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Commit 0c24604b authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

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: default avatarEric Dumazet <edumazet@google.com>
Cc: Kiran Kumar Kella <kkiran@broadcom.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 5f3600eb

include/linux/snmp.h

+1 −1
Original line number Diff line number Diff line
@@ -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

};

net/ipv4/proc.c

+1 −1
Original line number Diff line number Diff line
@@ -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

};

net/ipv4/tcp_input.c

+15 −17
Original line number Diff line number Diff line
@@ -5270,7 +5270,7 @@ 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,

static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,

				  const struct tcphdr *th, int syn_inerr)

{

	const u8 *hash_location;
@@ -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 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb, 
	 *	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) {