add knob to relax GSSAPI host credential check for multihomed hosts

bz#928, patch by Simon Wilkinson; ok dtucker
(kerberos/GSSAPI is not compiled by default on OpenBSD)

4 files changed, 56 insertions, 20 deletions

diff --git a/usr.bin/ssh/gss-serv.c b/usr.bin/ssh/gss-serv.c
index ffe41c8d6d0..f26db425964 100644
--- a/usr.bin/ssh/gss-serv.c
+++ b/usr.bin/ssh/gss-serv.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: gss-serv.c,v 1.28 2015/01/20 23:14:00 deraadt Exp $ */
+/* $OpenBSD: gss-serv.c,v 1.29 2015/05/22 03:50:02 djm Exp $ */
 
 /*
  * Copyright (c) 2001-2003 Simon Wilkinson. All rights reserved.
@@ -40,9 +40,12 @@
 #include "channels.h"
 #include "session.h"
 #include "misc.h"
+#include "servconf.h"
 
 #include "ssh-gss.h"
 
+extern ServerOptions options;
+
 static ssh_gssapi_client gssapi_client =
     { GSS_C_EMPTY_BUFFER, GSS_C_EMPTY_BUFFER,
     GSS_C_NO_CREDENTIAL, NULL, {NULL, NULL, NULL, NULL}};
@@ -95,25 +98,32 @@ ssh_gssapi_acquire_cred(Gssctxt *ctx)
 	char lname[NI_MAXHOST];
 	gss_OID_set oidset;
 
-	gss_create_empty_oid_set(&status, &oidset);
-	gss_add_oid_set_member(&status, ctx->oid, &oidset);
+	if (options.gss_strict_acceptor) {
+		gss_create_empty_oid_set(&status, &oidset);
+		gss_add_oid_set_member(&status, ctx->oid, &oidset);
 
-	if (gethostname(lname, sizeof(lname))) {
-		gss_release_oid_set(&status, &oidset);
-		return (-1);
-	}
+		if (gethostname(lname, MAXHOSTNAMELEN)) {
+			gss_release_oid_set(&status, &oidset);
+			return (-1);
+		}
+
+		if (GSS_ERROR(ssh_gssapi_import_name(ctx, lname))) {
+			gss_release_oid_set(&status, &oidset);
+			return (ctx->major);
+		}
+
+		if ((ctx->major = gss_acquire_cred(&ctx->minor,
+		    ctx->name, 0, oidset, GSS_C_ACCEPT, &ctx->creds,
+		    NULL, NULL)))
+			ssh_gssapi_error(ctx);
 
-	if (GSS_ERROR(ssh_gssapi_import_name(ctx, lname))) {
 		gss_release_oid_set(&status, &oidset);
 		return (ctx->major);
+	} else {
+		ctx->name = GSS_C_NO_NAME;
+		ctx->creds = GSS_C_NO_CREDENTIAL;
 	}
-
-	if ((ctx->major = gss_acquire_cred(&ctx->minor,
-	    ctx->name, 0, oidset, GSS_C_ACCEPT, &ctx->creds, NULL, NULL)))
-		ssh_gssapi_error(ctx);
-
-	gss_release_oid_set(&status, &oidset);
-	return (ctx->major);
+	return GSS_S_COMPLETE;
 }
 
