diff mbox

Trusty CVE-2016-5696

Message ID 945ecdd2-4d9a-c520-48cf-895f6a4b551d@canonical.com
State New
Headers show

Commit Message

Stefan Bader Aug. 12, 2016, 1:36 p.m. UTC 
On 12.08.2016 15:30, Tim Gardner wrote:
> In fact, how about this 4 patch set. the first 3 are clean cherry-picks, but required to provide
> the inline macros upon which the real fix depends.
> 
> [PATCH 1/4] kernel: Provide READ_ONCE and ASSIGN_ONCE
> [PATCH 2/4] kernel: Change ASSIGN_ONCE(val, x) to WRITE_ONCE(x, val)
> [PATCH 3/4] random32: add prandom_u32_max and convert open coded
> [PATCH 4/4] tcp: make challenge acks less predictable

Actually with the backport from Ben Hutchings that Seth mentioned...
diff mbox

Patch

From ffdc560eac34cd5968c2d73ca1d724f4818d890b 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

commit 75ff39ccc1bd5d3c455b6822ab09e533c551f758 upstream.

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>
[bwh: Backported to 3.2:
 - Adjust context
 - Use ACCESS_ONCE() instead of {READ,WRITE}_ONCE()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

CVE-2016-5696

[smb: Backported from f516fa21b9275c7fef99a1075f39394bcd677dfc bwh
 - Replaced prandom_u32_max by later implementation

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
---
 net/ipv4/tcp_input.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2cc1313..5380c00 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;
@@ -3288,13 +3288,20 @@  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;
 
 	if (now != challenge_timestamp) {
+		u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
+
 		challenge_timestamp = now;
-		challenge_count = 0;
-	}
-	if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
+		ACCESS_ONCE(challenge_count) =
+			half + (u32)(
+			((u64) prandom_u32() * sysctl_tcp_challenge_ack_limit)
+			>> 32);
+	}
+	count = ACCESS_ONCE(challenge_count);
+	if (count > 0) {
+		ACCESS_ONCE(challenge_count) = count - 1;
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
 		tcp_send_ack(sk);
 	}
-- 
1.9.1