diff -up openssh-7.4p1/auth-krb5.c.kuserok openssh-7.4p1/auth-krb5.c

--- openssh-7.4p1/auth-krb5.c.kuserok	2016-12-23 14:36:07.640465939 +0100

+++ openssh-7.4p1/auth-krb5.c	2016-12-23 14:36:07.644465936 +0100

@@ -56,6 +56,21 @@

 extern ServerOptions	 options;

+int

+ssh_krb5_kuserok(krb5_context krb5_ctx, krb5_principal krb5_user, const char *client,

+                 int k5login_exists)

+{

+	if (options.use_kuserok || !k5login_exists)

+		return krb5_kuserok(krb5_ctx, krb5_user, client);

+	else {

+		char kuser[65];

+

+		if (krb5_aname_to_localname(krb5_ctx, krb5_user, sizeof(kuser), kuser))

+			return 0;

+		return strcmp(kuser, client) == 0;

+	}

+}

+

 static int

 krb5_init(void *context)

 {

@@ -160,8 +175,9 @@ auth_krb5_password(Authctxt *authctxt, c

 	if (problem)

 		goto out;

-	if (!krb5_kuserok(authctxt->krb5_ctx, authctxt->krb5_user,

-	    authctxt->pw->pw_name)) {

+	/* Use !options.use_kuserok here to make ssh_krb5_kuserok() not

+	 * depend on the existance of .k5login */

+	if (!ssh_krb5_kuserok(authctxt->krb5_ctx, authctxt->krb5_user, authctxt->pw->pw_name, !options.use_kuserok)) {

 		problem = -1;

 		goto out;

 	}

diff -up openssh-7.4p1/gss-serv-krb5.c.kuserok openssh-7.4p1/gss-serv-krb5.c

--- openssh-7.4p1/gss-serv-krb5.c.kuserok	2016-12-23 14:36:07.640465939 +0100

+++ openssh-7.4p1/gss-serv-krb5.c	2016-12-23 14:36:07.644465936 +0100

@@ -67,6 +67,7 @@ static int ssh_gssapi_krb5_cmdok(krb5_pr

     int);

 static krb5_context krb_context = NULL;

+extern int ssh_krb5_kuserok(krb5_context, krb5_principal, const char *, int);

 /* Initialise the krb5 library, for the stuff that GSSAPI won't do */

@@ -92,6 +93,103 @@ ssh_gssapi_krb5_init(void)

  * Returns true if the user is OK to log in, otherwise returns 0

  */

+/* The purpose of the function is to find out if a Kerberos principal is

+ * allowed to log in as the given local user. This is a general problem with

+ * Kerberized services because by design the Kerberos principals are

+ * completely independent from the local user names. This is one of the

+ * reasons why Kerberos is working well on different operating systems like

+ * Windows and UNIX/Linux. Nevertheless a relationship between a Kerberos

+ * principal and a local user name must be established because otherwise every

+ * access would be granted for every principal with a valid ticket.

+ *

+ * Since it is a general issue libkrb5 provides some functions for

+ * applications to find out about the relationship between the Kerberos

+ * principal and a local user name. They are krb5_kuserok() and

+ * krb5_aname_to_localname().

+ *

+ * krb5_kuserok() can be used to "Determine if a principal is authorized to

+ * log in as a local user" (from the MIT Kerberos documentation of this

+ * function). Which is exactly what we are looking for and should be the

+ * preferred choice. It accepts the Kerberos principal and a local user name

+ * and let libkrb5 or its plugins determine if they relate to each other or

+ * not.

