Commit 083ae308 authored by Jason Baron's avatar Jason Baron Committed by David S. Miller
Browse files

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 f2b2c582 ("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: default avatarJason Baron <jbaron@akamai.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 005db31d

net/ipv4/tcp_input.c

+22 −17
Original line number Diff line number Diff line
@@ -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] */
@@ -3461,7 +3466,7 @@ 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,

	if (__tcp_oow_rate_limited(sock_net(sk),

				   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,

				   &tp->last_oow_ack_time))

		return;

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