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Commit 5f4f2cbe authored by Eric Dumazet's avatar Eric Dumazet Committed by Min Chong
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BACKPORT: tcp: make challenge acks less predictable 



(cherry picked from commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758)

Yue Cao claims that current host rate limiting of challenge ACKS
(RFC 5961) could leak enough information to allow a patient attacker
to hijack TCP sessions. He will soon provide details in an academic
paper.

This patch increases the default limit from 100 to 1000, and adds
some randomization so that the attacker can no longer hijack
sessions without spending a considerable amount of probes.

Based on initial analysis and patch from Linus.

Note that we also have per socket rate limiting, so it is tempting
to remove the host limit in the future.

v2: randomize the count of challenge acks per second, not the period.

Fixes: 282f23c6 ("tcp: implement RFC 5961 3.2")
Reported-by: default avatarYue Cao <ycao009@ucr.edu>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Acked-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
Change-Id: Ib46ba66f5e4a5a7c81bfccd7b0aa83c3d9e1b3bb
Bug: 30809774
parent 9e5b3354

net/ipv4/tcp_input.c

+9 −3
Original line number Diff line number Diff line
@@ -88,7 +88,7 @@ int sysctl_tcp_adv_win_scale __read_mostly = 1; 
EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);



/* rfc5961 challenge ack rate limiting */

int sysctl_tcp_challenge_ack_limit = 100;

int sysctl_tcp_challenge_ack_limit = 1000;



int sysctl_tcp_stdurg __read_mostly;

int sysctl_tcp_rfc1337 __read_mostly;
@@ -3325,12 +3325,18 @@ static void tcp_send_challenge_ack(struct sock *sk) 
	static u32 challenge_timestamp;

	static unsigned int challenge_count;

	u32 now = jiffies / HZ;

	u32 count;



	if (now != challenge_timestamp) {

		u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;



		challenge_timestamp = now;

		challenge_count = 0;

		WRITE_ONCE(challenge_count, half +

			   prandom_u32_max(sysctl_tcp_challenge_ack_limit));

	}

	if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {

	count = READ_ONCE(challenge_count);

	if (count > 0) {

		WRITE_ONCE(challenge_count, count - 1);

		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);

		tcp_send_ack(sk);

	}