+ *

+ * krb5_aname_to_localname() can use used to "Convert a principal name to a

+ * local name" (from the MIT Kerberos documentation of this function). It

+ * accepts a Kerberos principle and returns a local name and it is up to the

+ * application to do any additional checks. There are two issues using

+ * krb5_aname_to_localname(). First, since POSIX user names are case

+ * sensitive, the calling application in general has no other choice than

+ * doing a case-sensitive string comparison between the name returned by

+ * krb5_aname_to_localname() and the name used at the login prompt. When the

+ * users are provided by a case in-sensitive server, e.g. Active Directory,

+ * this might lead to login failures because the user typing the name at the

+ * login prompt might not be aware of the right case. Another issue might be

+ * caused if there are multiple alias names available for a single user. E.g.

+ * the canonical name of a user is user@group.department.example.com but there

+ * exists a shorter login name, e.g. user@example.com, to safe typing at the

+ * login prompt. Here krb5_aname_to_localname() can only return the canonical

+ * name, but if the short alias is used at the login prompt authentication

+ * will fail as well. All this can be avoided by using krb5_kuserok() and

+ * configuring krb5.conf or using a suitable plugin to meet the needs of the

+ * given environment.

+ *

+ * The Fedora and RHEL version of openssh contain two patches which modify the

+ * access control behavior:

+ *  - openssh-6.6p1-kuserok.patch

+ *  - openssh-6.6p1-force_krb.patch

+ *

+ * openssh-6.6p1-kuserok.patch adds a new option KerberosUseKuserok for

+ * sshd_config which controls if krb5_kuserok() is used to check if the

+ * principle is authorized or if krb5_aname_to_localname() should be used.

+ * The reason to add this patch was that krb5_kuserok() by default checks if

+ * a .k5login file exits in the users home-directory. With this the user can

+ * give access to his account for any given principal which might be

+ * in violation with company policies and it would be useful if this can be

+ * rejected. Nevertheless the patch ignores the fact that krb5_kuserok() does

+ * no only check .k5login but other sources as well and checking .k5login can

+ * be disabled for all applications in krb5.conf as well. With this new

+ * option KerberosUseKuserok set to 'no' (and this is the default for RHEL7

+ * and Fedora 21) openssh can only use krb5_aname_to_localname() with the

+ * restrictions mentioned above.

+ *

+ * openssh-6.6p1-force_krb.patch adds a ksu like behaviour to ssh, i.e. when

+ * using GSSAPI authentication only commands configured in the .k5user can be

+ * executed. Here the wrong assumption that krb5_kuserok() only checks

+ * .k5login is made as well. In contrast ksu checks .k5login directly and

+ * does not use krb5_kuserok() which might be more useful for the given

+ * purpose. Additionally this patch is not synced with

+ * openssh-6.6p1-kuserok.patch.

+ *

+ * The current patch tries to restore the usage of krb5_kuserok() so that e.g.

+ * localauth plugins can be used. It does so by adding a forth parameter to

+ * ssh_krb5_kuserok() which indicates whether .k5login exists or not. If it

+ * does not exists krb5_kuserok() is called even if KerberosUseKuserok is set

+ * to 'no' because the intent of the option is to not check .k5login and if it

+ * does not exists krb5_kuserok() returns a result without checking .k5login.

+ * If .k5login does exists and KerberosUseKuserok is 'no' we fall back to

+ * krb5_aname_to_localname(). This is in my point of view an acceptable

+ * limitation and does not break the current behaviour.

+ *

+ * Additionally with this patch ssh_krb5_kuserok() is called in

+ * ssh_gssapi_krb5_cmdok() instead of only krb5_aname_to_localname() is

+ * neither .k5login nor .k5users exists to allow plugin evaluation via

+ * krb5_kuserok() as well.

