tcp: enable per-socket rate limiting of all 'challenge acks'

The per-socket rate limit for 'challenge acks' was introduced in the
context of limiting ack loops:

commit f2b2c582e824 ("tcp: mitigate ACK loops for connections as tcp_sock")

And I think it can be extended to rate limit all 'challenge acks' on a
per-socket basis.

Since we have the global tcp_challenge_ack_limit, this patch allows for
tcp_challenge_ack_limit to be set to a large value and effectively rely on
the per-socket limit, or set tcp_challenge_ack_limit to a lower value and
still prevents a single connections from consuming the entire challenge ack
quota.

It further moves in the direction of eliminating the global limit at some
point, as Eric Dumazet has suggested. This a follow-up to:
Subject: tcp: make challenge acks less predictable

Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Yue Cao <ycao009@ucr.edu>
Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 22 insertions, 17 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 91868bb17818..42bf89aaf6a5 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3421,6 +3421,23 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
         return flag;
 }
 
+static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
+                                   u32 *last_oow_ack_time)
+{
+        if (*last_oow_ack_time) {
+                s32 elapsed = (s32)(tcp_time_stamp - *last_oow_ack_time);
+
+                if (0 <= elapsed && elapsed < sysctl_tcp_invalid_ratelimit) {
+                        NET_INC_STATS(net, mib_idx);
+                        return true;    /* rate-limited: don't send yet! */
+                }
+        }
+
+        *last_oow_ack_time = tcp_time_stamp;
+
+        return false;   /* not rate-limited: go ahead, send dupack now! */
+}
+
 /* Return true if we're currently rate-limiting out-of-window ACKs and
  * thus shouldn't send a dupack right now. We rate-limit dupacks in
  * response to out-of-window SYNs or ACKs to mitigate ACK loops or DoS
@@ -3434,21 +3451,9 @@ bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
         /* Data packets without SYNs are not likely part of an ACK loop. */
         if ((TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq) &&
             !tcp_hdr(skb)->syn)
-                goto not_rate_limited;
-
-        if (*last_oow_ack_time) {
-                s32 elapsed = (s32)(tcp_time_stamp - *last_oow_ack_time);
-
-                if (0 <= elapsed && elapsed < sysctl_tcp_invalid_ratelimit) {
-                        NET_INC_STATS(net, mib_idx);
-                        return true;    /* rate-limited: don't send yet! */
-                }
-        }
-
-        *last_oow_ack_time = tcp_time_stamp;
+                return false;
 
-not_rate_limited:
-        return false;   /* not rate-limited: go ahead, send dupack now! */
+        return __tcp_oow_rate_limited(net, mib_idx, last_oow_ack_time);
 }
 
 /* RFC 5961 7 [ACK Throttling] */
@@ -3461,9 +3466,9 @@ static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
         u32 count, 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))
+        if (__tcp_oow_rate_limited(sock_net(sk),
+                                   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
+                                   &tp->last_oow_ack_time))
                 return;
 
         /* Then check host-wide RFC 5961 rate limit. */