openssh/openssh-7.7p1-disable_short_DH_parameters.patch

# HG changeset patch
# Parent  681914438b1a02c1940d19204138e9b8eacfda7b

Raise minimal size of DH group parameters to 2048 bits like upstream did in
7.2. 1024b values are believed to be in breaking range for state adversaries
and the default moduli shipped with openssh have been around long enough to
make it more likely for them to be broken.

Also provide an option that allows the client to accept shorter (RFC4419
compliant) parameters.

CVE-2015-4000 (LOGJAM)
bsc#932483

Index: openssh-7.8p1/dh.c
===================================================================
--- openssh-7.8p1.orig/dh.c
+++ openssh-7.8p1/dh.c
@@ -43,6 +43,8 @@
 #include "misc.h"
 #include "ssherr.h"
 
+int dh_grp_min = DH_GRP_MIN;
+
 static int
 parse_prime(int linenum, char *line, struct dhgroup *dhg)
 {
Index: openssh-7.8p1/dh.h
===================================================================
--- openssh-7.8p1.orig/dh.h
+++ openssh-7.8p1/dh.h
@@ -50,6 +50,7 @@ u_int	 dh_estimate(int);
  * Max value from RFC4419.
  * Miniumum increased in light of DH precomputation attacks.
  */
+#define DH_GRP_MIN_RFC	1024
 #define DH_GRP_MIN	2048
 #define DH_GRP_MAX	8192
 
Index: openssh-7.8p1/kexgexc.c
===================================================================
--- openssh-7.8p1.orig/kexgexc.c
+++ openssh-7.8p1/kexgexc.c
@@ -51,6 +51,9 @@
 #include "sshbuf.h"
 #include "misc.h"
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 static int input_kex_dh_gex_group(int, u_int32_t, struct ssh *);
 static int input_kex_dh_gex_reply(int, u_int32_t, struct ssh *);
 
@@ -63,7 +66,7 @@ kexgex_client(struct ssh *ssh)
 
 	nbits = dh_estimate(kex->dh_need * 8);
 
-	kex->min = DH_GRP_MIN;
+	kex->min = dh_grp_min;
 	kex->max = DH_GRP_MAX;
 	kex->nbits = nbits;
 	if (datafellows & SSH_BUG_DHGEX_LARGE)
@@ -108,6 +111,12 @@ input_kex_dh_gex_group(int type, u_int32
 		goto out;
 	if ((bits = BN_num_bits(p)) < 0 ||
 	    (u_int)bits < kex->min || (u_int)bits > kex->max) {
+		if ((u_int)bits < kex->min && (u_int)bits >= DH_GRP_MIN_RFC)
+			logit("DH parameter offered by the server (%d bits) "
+			    "is considered insecure. "
+			    "You can lower the accepted the minimum "
+			    "via the KexDHMin option.",
+			    bits);
 		r = SSH_ERR_DH_GEX_OUT_OF_RANGE;
 		goto out;
 	}
Index: openssh-7.8p1/kexgexs.c
===================================================================
--- openssh-7.8p1.orig/kexgexs.c
+++ openssh-7.8p1/kexgexs.c
@@ -54,6 +54,9 @@
 #include "sshbuf.h"
 #include "misc.h"
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 static int input_kex_dh_gex_request(int, u_int32_t, struct ssh *);
 static int input_kex_dh_gex_init(int, u_int32_t, struct ssh *);
 
@@ -82,13 +85,19 @@ input_kex_dh_gex_request(int type, u_int
 	kex->nbits = nbits;
 	kex->min = min;
 	kex->max = max;
-	min = MAXIMUM(DH_GRP_MIN, min);
+	min = MAXIMUM(dh_grp_min, min);
 	max = MINIMUM(DH_GRP_MAX, max);
-	nbits = MAXIMUM(DH_GRP_MIN, nbits);
+	nbits = MAXIMUM(dh_grp_min, nbits);
 	nbits = MINIMUM(DH_GRP_MAX, nbits);
 
 	if (kex->max < kex->min || kex->nbits < kex->min ||
 	    kex->max < kex->nbits || kex->max < DH_GRP_MIN) {
+		if (kex->nbits < kex->min && kex->nbits >= DH_GRP_MIN_RFC)
+			logit("DH parameter requested by the client (%d bits) "
+			    "is considered insecure. "
+			    "You can lower the accepted minimum "
+			    "via the KexDHMin option.",
+			    kex->nbits);
 		r = SSH_ERR_DH_GEX_OUT_OF_RANGE;
 		goto out;
 	}
Index: openssh-7.8p1/readconf.c
===================================================================
--- openssh-7.8p1.orig/readconf.c
+++ openssh-7.8p1/readconf.c
@@ -67,6 +67,7 @@
 #include "uidswap.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "dh.h"
 
