netdev - [PATCH net 2/5] tcp: avoid collapses in tcp_prune

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Date:   Mon, 23 Jul 2018 09:28:18 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Juha-Matti Tilli <juha-matti.tilli@....fi>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net 2/5] tcp: avoid collapses in tcp_prune_queue() if possible

Right after a TCP flow is created, receiving tiny out of order
packets allways hit the condition :

if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
	tcp_clamp_window(sk);

tcp_clamp_window() increases sk_rcvbuf to match sk_rmem_alloc
(guarded by tcp_rmem[2])

Calling tcp_collapse_ofo_queue() in this case is not useful,
and offers a O(N^2) surface attack to malicious peers.

Better not attempt anything before full queue capacity is reached,
forcing attacker to spend lots of resource and allow us to more
easily detect the abuse.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>
---
 net/ipv4/tcp_input.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 64e45b279431886a50c8097593b9dbc9e5d75cc1..53289911362a2dea6b1e9d9ce630b29eed87ebb9 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5004,6 +5004,9 @@ static int tcp_prune_queue(struct sock *sk)
 	else if (tcp_under_memory_pressure(sk))
 		tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);
 
+	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
+		return 0;
+
 	tcp_collapse_ofo_queue(sk);
 	if (!skb_queue_empty(&sk->sk_receive_queue))
 		tcp_collapse(sk, &sk->sk_receive_queue, NULL,
-- 
2.18.0.233.g985f88cf7e-goog