_MIB_TCPABORTONCLOSE,              /* TCPAbortOnClose */
        LINUX_MIB_TCPABORTONMEMORY,             /* TCPAbortOnMemory */
@@ -238,6 +237,7 @@ enum
        LINUX_MIB_TCPOFODROP,                   /* TCPOFODrop */
        LINUX_MIB_TCPOFOMERGE,                  /* TCPOFOMerge */
        LINUX_MIB_TCPCHALLENGEACK,              /* TCPChallengeACK */
+       LINUX_MIB_TCPSYNCHALLENGE,              /* TCPSYNChallenge */
        __LINUX_MIB_MAX
 };
 

diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 3e8e78f12a38e635f9e0bb4d3174a21ef143156b..2a5240b2ea614a14665ef54a1ff65b9f553abf09 100644 (file)
--- 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 c841a8990377be4929fb580d9aff26003795961a..8aaec55361119249c6fc67af5f12f037e1c50504 100644 (file)
--- 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) {