[RFC SRU Trusty] tcp: make challenge acks less predictable

[Stefan Bader]

This is a rather quick (and not the most elegant) backport of the
upstream fix for Trusty (Precise will need the same or even more).
The Problem is that neither WRITE_ONCE nor prandom_u32_max do exist,
yet. The prandom_u32_max we might be able to just replace by the
actual function body. I am not sure I would trust myself enough to
try that for the WRITE_ONCE part.

-Stefan


>From 16961a6e38afb30c0afb41d766b3fa487fb017f4 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at 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 at ucr.edu>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Yuchung Cheng <ycheng at google.com>
Cc: Neal Cardwell <ncardwell at google.com>
Acked-by: Neal Cardwell <ncardwell at google.com>
Acked-by: Yuchung Cheng <ycheng at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>

CVE-2016-5696

(backported from commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758 upstream)
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 net/ipv4/tcp_input.c | 43 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2cc1313..4c244cd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -86,7 +86,7 @@ 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;
@@ -3282,23 +3282,58 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
 	return flag;
 }
 
+static __always_inline void __write_once_size(volatile void *p, void *res, int size)
+{
+	switch (size) {
+	case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
+	case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
+	case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
+	case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
+	default:
+		barrier();
+		__builtin_memcpy((void *)p, (const void *)res, size);
+		barrier();
+	}
+}
+
+#define WRITE_ONCE(x, val) \
+({							\
+	union { typeof(x) __val; char __c[1]; } __u =	\
+		{ .__val = (__force typeof(x)) (val) };	\
+	__write_once_size(&(x), __u.__c, sizeof(x));	\
+	__u.__val;					\
+})
+
+static inline u32 prandom_u32_max(u32 ep_ro)
+{
+	return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
+}
+
 /* RFC 5961 7 [ACK Throttling] */
 static void tcp_send_challenge_ack(struct sock *sk)
 {
 	/* unprotected vars, we dont care of overwrites */
 	static u32 challenge_timestamp;
 	static unsigned int challenge_count;
-	u32 now = jiffies / HZ;
+	u32 count, now = jiffies / HZ;
 
+	/* Check host-wide RFC 5961 rate limit. */
 	if (now != challenge_timestamp) {
+		u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
+
 		challenge_timestamp = now;
-		challenge_count = 0;
+		*((volatile unsigned int *) &challenge_count) =
+		WRITE_ONCE(challenge_count, half +
+			   prandom_u32_max(sysctl_tcp_challenge_ack_limit));
 	}
-	if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
+	count = ACCESS_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);
 	}
 }
+#undef WRITE_ONCE
 
 static void tcp_store_ts_recent(struct tcp_sock *tp)
 {
-- 
1.9.1