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Message-ID: <20130207175240.GA12520@www.outflux.net>
Date: Thu, 7 Feb 2013 09:52:40 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-kernel@...r.kernel.org
Cc: Rob Landley <rob@...dley.net>,
"David S. Miller" <davem@...emloft.net>,
Alexey Kuznetsov <kuznet@....inr.ac.ru>,
James Morris <jmorris@...ei.org>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
Patrick McHardy <kaber@...sh.net>,
Eric Dumazet <edumazet@...gle.com>,
Neil Horman <nhorman@...driver.com>,
Yuchung Cheng <ycheng@...gle.com>,
Shan Wei <davidshan@...cent.com>, linux-doc@...r.kernel.org,
netdev@...r.kernel.org, Willy Tarreau <w@....eu>
Subject: [PATCH] tcp: sysctl to disable TCP simultaneous connect
This is based on Willy Tarreau's patch from 2008[1]. The goal is to
close a corner-case of TCP that isn't used and poses a small DoS risk.
For systems that do not want to take any risk at all, this is a desirable
configuration knob.
It is possible for two clients to connect with crossed SYNs without
checking sequence numbers. As such, it might be possible to guess a source
port number to block a system from making connections to well-known
ports and IP addresses (e.g. auto-update checks) without requiring a
MiTM position.
The feature can now be disabled via sysctl:
$ echo 0 > /proc/sys/net/ipv4/tcp_simult_connect
$ echo ohai | nc -w 1 -p 50000 localhost 50000 -v -v -v
nc: connect to localhost port 50000 (tcp) timed out: Operation now in progress
nc: connect to localhost port 50000 (tcp) timed out: Operation now in progress
$ echo 1 > /proc/sys/net/ipv4/tcp_simult_connect
$ echo ohai | nc -w 1 -p 50000 localhost 50000 -v -v -v
Connection to localhost 50000 port [tcp/*] succeeded!
ohai
[1] http://thread.gmane.org/gmane.linux.network/107971
Cc: Willy Tarreau <w@....eu>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Documentation/networking/ip-sysctl.txt | 17 +++++++++++++++++
include/net/tcp.h | 1 +
net/ipv4/sysctl_net_ipv4.c | 9 +++++++++
net/ipv4/tcp_input.c | 3 ++-
4 files changed, 29 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index dbca661..2e5bd51 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -431,6 +431,23 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max
tcp_sack - BOOLEAN
Enable select acknowledgments (SACKS).
+tcp_simult_connect - BOOLEAN
+ Enables TCP simultaneous connect feature conforming to RFC793.
+ Strict implementation of RFC793 (TCP) requires support for a
+ feature called "simultaneous connect", which allows two clients to
+ connect to each other without anyone entering a listening state.
+ While almost never used, and supported by few OSes, Linux supports
+ this feature.
+
+ However, it introduces a weakness in the protocol which makes it
+ very easy for an attacker to prevent a client from connecting to
+ a known server. The attacker only has to guess the source port
+ to shut down the client connection during its establishment. The
+ impact is limited, but it may be used to prevent an antivirus
+ or IPS from fetching updates and not detecting an attack, or to
+ prevent an SSL gateway or browser from fetching a CRL.
+ Default: TRUE
+
tcp_slow_start_after_idle - BOOLEAN
If set, provide RFC2861 behavior and time out the congestion
window after an idle period. An idle period is defined at
diff --git a/include/net/tcp.h b/include/net/tcp.h
index aed42c7..ecd55d0 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -292,6 +292,7 @@ extern int sysctl_tcp_thin_dupack;
extern int sysctl_tcp_early_retrans;
extern int sysctl_tcp_limit_output_bytes;
extern int sysctl_tcp_challenge_ack_limit;
+extern int sysctl_tcp_simult_connect;
extern atomic_long_t tcp_memory_allocated;
extern struct percpu_counter tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index d84400b..01e475f 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -774,6 +774,15 @@ static struct ctl_table ipv4_table[] = {
.extra2 = &two,
},
{
+ .procname = "tcp_simult_connect",
+ .data = &sysctl_tcp_simult_connect,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &two,
+ },
+ {
.procname = "udp_mem",
.data = &sysctl_udp_mem,
.maxlen = sizeof(sysctl_udp_mem),
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 18f97ca..c71f8bb 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -102,6 +102,7 @@ int sysctl_tcp_thin_dupack __read_mostly;
int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;
int sysctl_tcp_early_retrans __read_mostly = 2;
+int sysctl_tcp_simult_connect __read_mostly = 1;
#define FLAG_DATA 0x01 /* Incoming frame contained data. */
#define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window update. */
@@ -5846,7 +5847,7 @@ discard:
tcp_paws_reject(&tp->rx_opt, 0))
goto discard_and_undo;
- if (th->syn) {
+ if (th->syn && sysctl_tcp_simult_connect) {
/* We see SYN without ACK. It is attempt of
* simultaneous connect with crossed SYNs.
* Particularly, it can be connect to self.
--
1.7.9.5
--
Kees Cook
Chrome OS Security
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