tcp: implement RFC 5961 3.2

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>

Documentation/networking/ip-sysctl.txt

@@ -565,6 +565,11 @@ tcp_limit_output_bytes - INTEGER
 	reduce the size of individual GSO packet (64KB being the max)
 	Default: 131072
 
+tcp_challenge_ack_limit - INTEGER
+	Limits number of Challenge ACK sent per second, as recommended
+	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
+	Default: 100
+
 UDP variables:
 
 udp_mem - vector of 3 INTEGERs: min, pressure, max

include/linux/snmp.h

@@ -237,6 +237,7 @@ enum
 	LINUX_MIB_TCPOFOQUEUE,			/* TCPOFOQueue */
 	LINUX_MIB_TCPOFODROP,			/* TCPOFODrop */
 	LINUX_MIB_TCPOFOMERGE,			/* TCPOFOMerge */
+	LINUX_MIB_TCPCHALLENGEACK,		/* TCPChallengeACK */
 	__LINUX_MIB_MAX
 };
 

include/net/tcp.h

@@ -254,6 +254,7 @@ extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
 extern int sysctl_tcp_early_retrans;
 extern int sysctl_tcp_limit_output_bytes;
+extern int sysctl_tcp_challenge_ack_limit;
 
 extern atomic_long_t tcp_memory_allocated;
 extern struct percpu_counter tcp_sockets_allocated;

net/ipv4/proc.c

@@ -261,6 +261,7 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPOFOQueue", LINUX_MIB_TCPOFOQUEUE),
 	SNMP_MIB_ITEM("TCPOFODrop", LINUX_MIB_TCPOFODROP),
 	SNMP_MIB_ITEM("TCPOFOMerge", LINUX_MIB_TCPOFOMERGE),
+	SNMP_MIB_ITEM("TCPChallengeACK", LINUX_MIB_TCPCHALLENGEACK),
 	SNMP_MIB_SENTINEL
 };
 

net/ipv4/sysctl_net_ipv4.c

@@ -605,6 +605,13 @@ static struct ctl_table ipv4_table[] = {
 		.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",

net/ipv4/tcp_input.c

@@ -88,6 +88,9 @@ int sysctl_tcp_app_win __read_mostly = 31;
 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;
@@ -5247,6 +5250,23 @@ static bool tcp_dma_try_early_copy(struct sock *sk, struct sk_buff *skb,
 }
 #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.
  */
@@ -5283,7 +5303,16 @@ static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 
 	/* Step 2: check RST bit */
 	if (th->rst) {
+		/* 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;
 	}