kernel: backport upstream challenge ACK fix (CVE-2016-5696)

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.

Backports upstream commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758
to the used LEDE kernel versions.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 76 insertions, 0 deletions

diff --git a/target/linux/generic/patches-4.1/096-tcp-make-challenge-acks-less-predictable.patch b/target/linux/generic/patches-4.1/096-tcp-make-challenge-acks-less-predictable.patch
new file mode 100644
index 0000000000..cf1da63b07
--- /dev/null
+++ b/target/linux/generic/patches-4.1/096-tcp-make-challenge-acks-less-predictable.patch
@@ -0,0 +1,76 @@
+From 75ff39ccc1bd5d3c455b6822ab09e533c551f758 Mon Sep 17 00:00:00 2001
+From: Eric Dumazet <edumazet@google.com>
+Date: Sun, 10 Jul 2016 10:04:02 +0200
+Subject: [PATCH] tcp: make challenge acks less predictable
+
+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: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
+Reported-by: Yue Cao <ycao009@ucr.edu>
+Signed-off-by: Eric Dumazet <edumazet@google.com>
+Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
+Cc: Yuchung Cheng <ycheng@google.com>
+Cc: Neal Cardwell <ncardwell@google.com>
+Acked-by: Neal Cardwell <ncardwell@google.com>
+Acked-by: Yuchung Cheng <ycheng@google.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ net/ipv4/tcp_input.c | 15 ++++++++++-----
+ 1 file changed, 10 insertions(+), 5 deletions(-)
+
+--- a/net/ipv4/tcp_input.c
++++ b/net/ipv4/tcp_input.c
+@@ -89,7 +89,7 @@ int sysctl_tcp_adv_win_scale __read_most
+ 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;
+@@ -3380,7 +3380,7 @@ static void tcp_send_challenge_ack(struc
+ 	static u32 challenge_timestamp;
+ 	static unsigned int challenge_count;
+ 	struct tcp_sock *tp = tcp_sk(sk);
+-	u32 now;
++	u32 count, now;
+ 
+ 	/* First check our per-socket dupack rate limit. */
+ 	if (tcp_oow_rate_limited(sock_net(sk), skb,
+@@ -3388,13 +3388,18 @@ static void tcp_send_challenge_ack(struc
+ 				 &tp->last_oow_ack_time))
+ 		return;
+ 
+-	/* Then check the check host-wide RFC 5961 rate limit. */
++	/* Then check host-wide RFC 5961 rate limit. */
+ 	now = jiffies / HZ;
+ 	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);
+ 	}