Add a sshd_config DisableForwaring option that disables X11, agent,

TCP, tunnel and Unix domain socket forwarding, as well as anything
else we might implement in the future.

This, like the 'restrict' authorized_keys flag, is intended to be a
simple and future-proof way of restricting an account. Suggested as
a complement to 'restrict' by Jann Horn; ok markus@

5 files changed, 29 insertions, 12 deletions

diff --git a/usr.bin/ssh/servconf.c b/usr.bin/ssh/servconf.c
index 2a3c5d3dc9c..f9b7812f4c5 100644
--- a/usr.bin/ssh/servconf.c
+++ b/usr.bin/ssh/servconf.c
@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.300 2016/11/23 23:14:15 markus Exp $ */
+/* $OpenBSD: servconf.c,v 1.301 2016/11/30 03:00:05 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -153,6 +153,7 @@ initialize_server_options(ServerOptions *options)
 	options->ip_qos_bulk = -1;
 	options->version_addendum = NULL;
 	options->fingerprint_hash = -1;
+	options->disable_forwarding = -1;
 }
 
 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */
@@ -313,6 +314,8 @@ fill_default_server_options(ServerOptions *options)
 		options->fwd_opts.streamlocal_bind_unlink = 0;
 	if (options->fingerprint_hash == -1)
 		options->fingerprint_hash = SSH_FP_HASH_DEFAULT;
+	if (options->disable_forwarding == -1)
+		options->disable_forwarding = 0;
 
 	assemble_algorithms(options);
 
@@ -384,7 +387,7 @@ typedef enum {
 	sAuthorizedKeysCommand, sAuthorizedKeysCommandUser,
 	sAuthenticationMethods, sHostKeyAgent, sPermitUserRC,
 	sStreamLocalBindMask, sStreamLocalBindUnlink,
-	sAllowStreamLocalForwarding, sFingerprintHash,
+	sAllowStreamLocalForwarding, sFingerprintHash, sDisableForwarding,
 	sDeprecated, sIgnore, sUnsupported
 } ServerOpCodes;
 
@@ -511,6 +514,7 @@ static struct {
 	{ "streamlocalbindunlink", sStreamLocalBindUnlink, SSHCFG_ALL },
 	{ "allowstreamlocalforwarding", sAllowStreamLocalForwarding, SSHCFG_ALL },
 	{ "fingerprinthash", sFingerprintHash, SSHCFG_GLOBAL },
+	{ "disableforwarding", sDisableForwarding, SSHCFG_ALL },
 	{ NULL, sBadOption, 0 }
 };
 
@@ -1304,6 +1308,10 @@ process_server_config_line(ServerOptions *options, char *line,
 		intptr = &options->allow_agent_forwarding;
 		goto parse_flag;
 
+	case sDisableForwarding:
+		intptr = &options->disable_forwarding;
+		goto parse_flag;
+
 	case sUsePrivilegeSeparation:
 		intptr = &use_privsep;
 		multistate_ptr = multistate_privsep;
@@ -1913,6 +1921,7 @@ copy_set_server_options(ServerOptions *dst, ServerOptions *src, int preauth)
 	M_CP_INTOPT(allow_tcp_forwarding);
 	M_CP_INTOPT(allow_streamlocal_forwarding);
 	M_CP_INTOPT(allow_agent_forwarding);
+	M_CP_INTOPT(disable_forwarding);
 	M_CP_INTOPT(permit_tun);
 	M_CP_INTOPT(fwd_opts.gateway_ports);
 	M_CP_INTOPT(fwd_opts.streamlocal_bind_unlink);
@@ -2204,6 +2213,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_fmtint(sUseDNS, o->use_dns);
 	dump_cfg_fmtint(sAllowTcpForwarding, o->allow_tcp_forwarding);
 	dump_cfg_fmtint(sAllowAgentForwarding, o->allow_agent_forwarding);
+	dump_cfg_fmtint(sDisableForwarding, o->disable_forwarding);
 	dump_cfg_fmtint(sAllowStreamLocalForwarding, o->allow_streamlocal_forwarding);
 	dump_cfg_fmtint(sStreamLocalBindUnlink, o->fwd_opts.streamlocal_bind_unlink);
 	dump_cfg_fmtint(sUsePrivilegeSeparation, use_privsep);
diff --git a/usr.bin/ssh/servconf.h b/usr.bin/ssh/servconf.h
index 74230a1b07e..712e7563be8 100644
--- a/usr.bin/ssh/servconf.h
+++ b/usr.bin/ssh/servconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.122 2016/08/19 03:18:06 djm Exp $ */
+/* $OpenBSD: servconf.h,v 1.123 2016/11/30 03:00:05 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -125,6 +125,7 @@ typedef struct {
 	int	allow_tcp_forwarding; /* One of FORWARD_* */
 	int	allow_streamlocal_forwarding; /* One of FORWARD_* */
 	int	allow_agent_forwarding;
+	int	disable_forwarding;
 	u_int num_allow_users;
 	char   *allow_users[MAX_ALLOW_USERS];
 	u_int num_deny_users;
diff --git a/usr.bin/ssh/serverloop.c b/usr.bin/ssh/serverloop.c
index 15d7d400265..679ad13c044 100644
--- a/usr.bin/ssh/serverloop.c
+++ b/usr.bin/ssh/serverloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: serverloop.c,v 1.187 2016/10/23 22:04:05 dtucker Exp $ */
+/* $OpenBSD: serverloop.c,v 1.188 2016/11/30 03:00:05 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -440,7 +440,7 @@ server_request_direct_tcpip(void)
 