 /* Format of the configuration file:
 
@@ -167,7 +168,7 @@ typedef enum {
 	oTunnel, oTunnelDevice,
 	oLocalCommand, oPermitLocalCommand, oRemoteCommand,
 	oVisualHostKey,
-	oKexAlgorithms, oIPQoS, oRequestTTY, oIgnoreUnknown, oProxyUseFdpass,
+	oKexAlgorithms, oKexDHMin, oIPQoS, oRequestTTY, oIgnoreUnknown, oProxyUseFdpass,
 	oCanonicalDomains, oCanonicalizeHostname, oCanonicalizeMaxDots,
 	oCanonicalizeFallbackLocal, oCanonicalizePermittedCNAMEs,
 	oStreamLocalBindMask, oStreamLocalBindUnlink, oRevokedHostKeys,
@@ -291,6 +292,7 @@ static struct {
 	{ "remotecommand", oRemoteCommand },
 	{ "visualhostkey", oVisualHostKey },
 	{ "kexalgorithms", oKexAlgorithms },
+	{ "kexdhmin", oKexDHMin },
 	{ "ipqos", oIPQoS },
 	{ "requesttty", oRequestTTY },
 	{ "proxyusefdpass", oProxyUseFdpass },
@@ -312,6 +314,9 @@ static struct {
 	{ NULL, oBadOption }
 };
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 /*
  * Adds a local TCP/IP port forward to options.  Never returns if there is an
  * error.
@@ -1206,6 +1211,10 @@ parse_int:
 			options->kex_algorithms = xstrdup(arg);
 		break;
 
+	case oKexDHMin:
+		intptr = &options->kex_dhmin;
+		goto parse_int;
+
 	case oHostKeyAlgorithms:
 		charptr = &options->hostkeyalgorithms;
 parse_keytypes:
@@ -1835,6 +1844,7 @@ initialize_options(Options * options)
 	options->ciphers = NULL;
 	options->macs = NULL;
 	options->kex_algorithms = NULL;
+	options->kex_dhmin = -1;
 	options->hostkeyalgorithms = NULL;
 	options->num_identity_files = 0;
 	options->num_certificate_files = 0;
@@ -1988,6 +1998,13 @@ fill_default_options(Options * options)
 		options->connection_attempts = 1;
 	if (options->number_of_password_prompts == -1)
 		options->number_of_password_prompts = 3;
+	if (options->kex_dhmin == -1)
+		options->kex_dhmin = DH_GRP_MIN;
+	else {
+		options->kex_dhmin = MAXIMUM(options->kex_dhmin, DH_GRP_MIN_RFC);
+		options->kex_dhmin = MINIMUM(options->kex_dhmin, DH_GRP_MAX);
+	}
+	dh_grp_min = options->kex_dhmin;
 	/* options->hostkeyalgorithms, default set in myproposals.h */
 	if (options->add_keys_to_agent == -1)
 		options->add_keys_to_agent = 0;
Index: openssh-7.8p1/readconf.h
===================================================================
--- openssh-7.8p1.orig/readconf.h
+++ openssh-7.8p1/readconf.h
@@ -67,6 +67,7 @@ typedef struct {
 	char   *macs;		/* SSH2 macs in order of preference. */
 	char   *hostkeyalgorithms;	/* SSH2 server key types in order of preference. */
 	char   *kex_algorithms;	/* SSH2 kex methods in order of preference. */
+	int     kex_dhmin;	/* minimum bit length of the DH group parameter */
 	char   *hostname;	/* Real host to connect. */
 	char   *host_key_alias;	/* hostname alias for .ssh/known_hosts */
 	char   *proxy_command;	/* Proxy command for connecting the host. */
Index: openssh-7.8p1/servconf.c
===================================================================
--- openssh-7.8p1.orig/servconf.c
+++ openssh-7.8p1/servconf.c
@@ -64,6 +64,10 @@
 #include "auth.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "dh.h"
+
+/* import from dh.c */
+extern int dh_grp_min;
 
 static void add_listen_addr(ServerOptions *, const char *,
     const char *, int);
@@ -146,6 +150,7 @@ initialize_server_options(ServerOptions
 	options->ciphers = NULL;
 	options->macs = NULL;
 	options->kex_algorithms = NULL;
+	options->kex_dhmin = -1;
 	options->fwd_opts.gateway_ports = -1;
 	options->fwd_opts.streamlocal_bind_mask = (mode_t)-1;
 	options->fwd_opts.streamlocal_bind_unlink = -1;
@@ -263,6 +268,14 @@ fill_default_server_options(ServerOption
 	if (options->use_pam_check_locks == -1)
 		options->use_pam_check_locks = 0;
 