+ *

+ * I tried to keep the patch as minimal as possible, nevertheless I see some

+ * areas for improvement which, if they make sense, have to be evaluated

+ * carefully because they might change existing behaviour and cause breaks

+ * during upgrade:

+ * - I wonder if disabling .k5login usage make sense in sshd or if it should

+ *   be better disabled globally in krb5.conf

+ * - if really needed openssh-6.6p1-kuserok.patch should be fixed to really

+ *   only disable checking .k5login and maybe .k5users

+ * - the ksu behaviour should be configurable and maybe check the .k5login and

+ *   .k5users files directly like ksu itself does

+ * - to make krb5_aname_to_localname() more useful an option for sshd to use

+ *   the canonical name (the one returned by getpwnam()) instead of the name

+ *   given at the login prompt might be useful */

+

 static int

 ssh_gssapi_krb5_userok(ssh_gssapi_client *client, char *name)

 {

@@ -116,7 +214,8 @@ ssh_gssapi_krb5_userok(ssh_gssapi_client

 	/* NOTE: .k5login and .k5users must opened as root, not the user,

 	 * because if they are on a krb5-protected filesystem, user credentials

 	 * to access these files aren't available yet. */

-	if (krb5_kuserok(krb_context, princ, name) && k5login_exists) {

+	if (ssh_krb5_kuserok(krb_context, princ, name, k5login_exists)

+			&& k5login_exists) {

 		retval = 1;

 		logit("Authorized to %s, krb5 principal %s (krb5_kuserok)",

 		    name, (char *)client->displayname.value);

@@ -190,9 +289,8 @@ ssh_gssapi_krb5_cmdok(krb5_principal pri

 	snprintf(file, sizeof(file), "%s/.k5users", pw->pw_dir);

 	/* If both .k5login and .k5users DNE, self-login is ok. */

 	if (!k5login_exists && (access(file, F_OK) == -1)) {

-		return (krb5_aname_to_localname(krb_context, principal,

-		    sizeof(kuser), kuser) == 0) &&

-		    (strcmp(kuser, luser) == 0);

+                return ssh_krb5_kuserok(krb_context, principal, luser,

+                                        k5login_exists);

 	}

 	if ((fp = fopen(file, "r")) == NULL) {

 		int saved_errno = errno;

diff -up openssh-7.4p1/servconf.c.kuserok openssh-7.4p1/servconf.c

--- openssh-7.4p1/servconf.c.kuserok	2016-12-23 14:36:07.630465944 +0100

+++ openssh-7.4p1/servconf.c	2016-12-23 15:11:52.278133344 +0100

@@ -167,6 +167,7 @@ initialize_server_options(ServerOptions

 	options->version_addendum = NULL;

 	options->fingerprint_hash = -1;

 	options->disable_forwarding = -1;

+	options->use_kuserok = -1;

 }

 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */

@@ -342,6 +343,8 @@ fill_default_server_options(ServerOption

 		options->fingerprint_hash = SSH_FP_HASH_DEFAULT;

 	if (options->disable_forwarding == -1)

 		options->disable_forwarding = 0;

+	if (options->use_kuserok == -1)

+		options->use_kuserok = 1;

 	assemble_algorithms(options);

@@ -399,7 +402,7 @@ typedef enum {

 	sPermitRootLogin, sLogFacility, sLogLevel,

 	sRhostsRSAAuthentication, sRSAAuthentication,

 	sKerberosAuthentication, sKerberosOrLocalPasswd, sKerberosTicketCleanup,

-	sKerberosGetAFSToken,

+	sKerberosGetAFSToken, sKerberosUseKuserok,

 	sKerberosTgtPassing, sChallengeResponseAuthentication,

 	sPasswordAuthentication, sKbdInteractiveAuthentication,

 	sListenAddress, sAddressFamily,

@@ -478,11 +481,13 @@ static struct {

 #else

 	{ "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL },

 #endif

+	{ "kerberosusekuserok", sKerberosUseKuserok, SSHCFG_ALL },

 #else

 	{ "kerberosauthentication", sUnsupported, SSHCFG_ALL },

 	{ "kerberosorlocalpasswd", sUnsupported, SSHCFG_GLOBAL },

 	{ "kerberosticketcleanup", sUnsupported, SSHCFG_GLOBAL },

 	{ "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL },

+	{ "kerberosusekuserok", sUnsupported, SSHCFG_ALL },

 #endif

 	{ "kerberostgtpassing", sUnsupported, SSHCFG_GLOBAL },

 	{ "afstokenpassing", sUnsupported, SSHCFG_GLOBAL },

@@ -1644,6 +1649,10 @@ process_server_config_line(ServerOptions

 		*activep = value;

 		break;