 	/* XXX fine grained permissions */
 	if ((options.allow_tcp_forwarding & FORWARD_LOCAL) != 0 &&
-	    !no_port_forwarding_flag) {
+	    !no_port_forwarding_flag && !options.disable_forwarding) {
 		c = channel_connect_to_port(target, target_port,
 		    "direct-tcpip", "direct-tcpip");
 	} else {
@@ -472,7 +472,7 @@ server_request_direct_streamlocal(void)
 
 	/* XXX fine grained permissions */
 	if ((options.allow_streamlocal_forwarding & FORWARD_LOCAL) != 0 &&
-	    !no_port_forwarding_flag) {
+	    !no_port_forwarding_flag && !options.disable_forwarding) {
 		c = channel_connect_to_path(target,
 		    "direct-streamlocal@openssh.com", "direct-streamlocal");
 	} else {
@@ -710,7 +710,7 @@ server_input_global_request(int type, u_int32_t seq, void *ctxt)
 
 		/* check permissions */
 		if ((options.allow_tcp_forwarding & FORWARD_REMOTE) == 0 ||
-		    no_port_forwarding_flag ||
+		    no_port_forwarding_flag || options.disable_forwarding ||
 		    (!want_reply && fwd.listen_port == 0) ||
 		    (fwd.listen_port != 0 &&
 		     !bind_permitted(fwd.listen_port, pw->pw_uid))) {
@@ -748,7 +748,7 @@ server_input_global_request(int type, u_int32_t seq, void *ctxt)
 
 		/* check permissions */
 		if ((options.allow_streamlocal_forwarding & FORWARD_REMOTE) == 0
-		    || no_port_forwarding_flag) {
+		    || no_port_forwarding_flag || options.disable_forwarding) {
 			success = 0;
 			packet_send_debug("Server has disabled port forwarding.");
 		} else {
diff --git a/usr.bin/ssh/session.c b/usr.bin/ssh/session.c
index 038b7e41781..61d2e42788a 100644
--- a/usr.bin/ssh/session.c
+++ b/usr.bin/ssh/session.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: session.c,v 1.285 2016/08/23 16:21:45 otto Exp $ */
+/* $OpenBSD: session.c,v 1.286 2016/11/30 03:00:05 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -239,7 +239,7 @@ do_authenticated(Authctxt *authctxt)
 
 	/* setup the channel layer */
 	/* XXX - streamlocal? */
-	if (no_port_forwarding_flag ||
+	if (no_port_forwarding_flag || options.disable_forwarding ||
 	    (options.allow_tcp_forwarding & FORWARD_LOCAL) == 0)
 		channel_disable_adm_local_opens();
 	else
diff --git a/usr.bin/ssh/sshd_config.5 b/usr.bin/ssh/sshd_config.5
index d5dff472867..beb245cf5f8 100644
--- a/usr.bin/ssh/sshd_config.5
+++ b/usr.bin/ssh/sshd_config.5
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: sshd_config.5,v 1.238 2016/11/23 23:14:15 markus Exp $
-.Dd $Mdocdate: November 23 2016 $
+.\" $OpenBSD: sshd_config.5,v 1.239 2016/11/30 03:00:05 djm Exp $
+.Dd $Mdocdate: November 30 2016 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -565,6 +565,12 @@ and finally
 See PATTERNS in
 .Xr ssh_config 5
 for more information on patterns.
+.It Cm DisableForwarding
+Disables all forwarding features, including X11,
+.Xr ssh-agent 1 ,
+TCP and StreamLocal.
+This option overrides all other forwarding-related options and may
+simplify restricted configurations.
 .It Cm FingerprintHash
 Specifies the hash algorithm used when logging key fingerprints.
 Valid options are: