[linux] 01/02: tcp: make challenge acks less predictable (CVE-2016-5696)

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benh pushed a commit to branch wheezy-security
in repository linux.

commit f383788fb866fc61daf26836bccd92ebf7a6f02f
Author: Ben Hutchings <ben at decadent.org.uk>
Date:   Fri Aug 12 16:09:48 2016 +0100

    tcp: make challenge acks less predictable (CVE-2016-5696)
---
 debian/changelog                                   |  1 +
 .../tcp-make-challenge-acks-less-predictable.patch | 75 ++++++++++++++++++++++
 debian/patches/series                              |  1 +
 3 files changed, 77 insertions(+)

diff --git a/debian/changelog b/debian/changelog
index de8c233..1c1c31d 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -5,6 +5,7 @@ linux (3.2.81-2) UNRELEASED; urgency=medium
   * KEYS: potential uninitialized variable (CVE-2016-4470)
   * HID: hiddev: validate num_values for HIDIOCGUSAGES, HIDIOCSUSAGES commands
     (CVE-2016-5829)
+  * tcp: make challenge acks less predictable (CVE-2016-5696)
 
  -- Ben Hutchings <ben at decadent.org.uk>  Fri, 17 Jun 2016 23:35:41 +0100
 
diff --git a/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch b/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch
new file mode 100644
index 0000000..9602ab1
--- /dev/null
+++ b/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch
@@ -0,0 +1,75 @@
+From: Eric Dumazet <edumazet at google.com>
+Date: Sun, 10 Jul 2016 10:04:02 +0200
+Subject: tcp: make challenge acks less predictable
+Origin: https://git.kernel.org/linus/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: 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>
+[bwh: Backported to 3.2:
+ - Adjust context
+ - Use ACCESS_ONCE() instead of {READ,WRITE}_ONCE()
+ - Open-code prandom_u32_max()]
+Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
+---
+ net/ipv4/tcp_input.c | 17 ++++++++++++-----
+ 1 file changed, 12 insertions(+), 5 deletions(-)
+
+--- a/net/ipv4/tcp_input.c
++++ b/net/ipv4/tcp_input.c
+@@ -87,7 +87,7 @@ 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;
+@@ -3715,13 +3715,20 @@ static void tcp_send_challenge_ack(struc
+ 	/* 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) random32() * 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);
+ 	}
diff --git a/debian/patches/series b/debian/patches/series
index 4b7a0cb..cb2d7f3 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -1118,6 +1118,7 @@ bugfix/all/keys-potential-uninitialized-variable.patch
 bugfix/all/hid-hiddev-validate-num_values-for-hidiocgusages-hid.patch
 
 # ABI maintenance
+bugfix/all/tcp-make-challenge-acks-less-predictable.patch
 debian/perf-hide-abi-change-in-3.2.30.patch
 debian/hid-avoid-ABI-change-in-3.2.31.patch
 debian/xfrm-avoid-ABI-change-in-3.2.31.patch

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