Expand ssh_config's StrictModes option with two new settings:

StrictModes=accept-new will automatically accept hitherto-unseen keys
but will refuse connections for changed or invalid hostkeys.

StrictModes=off is the same as StrictModes=no

Motivation:

StrictModes=no combines two behaviours for host key processing:
automatically learning new hostkeys and continuing to connect to hosts
with invalid/changed hostkeys. The latter behaviour is quite dangerous
since it removes most of the protections the SSH protocol is supposed to
provide.

Quite a few users want to automatically learn hostkeys however, so
this makes that feature available with less danger.

At some point in the future, StrictModes=no will change to be a synonym
for accept-new, with its current behaviour remaining available via
StrictModes=off.

bz#2400, suggested by Michael Samuel; ok markus

4 files changed, 52 insertions, 22 deletions

diff --git a/usr.bin/ssh/readconf.c b/usr.bin/ssh/readconf.c
index b9a7f7c30cb..51bd866638e 100644
--- a/usr.bin/ssh/readconf.c
+++ b/usr.bin/ssh/readconf.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.277 2017/05/30 18:58:37 bluhm Exp $ */
+/* $OpenBSD: readconf.c,v 1.278 2017/09/03 23:33:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -736,6 +736,16 @@ static const struct multistate multistate_yesnoask[] = {
 	{ "ask",			2 },
 	{ NULL, -1 }
 };
+static const struct multistate multistate_strict_hostkey[] = {
+	{ "true",			SSH_STRICT_HOSTKEY_YES },
+	{ "false",			SSH_STRICT_HOSTKEY_OFF },
+	{ "yes",			SSH_STRICT_HOSTKEY_YES },
+	{ "no",				SSH_STRICT_HOSTKEY_OFF },
+	{ "ask",			SSH_STRICT_HOSTKEY_ASK },
+	{ "off",			SSH_STRICT_HOSTKEY_OFF },
+	{ "accept-new",			SSH_STRICT_HOSTKEY_NEW },
+	{ NULL, -1 }
+};
 static const struct multistate multistate_yesnoaskconfirm[] = {
 	{ "true",			1 },
 	{ "false",			0 },
@@ -969,7 +979,7 @@ parse_time:
 
 	case oStrictHostKeyChecking:
 		intptr = &options->strict_host_key_checking;
-		multistate_ptr = multistate_yesnoask;
+		multistate_ptr = multistate_strict_hostkey;
 		goto parse_multistate;
 
 	case oCompression:
@@ -1912,7 +1922,7 @@ fill_default_options(Options * options)
 	if (options->check_host_ip == -1)
 		options->check_host_ip = 1;
 	if (options->strict_host_key_checking == -1)
-		options->strict_host_key_checking = 2;	/* 2 is default */
+		options->strict_host_key_checking = SSH_STRICT_HOSTKEY_ASK;
 	if (options->compression == -1)
 		options->compression = 0;
 	if (options->tcp_keep_alive == -1)
@@ -2312,9 +2322,10 @@ fmt_intarg(OpCodes code, int val)
 	case oAddressFamily:
 		return fmt_multistate_int(val, multistate_addressfamily);
 	case oVerifyHostKeyDNS:
-	case oStrictHostKeyChecking:
 	case oUpdateHostkeys:
 		return fmt_multistate_int(val, multistate_yesnoask);
+	case oStrictHostKeyChecking:
+		return fmt_multistate_int(val, multistate_strict_hostkey);
 	case oControlMaster:
 		return fmt_multistate_int(val, multistate_controlmaster);
 	case oTunnel:
diff --git a/usr.bin/ssh/readconf.h b/usr.bin/ssh/readconf.h
index 94dd427f57d..22fe5c1873c 100644
--- a/usr.bin/ssh/readconf.h
+++ b/usr.bin/ssh/readconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.h,v 1.122 2017/05/30 18:58:37 bluhm Exp $ */
+/* $OpenBSD: readconf.h,v 1.123 2017/09/03 23:33:13 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -190,6 +190,11 @@ typedef struct {
 #define SSH_UPDATE_HOSTKEYS_YES	1
 #define SSH_UPDATE_HOSTKEYS_ASK	2
 
