tcp: mitigate ACK loops for connections as tcp_sock

Ensure that in state ESTABLISHED, where the connection is represented
by a tcp_sock, we rate limit dupacks in response to incoming packets
(a) with TCP timestamps that fail PAWS checks, or (b) with sequence
numbers or ACK numbers that are out of the acceptable window.

We do not send a dupack in response to out-of-window packets if it has
been less than sysctl_tcp_invalid_ratelimit (default 500ms) since we
last sent a dupack in response to an out-of-window packet.

There is already a similar (although global) rate-limiting mechanism
for "challenge ACKs". When deciding whether to send a challence ACK,
we first consult the new per-connection rate limit, and then the
global rate limit.

Reported-by: Avery Fay <avery@mixpanel.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 22 insertions, 7 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9401aa43b814..8fdd27b17306 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3322,13 +3322,22 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
 }
 
 /* RFC 5961 7 [ACK Throttling] */
-static void tcp_send_challenge_ack(struct sock *sk)
+static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
 {
         /* unprotected vars, we dont care of overwrites */
         static u32 challenge_timestamp;
         static unsigned int challenge_count;
-        u32 now = jiffies / HZ;
+        struct tcp_sock *tp = tcp_sk(sk);
+        u32 now;
+
+        /* First check our per-socket dupack rate limit. */
+        if (tcp_oow_rate_limited(sock_net(sk), skb,
+                                 LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
+                                 &tp->last_oow_ack_time))
+                return;
 
+        /* Then check the check host-wide RFC 5961 rate limit. */
+        now = jiffies / HZ;
         if (now != challenge_timestamp) {
                 challenge_timestamp = now;
                 challenge_count = 0;
@@ -3424,7 +3433,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
         if (before(ack, prior_snd_una)) {
                 /* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
                 if (before(ack, prior_snd_una - tp->max_window)) {
-                        tcp_send_challenge_ack(sk);
+                        tcp_send_challenge_ack(sk, skb);
                         return -1;
                 }
                 goto old_ack;
@@ -4993,7 +5002,10 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
             tcp_paws_discard(sk, skb)) {
                 if (!th->rst) {
                         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
-                        tcp_send_dupack(sk, skb);
+                        if (!tcp_oow_rate_limited(sock_net(sk), skb,
+                                                  LINUX_MIB_TCPACKSKIPPEDPAWS,
+                                                  &tp->last_oow_ack_time))
+                                tcp_send_dupack(sk, skb);
                         goto discard;
                 }
                 /* Reset is accepted even if it did not pass PAWS. */
@@ -5010,7 +5022,10 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                 if (!th->rst) {
                         if (th->syn)
                                 goto syn_challenge;
-                        tcp_send_dupack(sk, skb);
+                        if (!tcp_oow_rate_limited(sock_net(sk), skb,
+                                                  LINUX_MIB_TCPACKSKIPPEDSEQ,
+                                                  &tp->last_oow_ack_time))
+                                tcp_send_dupack(sk, skb);
                 }
                 goto discard;
         }
@@ -5026,7 +5041,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
                 if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt)
                         tcp_reset(sk);
                 else
-                        tcp_send_challenge_ack(sk);
+                        tcp_send_challenge_ack(sk, skb);
                 goto discard;
         }
 
@@ -5040,7 +5055,7 @@ syn_challenge:
                 if (syn_inerr)
                         TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
-                tcp_send_challenge_ack(sk);
+                tcp_send_challenge_ack(sk, skb);
                 goto discard;
         }