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From 0af29b4f03b40fe0980c31d18e352f2ef19ec703 Mon Sep 17 00:00:00 2001
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From: Eric Dumazet <edumazet@google.com>
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Date: Sun, 10 Jul 2016 10:04:02 +0200
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Subject: [PATCH] tcp: make challenge acks less predictable
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Yue Cao claims that current host rate limiting of challenge ACKS
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(RFC 5961) could leak enough information to allow a patient attacker
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to hijack TCP sessions. He will soon provide details in an academic
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paper.
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This patch increases the default limit from 100 to 1000, and adds
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some randomization so that the attacker can no longer hijack
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sessions without spending a considerable amount of probes.
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Based on initial analysis and patch from Linus.
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Note that we also have per socket rate limiting, so it is tempting
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to remove the host limit in the future.
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v2: randomize the count of challenge acks per second, not the period.
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Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
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Reported-by: Yue Cao <ycao009@ucr.edu>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Yuchung Cheng <ycheng@google.com>
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Cc: Neal Cardwell <ncardwell@google.com>
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Acked-by: Neal Cardwell <ncardwell@google.com>
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Acked-by: Yuchung Cheng <ycheng@google.com>
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Signed-off-by: David S. Miller <davem@davemloft.net>
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---
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net/ipv4/tcp_input.c | 15 ++++++++++-----
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1 file changed, 10 insertions(+), 5 deletions(-)
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diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
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index d6c8f4cd0800..91868bb17818 100644
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--- a/net/ipv4/tcp_input.c
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+++ b/net/ipv4/tcp_input.c
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@@ -87,7 +87,7 @@ int sysctl_tcp_adv_win_scale __read_mostly = 1;
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EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
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/* rfc5961 challenge ack rate limiting */
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-int sysctl_tcp_challenge_ack_limit = 100;
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+int sysctl_tcp_challenge_ack_limit = 1000;
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int sysctl_tcp_stdurg __read_mostly;
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int sysctl_tcp_rfc1337 __read_mostly;
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@@ -3458,7 +3458,7 @@ static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
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static u32 challenge_timestamp;
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static unsigned int challenge_count;
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struct tcp_sock *tp = tcp_sk(sk);
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- u32 now;
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+ u32 count, now;
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/* First check our per-socket dupack rate limit. */
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if (tcp_oow_rate_limited(sock_net(sk), skb,
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@@ -3466,13 +3466,18 @@ static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
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&tp->last_oow_ack_time))
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return;
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- /* Then check the check host-wide RFC 5961 rate limit. */
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+ /* Then check host-wide RFC 5961 rate limit. */
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now = jiffies / HZ;
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if (now != challenge_timestamp) {
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+ u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
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+
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challenge_timestamp = now;
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- challenge_count = 0;
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+ WRITE_ONCE(challenge_count, half +
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+ prandom_u32_max(sysctl_tcp_challenge_ack_limit));
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}
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- if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
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+ count = READ_ONCE(challenge_count);
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+ if (count > 0) {
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+ WRITE_ONCE(challenge_count, count - 1);
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NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
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tcp_send_ack(sk);
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}
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--
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2.5.5
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