OpenSSH 6.5 and 6.6 have a bug that causes ~0.2% of connections

using the curve25519-sha256@libssh.org KEX exchange method to fail
when connecting with something that implements the spec properly.

Disable this KEX method when speaking to one of the affected
versions.

reported by Aris Adamantiadis; ok markus@

5 files changed, 28 insertions, 7 deletions

diff --git a/usr.bin/ssh/compat.c b/usr.bin/ssh/compat.c
index 52ce971e541..5f9e363eeec 100644
--- a/usr.bin/ssh/compat.c
+++ b/usr.bin/ssh/compat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.c,v 1.82 2013/12/30 23:52:27 djm Exp $ */
+/* $OpenBSD: compat.c,v 1.83 2014/04/18 23:52:25 djm Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *
@@ -93,6 +93,8 @@ compat_datafellows(const char *version)
 		{ "Sun_SSH_1.0*",	SSH_BUG_NOREKEY|SSH_BUG_EXTEOF},
 		{ "OpenSSH_4*",		0 },
 		{ "OpenSSH_5*",		SSH_NEW_OPENSSH|SSH_BUG_DYNAMIC_RPORT},
+		{ "OpenSSH_6.5*,"
+		  "OpenSSH_6.6",	SSH_NEW_OPENSSH|SSH_BUG_CURVE25519PAD},
 		{ "OpenSSH*",		SSH_NEW_OPENSSH },
 		{ "*MindTerm*",		0 },
 		{ "2.1.0*",		SSH_BUG_SIGBLOB|SSH_BUG_HMAC|
@@ -249,7 +251,6 @@ compat_cipher_proposal(char *cipher_prop)
 	return cipher_prop;
 }
 
-
 char *
 compat_pkalg_proposal(char *pkalg_prop)
 {
@@ -263,3 +264,16 @@ compat_pkalg_proposal(char *pkalg_prop)
 	return pkalg_prop;
 }
 
+char *
+compat_kex_proposal(char *kex_prop)
+{
+	if (!(datafellows & SSH_BUG_CURVE25519PAD))
+		return kex_prop;
+	debug2("%s: original KEX proposal: %s", __func__, kex_prop);
+	kex_prop = filter_proposal(kex_prop, "curve25519-sha256@libssh.org");
+	debug2("%s: compat KEX proposal: %s", __func__, kex_prop);
+	if (*kex_prop == '\0')
+		fatal("No supported key exchange algorithms found");
+	return kex_prop;
+}
+
diff --git a/usr.bin/ssh/compat.h b/usr.bin/ssh/compat.h
index b174fa1719c..2e25d5ba991 100644
--- a/usr.bin/ssh/compat.h
+++ b/usr.bin/ssh/compat.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.h,v 1.44 2013/12/30 23:52:27 djm Exp $ */
+/* $OpenBSD: compat.h,v 1.45 2014/04/18 23:52:25 djm Exp $ */
 
 /*
  * Copyright (c) 1999, 2000, 2001 Markus Friedl.  All rights reserved.
@@ -59,6 +59,7 @@
 #define SSH_BUG_RFWD_ADDR	0x02000000
 #define SSH_NEW_OPENSSH		0x04000000
 #define SSH_BUG_DYNAMIC_RPORT	0x08000000
+#define SSH_BUG_CURVE25519PAD	0x10000000
 
 void     enable_compat13(void);
 void     enable_compat20(void);
@@ -66,6 +67,7 @@ void     compat_datafellows(const char *);
 int	 proto_spec(const char *);
 char	*compat_cipher_proposal(char *);
 char	*compat_pkalg_proposal(char *);
+char	*compat_kex_proposal(char *);
 
 extern int compat13;
 extern int compat20;
diff --git a/usr.bin/ssh/sshconnect2.c b/usr.bin/ssh/sshconnect2.c
index 4848cd48af8..8e7e7ca75b7 100644
--- a/usr.bin/ssh/sshconnect2.c
+++ b/usr.bin/ssh/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.205 2014/03/27 23:01:27 markus Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.206 2014/04/18 23:52:25 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -190,6 +190,8 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
 	}
 	if (options.kex_algorithms != NULL)
 		myproposal[PROPOSAL_KEX_ALGS] = options.kex_algorithms;
+	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
+	    myproposal[PROPOSAL_KEX_ALGS]);
 
 	if (options.rekey_limit || options.rekey_interval)
 		packet_set_rekey_limits((u_int32_t)options.rekey_limit,
diff --git a/usr.bin/ssh/sshd.c b/usr.bin/ssh/sshd.c
index e15dedd421a..cf013ee5af7 100644
--- a/usr.bin/ssh/sshd.c
+++ b/usr.bin/ssh/sshd.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshd.c,v 1.423 2014/04/12 04:55:53 djm Exp $ */
+/* $OpenBSD: sshd.c,v 1.424 2014/04/18 23:52:25 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2292,6 +2292,9 @@ do_ssh2_kex(void)
 	if (options.kex_algorithms != NULL)
 		myproposal[PROPOSAL_KEX_ALGS] = options.kex_algorithms;
 
+	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
+	    myproposal[PROPOSAL_KEX_ALGS]);
+
 	if (options.rekey_limit || options.rekey_interval)
 		packet_set_rekey_limits((u_int32_t)options.rekey_limit,
 		    (time_t)options.rekey_interval);
diff --git a/usr.bin/ssh/version.h b/usr.bin/ssh/version.h
index 5fb6a69cf24..fa2e4b07bd3 100644
--- a/usr.bin/ssh/version.h
+++ b/usr.bin/ssh/version.h
@@ -1,3 +1,3 @@
-/* $OpenBSD: version.h,v 1.70 2014/02/27 22:57:40 djm Exp $ */
+/* $OpenBSD: version.h,v 1.71 2014/04/18 23:52:25 djm Exp $ */
 
-#define SSH_VERSION	"OpenSSH_6.6"
+#define SSH_VERSION	"OpenSSH_6.7"