+	case sKerberosUseKuserok:

+		intptr = &options->use_kuserok;

+		goto parse_flag;

+

 	case sPermitOpen:

 		arg = strdelim(&cp;;

 		if (!arg || *arg == '\0')

@@ -2016,6 +2025,7 @@ copy_set_server_options(ServerOptions *d

 	M_CP_INTOPT(client_alive_interval);

 	M_CP_INTOPT(ip_qos_interactive);

 	M_CP_INTOPT(ip_qos_bulk);

+	M_CP_INTOPT(use_kuserok);

 	M_CP_INTOPT(rekey_limit);

 	M_CP_INTOPT(rekey_interval);

@@ -2309,6 +2319,7 @@ dump_config(ServerOptions *o)

 	dump_cfg_fmtint(sStreamLocalBindUnlink, o->fwd_opts.streamlocal_bind_unlink);

 	dump_cfg_fmtint(sUsePrivilegeSeparation, use_privsep);

 	dump_cfg_fmtint(sFingerprintHash, o->fingerprint_hash);

+	dump_cfg_fmtint(sKerberosUseKuserok, o->use_kuserok);

 	/* string arguments */

 	dump_cfg_string(sPidFile, o->pid_file);

diff -up openssh-7.4p1/servconf.h.kuserok openssh-7.4p1/servconf.h

--- openssh-7.4p1/servconf.h.kuserok	2016-12-23 14:36:07.630465944 +0100

+++ openssh-7.4p1/servconf.h	2016-12-23 14:36:07.645465936 +0100

@@ -174,6 +174,7 @@ typedef struct {

 	int	num_permitted_opens;

+	int	use_kuserok;

 	char   *chroot_directory;

 	char   *revoked_keys_file;

 	char   *trusted_user_ca_keys;

diff -up openssh-7.4p1/sshd_config.5.kuserok openssh-7.4p1/sshd_config.5

--- openssh-7.4p1/sshd_config.5.kuserok	2016-12-23 14:36:07.637465940 +0100

+++ openssh-7.4p1/sshd_config.5	2016-12-23 15:14:03.117162222 +0100

@@ -850,6 +850,10 @@ Specifies whether to automatically destr

 file on logout.

 The default is

 .Cm yes .

+.It Cm KerberosUseKuserok

+Specifies whether to look at .k5login file for user's aliases.

+The default is

+.Cm yes .

 .It Cm KexAlgorithms

 Specifies the available KEX (Key Exchange) algorithms.

 Multiple algorithms must be comma-separated.

@@ -1078,6 +1082,7 @@ Available keywords are

 .Cm IPQoS ,

 .Cm KbdInteractiveAuthentication ,

 .Cm KerberosAuthentication ,

+.Cm KerberosUseKuserok ,

 .Cm MaxAuthTries ,

 .Cm MaxSessions ,

 .Cm PasswordAuthentication ,

diff -up openssh-7.4p1/sshd_config.kuserok openssh-7.4p1/sshd_config

--- openssh-7.4p1/sshd_config.kuserok	2016-12-23 14:36:07.631465943 +0100

+++ openssh-7.4p1/sshd_config	2016-12-23 14:36:07.646465935 +0100

@@ -73,6 +73,7 @@ ChallengeResponseAuthentication no

 #KerberosOrLocalPasswd yes

 #KerberosTicketCleanup yes

 #KerberosGetAFSToken no

+#KerberosUseKuserok yes

 # GSSAPI options

 GSSAPIAuthentication yes