Apply upstream-recommended patch to fix bignum encoding for curve25519-sha256@libssh.org, fixing occasional key exchange failures.

2 files changed, 162 insertions, 0 deletions

diff --git a/debian/patches/curve25519-sha256-bignum-encoding.patch b/debian/patches/curve25519-sha256-bignum-encoding.patch
new file mode 100644
index 000000000..ccb66048d
--- /dev/null
+++ b/debian/patches/curve25519-sha256-bignum-encoding.patch
@@ -0,0 +1,161 @@
+From 02883061577ec43ff8d0e8f0cf486bc5131db507 Mon Sep 17 00:00:00 2001
+From: Damien Miller <djm@mindrot.org>
+Date: Sun, 20 Apr 2014 13:47:45 +1000
+Subject: bad bignum encoding for curve25519-sha256@libssh.org
+
+Hi,
+
+So I screwed up when writing the support for the curve25519 KEX method
+that doesn't depend on OpenSSL's BIGNUM type - a bug in my code left
+leading zero bytes where they should have been skipped. The impact of
+this is that OpenSSH 6.5 and 6.6 will fail during key exchange with a
+peer that implements curve25519-sha256@libssh.org properly about 0.2%
+of the time (one in every 512ish connections).
+
+We've fixed this for OpenSSH 6.7 by avoiding the curve25519-sha256
+key exchange for previous versions, but I'd recommend distributors
+of OpenSSH apply this patch so the affected code doesn't become
+too entrenched in LTS releases.
+
+The patch fixes the bug and makes OpenSSH identify itself as 6.6.1 so as
+to distinguish itself from the incorrect versions so the compatibility
+code to disable the affected KEX isn't activated.
+
+I've committed this on the 6.6 branch too.
+
+Apologies for the hassle.
+
+-d
+
+Origin: upstream, https://lists.mindrot.org/pipermail/openssh-unix-dev/2014-April/032494.html
+Forwarded: not-needed
+Last-Update: 2014-04-21
+
+Patch-Name: curve25519-sha256-bignum-encoding.patch
+---
+ bufaux.c      |  5 ++++-
+ compat.c      | 17 ++++++++++++++++-
+ compat.h      |  2 ++
+ sshconnect2.c |  2 ++
+ sshd.c        |  3 +++
+ version.h     |  2 +-
+ 6 files changed, 28 insertions(+), 3 deletions(-)
+
+diff --git a/bufaux.c b/bufaux.c
+index e24b5fc..f6a6f2a 100644
+--- a/bufaux.c
++++ b/bufaux.c
+@@ -1,4 +1,4 @@
+-/* $OpenBSD: bufaux.c,v 1.56 2014/02/02 03:44:31 djm Exp $ */
++/* $OpenBSD: bufaux.c,v 1.57 2014/04/16 23:22:45 djm Exp $ */
+ /*
+  * Author: Tatu Ylonen <ylo@cs.hut.fi>
+  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
+@@ -372,6 +372,9 @@ buffer_put_bignum2_from_string(Buffer *buffer, const u_char *s, u_int l)
+ 
+        if (l > 8 * 1024)
+                fatal("%s: length %u too long", __func__, l);
++       /* Skip leading zero bytes */
++       for (; l > 0 && *s == 0; l--, s++)
++               ;
+        p = buf = xmalloc(l + 1);
+        /*
+         * If most significant bit is set then prepend a zero byte to
+diff --git a/compat.c b/compat.c
+index 9d9fabe..2709dc5 100644
+--- a/compat.c
++++ b/compat.c
+@@ -95,6 +95,9 @@ 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_6.6.1*",     SSH_NEW_OPENSSH},
++               { "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|
+@@ -251,7 +254,6 @@ compat_cipher_proposal(char *cipher_prop)
+        return cipher_prop;
+ }
+ 
+-
+ char *
+ compat_pkalg_proposal(char *pkalg_prop)
+ {
+@@ -265,3 +267,16 @@ 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/compat.h b/compat.h
+index b174fa1..a6c3f3d 100644
+--- a/compat.h
++++ b/compat.h
+@@ -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/sshconnect2.c b/sshconnect2.c
+index 66cb035..1a4e551 100644
+--- a/sshconnect2.c
++++ b/sshconnect2.c
+@@ -220,6 +220,8 @@ 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]);
+ 
+ #ifdef GSSAPI
+        /* If we've got GSSAPI algorithms, then we also support the
+diff --git a/sshd.c b/sshd.c
+index 0964491..fe78d7b 100644
+--- a/sshd.c
++++ b/sshd.c
+@@ -2534,6 +2534,9 @@ 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/version.h b/version.h
+index a97c337..0659576 100644
+--- a/version.h
++++ b/version.h
+@@ -1,6 +1,6 @@
+ /* $OpenBSD: version.h,v 1.70 2014/02/27 22:57:40 djm Exp $ */
+ 
+-#define SSH_VERSION    "OpenSSH_6.6"
++#define SSH_VERSION    "OpenSSH_6.6.1"
+ 
+ #define SSH_PORTABLE   "p1"
+ #define SSH_RELEASE_MINIMUM    SSH_VERSION SSH_PORTABLE
diff --git a/debian/patches/series b/debian/patches/series
index de7c9902d..c554b34ca 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -27,3 +27,4 @@ gnome-ssh-askpass2-icon.patch
 sigstop.patch
 debian-config.patch
 sshfp_with_server_cert_upstr
+curve25519-sha256-bignum-encoding.patch