+#define SSH_STRICT_HOSTKEY_OFF	0
+#define SSH_STRICT_HOSTKEY_NEW	1
+#define SSH_STRICT_HOSTKEY_YES	2
+#define SSH_STRICT_HOSTKEY_ASK	3
+
 void     initialize_options(Options *);
 void     fill_default_options(Options *);
 void	 fill_default_options_for_canonicalization(Options *);
diff --git a/usr.bin/ssh/ssh_config.5 b/usr.bin/ssh/ssh_config.5
index 15ca0b4f99b..3823da6f34b 100644
--- a/usr.bin/ssh/ssh_config.5
+++ b/usr.bin/ssh/ssh_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: ssh_config.5,v 1.253 2017/07/23 23:37:02 djm Exp $
-.Dd $Mdocdate: July 23 2017 $
+.\" $OpenBSD: ssh_config.5,v 1.254 2017/09/03 23:33:13 djm Exp $
+.Dd $Mdocdate: September 3 2017 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -1459,9 +1459,17 @@ frequently made.
 This option forces the user to manually
 add all new hosts.
 If this flag is set to
-.Cm no ,
-ssh will automatically add new host keys to the
-user known hosts files.
+.Dq accept-new
+then ssh will automatically add new new host keys to the user
+known hosts files, but will not permit connections to hosts with
+changed host keys.
+If this flag is set to
+.Dq no
+or
+.Dq off ,
+ssh will automatically add new host keys to the user known hosts files,
+and allow connections to hosts with changed hostkeys to proceed subject
+to some restrictions.
 If this flag is set to
 .Cm ask
 (the default),
diff --git a/usr.bin/ssh/sshconnect.c b/usr.bin/ssh/sshconnect.c
index 7dad4f418de..a3bcf58ac53 100644
--- a/usr.bin/ssh/sshconnect.c
+++ b/usr.bin/ssh/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.284 2017/09/01 05:53:56 djm Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.285 2017/09/03 23:33:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -863,7 +863,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 		if (readonly || want_cert)
 			goto fail;
 		/* The host is new. */
-		if (options.strict_host_key_checking == 1) {
+		if (options.strict_host_key_checking ==
+		    SSH_STRICT_HOSTKEY_YES) {
 			/*
 			 * User has requested strict host key checking.  We
 			 * will not add the host key automatically.  The only
@@ -872,7 +873,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 			error("No %s host key is known for %.200s and you "
 			    "have requested strict checking.", type, host);
 			goto fail;
-		} else if (options.strict_host_key_checking == 2) {
+		} else if (options.strict_host_key_checking ==
+		    SSH_STRICT_HOSTKEY_ASK) {
 			char msg1[1024], msg2[1024];
 
 			if (show_other_keys(host_hostkeys, host_key))
@@ -916,8 +918,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 			hostkey_trusted = 1; /* user explicitly confirmed */
 		}
 		/*
-		 * If not in strict mode, add the key automatically to the
-		 * local known_hosts file.
+		 * If in "new" or "off" strict mode, add the key automatically
+		 * to the local known_hosts file.
 		 */
 		if (options.check_host_ip && ip_status == HOST_NEW) {
 			snprintf(hostline, sizeof(hostline), "%s,%s", host, ip);
@@ -959,7 +961,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 		 * If strict host key checking is in use, the user will have
 		 * to edit the key manually and we can only abort.
 		 */
-		if (options.strict_host_key_checking) {
+		if (options.strict_host_key_checking !=
+		    SSH_STRICT_HOSTKEY_OFF) {
 			error("%s host key for %.200s was revoked and you have "
 			    "requested strict checking.", type, host);
 			goto fail;
@@ -1012,7 +1015,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 		 * If strict host key checking is in use, the user will have
 		 * to edit the key manually and we can only abort.
 		 */
-		if (options.strict_host_key_checking) {
+		if (options.strict_host_key_checking !=
+		    SSH_STRICT_HOSTKEY_OFF) {
 			error("%s host key for %.200s has changed and you have "
 			    "requested strict checking.", type, host);
 			goto fail;
@@ -1099,15 +1103,17 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
 			    "\nMatching host key in %s:%lu",
 			    host_found->file, host_found->line);
 		}
-		if (options.strict_host_key_checking == 1) {
-			logit("%s", msg);
-			error("Exiting, you have requested strict checking.");
-			goto fail;
-		} else if (options.strict_host_key_checking == 2) {
+		if (options.strict_host_key_checking ==
+		    SSH_STRICT_HOSTKEY_ASK) {
 			strlcat(msg, "\nAre you sure you want "
 			    "to continue connecting (yes/no)? ", sizeof(msg));
 			if (!confirm(msg))
 				goto fail;
+		} else if (options.strict_host_key_checking !=
+		    SSH_STRICT_HOSTKEY_OFF) {
+			logit("%s", msg);
+			error("Exiting, you have requested strict checking.");
+			goto fail;
 		} else {
 			logit("%s", msg);
 		}