+	if (options->kex_dhmin == -1)
+		options->kex_dhmin = DH_GRP_MIN;
+	else {
+		options->kex_dhmin = MAXIMUM(options->kex_dhmin, DH_GRP_MIN_RFC);
+		options->kex_dhmin = MINIMUM(options->kex_dhmin, DH_GRP_MAX);
+	}
+	dh_grp_min = options->kex_dhmin;
+
 	/* Standard Options */
 	if (options->num_host_key_files == 0) {
 		/* fill default hostkeys for protocols */
@@ -490,7 +503,7 @@ typedef enum {
 	sHostCertificate,
 	sRevokedKeys, sTrustedUserCAKeys, sAuthorizedPrincipalsFile,
 	sAuthorizedPrincipalsCommand, sAuthorizedPrincipalsCommandUser,
-	sKexAlgorithms, sIPQoS, sVersionAddendum,
+	sKexAlgorithms, sKexDHMin, sIPQoS, sVersionAddendum,
 	sAuthorizedKeysCommand, sAuthorizedKeysCommandUser,
 	sAuthenticationMethods, sHostKeyAgent, sPermitUserRC,
 	sStreamLocalBindMask, sStreamLocalBindUnlink,
@@ -631,6 +644,7 @@ static struct {
 	{ "trustedusercakeys", sTrustedUserCAKeys, SSHCFG_ALL },
 	{ "authorizedprincipalsfile", sAuthorizedPrincipalsFile, SSHCFG_ALL },
 	{ "kexalgorithms", sKexAlgorithms, SSHCFG_GLOBAL },
+	{ "kexdhmin", sKexDHMin },
 	{ "ipqos", sIPQoS, SSHCFG_ALL },
 	{ "authorizedkeyscommand", sAuthorizedKeysCommand, SSHCFG_ALL },
 	{ "authorizedkeyscommanduser", sAuthorizedKeysCommandUser, SSHCFG_ALL },
@@ -1726,6 +1740,10 @@ process_server_config_line(ServerOptions
 			options->kex_algorithms = xstrdup(arg);
 		break;
 
+	case sKexDHMin:
+		intptr = &options->kex_dhmin;
+		goto parse_int;
+
 	case sSubsystem:
 		if (options->num_subsystems >= MAX_SUBSYSTEMS) {
 			fatal("%s line %d: too many subsystems defined.",
@@ -2540,6 +2558,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_int(sClientAliveInterval, o->client_alive_interval);
 	dump_cfg_int(sClientAliveCountMax, o->client_alive_count_max);
 	dump_cfg_oct(sStreamLocalBindMask, o->fwd_opts.streamlocal_bind_mask);
+	dump_cfg_int(sKexDHMin, o->kex_dhmin);
 
 	/* formatted integer arguments */
 	dump_cfg_fmtint(sPermitRootLogin, o->permit_root_login);
Index: openssh-7.8p1/servconf.h
===================================================================
--- openssh-7.8p1.orig/servconf.h
+++ openssh-7.8p1/servconf.h
@@ -103,6 +103,7 @@ typedef struct {
 	char   *ciphers;	/* Supported SSH2 ciphers. */
 	char   *macs;		/* Supported SSH2 macs. */
 	char   *kex_algorithms;	/* SSH2 kex methods in order of preference. */
+	int     kex_dhmin;	/* minimum bit length of the DH group parameter */
 	struct ForwardOptions fwd_opts;	/* forwarding options */
 	SyslogFacility log_facility;	/* Facility for system logging. */
 	LogLevel log_level;	/* Level for system logging. */
Index: openssh-7.8p1/ssh_config
===================================================================
--- openssh-7.8p1.orig/ssh_config
+++ openssh-7.8p1/ssh_config
@@ -17,6 +17,11 @@
 # list of available options, their meanings and defaults, please see the
 # ssh_config(5) man page.
 
+# Minimum accepted size of the DH parameter p. By default this is set to 1024
+# to maintain compatibility with RFC4419, but should be set higher.
+# Upstream default is identical to setting this to 2048.
+#KexDHMin 1024
+
 Host *
 #   ForwardAgent no
 #   ForwardX11 no
Index: openssh-7.8p1/ssh_config.0
===================================================================
--- openssh-7.8p1.orig/ssh_config.0
+++ openssh-7.8p1/ssh_config.0
@@ -595,6 +595,23 @@ DESCRIPTION
              The list of available key exchange algorithms may also be
              obtained using "ssh -Q kex".
 
+     KexDHMin
+             Specifies the minimum accepted bit length of the DH group
+             parameter p.
+
+             As per RFC4419, this is 1024 bits, however this has increasingly
+             been seen as insecure, which prompted the change to 2048 bits.
+             Setting this option allows the client to accept parameters shorter
+             than the current minimum, down to the RFC specified 1024 bits.
+             Using this option may be needed when connecting to servers that
+             only know short DH group parameters.
+
+             Note, that while by default this option is set to 1024 to maintain
+             maximum backward compatibility, using it can severly impact
+             security and thus should be viewed as a temporary fix of last
+             resort and all efforts should be made to fix the (broken)
+             counterparty.
+
      LocalCommand
              Specifies a command to execute on the local machine after
              successfully connecting to the server.  The command string
Index: openssh-7.8p1/ssh_config.5
===================================================================
--- openssh-7.8p1.orig/ssh_config.5
+++ openssh-7.8p1/ssh_config.5
@@ -1025,6 +1025,22 @@ diffie-hellman-group14-sha1
 .Pp
 The list of available key exchange algorithms may also be obtained using
 .Qq ssh -Q kex .
+.It Cm KexDHMin
+Specifies the minimum accepted bit length of the DH group
+parameter p.
+.Pp
+As per RFC4419, this is 1024 bits, however this has increasingly
+been seen as insecure, which prompted the change to 2048 bits.
+Setting this option allows the client to accept parameters shorter
+than the current minimum, down to the RFC specified 1024 bits.
+Using this option may be needed when connecting to servers that
+only know short DH group parameters.
+.Pp
+Note, that while by default this option is set to 1024 to maintain
+maximum backward compatibility, using it can severly impact
+security and thus should be viewed as a temporary fix of last
+resort and all efforts should be made to fix the (broken)
+counterparty.
 .It Cm LocalCommand
 Specifies a command to execute on the local machine after successfully
 connecting to the server.
Index: openssh-7.8p1/sshd_config
===================================================================
--- openssh-7.8p1.orig/sshd_config
+++ openssh-7.8p1/sshd_config
@@ -19,6 +19,13 @@
 #HostKey /etc/ssh/ssh_host_ecdsa_key
 #HostKey /etc/ssh/ssh_host_ed25519_key
 
+# Minimum accepted size of the DH parameter p. The default, which replicates
+# upstream behaviour, is 2048. To maintain compatibility with RFC4419 it should
+# be set to 1024.
+# You will also need to add a HostKey directive to load a DSA key (typically
+# located in /etc/ssh/ssh_host_dsa_key), which is not loaded by default.
+#KexDHMin 2048
+
 # Ciphers and keying
 #RekeyLimit default none
 
Index: openssh-7.8p1/sshd_config.0
===================================================================
--- openssh-7.8p1.orig/sshd_config.0
+++ openssh-7.8p1/sshd_config.0
@@ -545,6 +545,23 @@ DESCRIPTION
              The list of available key exchange algorithms may also be
              obtained using "ssh -Q kex".
 
+     KexDHMin
+             Specifies the minimum accepted bit length of the DH group
+             parameter p.
+
+             As per RFC4419, this is 1024 bits, however this has increasingly
+             been seen as insecure, which prompted the change to 2048 bits.
+             Setting this option allows the server to accept parameters shorter
+             than the current minimum, down to the RFC specified 1024 bits.
+             Using this option may be needed when some of the connectiong
+             clients only know short DH group parameters.
+
+             Note, that while by default this option is set to 1024 to maintain
+             maximum backward compatibility, using it can severly impact
+             security and thus should be viewed as a temporary fix of last
+             resort and all efforts should be made to fix the (broken)
+             counterparty.
+
      ListenAddress
              Specifies the local addresses sshd(8) should listen on.  The
              following forms may be used:
Index: openssh-7.8p1/sshd_config.5
===================================================================
--- openssh-7.8p1.orig/sshd_config.5
+++ openssh-7.8p1/sshd_config.5
@@ -912,6 +912,22 @@ diffie-hellman-group14-sha256,diffie-hel
 .Pp
 The list of available key exchange algorithms may also be obtained using
 .Qq ssh -Q kex .
+.It Cm KexDHMin
+Specifies the minimum accepted bit length of the DH group
+parameter p.
+.Pp
+As per RFC4419, this is 1024 bits, however this has increasingly
+been seen as insecure, which prompted the change to 2048 bits.
+Setting this option allows the server to accept parameters shorter
+than the current minimum, down to the RFC specified 1024 bits.
+Using this option may be needed when some of the connectiong
+clients only know short DH group parameters.
+.Pp
+Note, that while by default this option is set to 1024 to maintain
+maximum backward compatibility, using it can severly impact
+security and thus should be viewed as a temporary fix of last
+resort and all efforts should be made to fix the (broken)
+counterparty.
 .It Cm ListenAddress
 Specifies the local addresses
 .Xr sshd 8