 /* Privileged */
diff --git a/usr.bin/ssh/servconf.c b/usr.bin/ssh/servconf.c
index d64ba6b7533..ea254b06e60 100644
--- a/usr.bin/ssh/servconf.c
+++ b/usr.bin/ssh/servconf.c
@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.270 2015/05/21 06:43:30 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.271 2015/05/22 03:50:02 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -107,6 +107,7 @@ initialize_server_options(ServerOptions *options)
 	options->kerberos_get_afs_token = -1;
 	options->gss_authentication=-1;
 	options->gss_cleanup_creds = -1;
+	options->gss_strict_acceptor = -1;
 	options->password_authentication = -1;
 	options->kbd_interactive_authentication = -1;
 	options->challenge_response_authentication = -1;
@@ -260,6 +261,8 @@ fill_default_server_options(ServerOptions *options)
 		options->gss_authentication = 0;
 	if (options->gss_cleanup_creds == -1)
 		options->gss_cleanup_creds = 1;
+	if (options->gss_strict_acceptor == -1)
+		options->gss_strict_acceptor = 0;
 	if (options->password_authentication == -1)
 		options->password_authentication = 1;
 	if (options->kbd_interactive_authentication == -1)
@@ -368,7 +371,8 @@ typedef enum {
 	sBanner, sUseDNS, sHostbasedAuthentication,
 	sHostbasedUsesNameFromPacketOnly, sHostbasedAcceptedKeyTypes,
 	sClientAliveInterval, sClientAliveCountMax, sAuthorizedKeysFile,
-	sGssAuthentication, sGssCleanupCreds, sAcceptEnv, sPermitTunnel,
+	sGssAuthentication, sGssCleanupCreds, sGssStrictAcceptor,
+	sAcceptEnv, sPermitTunnel,
 	sMatch, sPermitOpen, sForceCommand, sChrootDirectory,
 	sUsePrivilegeSeparation, sAllowAgentForwarding,
 	sHostCertificate,
@@ -428,9 +432,11 @@ static struct {
 #ifdef GSSAPI
 	{ "gssapiauthentication", sGssAuthentication, SSHCFG_ALL },
 	{ "gssapicleanupcredentials", sGssCleanupCreds, SSHCFG_GLOBAL },
+	{ "gssapistrictacceptorcheck", sGssStrictAcceptor, SSHCFG_GLOBAL },
 #else
 	{ "gssapiauthentication", sUnsupported, SSHCFG_ALL },
 	{ "gssapicleanupcredentials", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapistrictacceptorcheck", sUnsupported, SSHCFG_GLOBAL },
 #endif
 	{ "passwordauthentication", sPasswordAuthentication, SSHCFG_ALL },
 	{ "kbdinteractiveauthentication", sKbdInteractiveAuthentication, SSHCFG_ALL },
@@ -1165,6 +1171,10 @@ process_server_config_line(ServerOptions *options, char *line,
 		intptr = &options->gss_cleanup_creds;
 		goto parse_flag;
 
+	case sGssStrictAcceptor:
+		intptr = &options->gss_strict_acceptor;
+		goto parse_flag;
+
 	case sPasswordAuthentication:
 		intptr = &options->password_authentication;
 		goto parse_flag;
diff --git a/usr.bin/ssh/servconf.h b/usr.bin/ssh/servconf.h
index 4c4867b010e..0b72d492ae2 100644
--- a/usr.bin/ssh/servconf.h
+++ b/usr.bin/ssh/servconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.118 2015/05/21 06:43:31 djm Exp $ */
+/* $OpenBSD: servconf.h,v 1.119 2015/05/22 03:50:02 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -118,6 +118,7 @@ typedef struct {
 						 * authenticated with Kerberos. */
 	int     gss_authentication;	/* If true, permit GSSAPI authentication */
 	int     gss_cleanup_creds;	/* If true, destroy cred cache on logout */
+	int     gss_strict_acceptor;	/* If true, restrict the GSSAPI acceptor name */
 	int     password_authentication;	/* If true, permit password
 						 * authentication. */
 	int     kbd_interactive_authentication;	/* If true, permit */
diff --git a/usr.bin/ssh/sshd_config.5 b/usr.bin/ssh/sshd_config.5
index 507a8bf3354..72403e53e8d 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 1.202 2015/05/21 06:43:31 djm Exp $
-.Dd $Mdocdate: May 21 2015 $
+.\" $OpenBSD: sshd_config.5,v 1.203 2015/05/22 03:50:02 djm Exp $
+.Dd $Mdocdate: May 22 2015 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -623,6 +623,21 @@ on logout.
 The default is
 .Dq yes .
 Note that this option applies to protocol version 2 only.
+.It Cm GSSAPIStrictAcceptorCheck
+Determines whether to be strict about the identity of the GSSAPI acceptor
+a client authenticates against.
+If set to
+.Dq yes
+then the client must authenticate against the
+.Pa host
+service on the current hostname.
+If set to
+.Dq no
+then the client may authenticate against any service key stored in the
+machine's default store.
+This facility is provided to assist with operation on multi homed machines.
+The default is
+.Dq yes .
 .It Cm HostbasedAcceptedKeyTypes
 Specifies the key types that will be accepted for hostbased authentication
 as a comma-separated pattern list.