openssh/openssh-6.6p1-kuserok.patch

diff -up openssh-7.0p1/auth-krb5.c.kuserok openssh-7.0p1/auth-krb5.c
--- openssh-7.0p1/auth-krb5.c.kuserok	2015-08-11 10:57:29.000000000 +0200
+++ openssh-7.0p1/auth-krb5.c	2015-08-12 11:26:21.874536127 +0200
@@ -55,6 +55,21 @@
 
 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)
 {
@@ -158,8 +173,9 @@ 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-7.0p1/gss-serv-krb5.c.kuserok openssh-7.0p1/gss-serv-krb5.c
--- openssh-7.0p1/gss-serv-krb5.c.kuserok	2015-08-12 11:26:21.868536137 +0200
+++ openssh-7.0p1/gss-serv-krb5.c	2015-08-12 11:26:21.875536126 +0200
@@ -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);
@@ -171,9 +270,8 @@ 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-7.0p1/servconf.c.kuserok openssh-7.0p1/servconf.c
--- openssh-7.0p1/servconf.c.kuserok	2015-08-12 11:26:21.865536141 +0200
+++ openssh-7.0p1/servconf.c	2015-08-12 11:27:14.126454598 +0200
@@ -172,6 +172,7 @@ initialize_server_options(ServerOptions
 	options->ip_qos_bulk = -1;
 	options->version_addendum = NULL;
 	options->fingerprint_hash = -1;
+	options->use_kuserok = -1;
 }
 
 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */
@@ -350,6 +351,8 @@ fill_default_server_options(ServerOption
 		options->fwd_opts.streamlocal_bind_unlink = 0;
 	if (options->fingerprint_hash == -1)
 		options->fingerprint_hash = SSH_FP_HASH_DEFAULT;
+	if (options->use_kuserok == -1)
+		options->use_kuserok = 1;
 
 	if (kex_assemble_names(KEX_SERVER_ENCRYPT, &options->ciphers) != 0 ||
 	    kex_assemble_names(KEX_SERVER_MAC, &options->macs) != 0 ||
@@ -404,7 +407,7 @@ typedef enum {
 	sKeyRegenerationTime, sPermitRootLogin, sLogFacility, sLogLevel,
 	sRhostsRSAAuthentication, sRSAAuthentication,
 	sKerberosAuthentication, sKerberosOrLocalPasswd, sKerberosTicketCleanup,
-	sKerberosGetAFSToken,
+	sKerberosGetAFSToken, sKerberosUseKuserok,
 	sKerberosTgtPassing, sChallengeResponseAuthentication,
 	sPasswordAuthentication, sKbdInteractiveAuthentication,
 	sListenAddress, sAddressFamily,
@@ -483,11 +486,13 @@ 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 },
@@ -1671,6 +1676,10 @@ 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')
@@ -2023,6 +2032,7 @@ copy_set_server_options(ServerOptions *d
 	M_CP_INTOPT(max_authtries);
 	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);
 
@@ -2304,6 +2314,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_fmtint(sAllowStreamLocalForwarding, o->allow_streamlocal_forwarding);
 	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-7.0p1/servconf.h.kuserok openssh-7.0p1/servconf.h
--- openssh-7.0p1/servconf.h.kuserok	2015-08-12 11:26:21.865536141 +0200
+++ openssh-7.0p1/servconf.h	2015-08-12 11:26:21.876536124 +0200
@@ -180,6 +180,7 @@ typedef struct {
 
 	int	num_permitted_opens;
 
+	int	use_kuserok;
 	char   *chroot_directory;
 	char   *revoked_keys_file;
 	char   *trusted_user_ca_keys;
diff -up openssh-7.0p1/sshd_config.5.kuserok openssh-7.0p1/sshd_config.5
--- openssh-7.0p1/sshd_config.5.kuserok	2015-08-12 11:26:21.867536138 +0200
+++ openssh-7.0p1/sshd_config.5	2015-08-12 11:26:21.877536123 +0200
@@ -872,6 +872,10 @@ Specifies whether to automatically destr
 file on logout.
 The default is
 .Dq yes .
+.It Cm KerberosUseKuserok
+Specifies whether to look at .k5login file for user's aliases.
+The default is
+.Dq yes .
 .It Cm KexAlgorithms
 Specifies the available KEX (Key Exchange) algorithms.
 Multiple algorithms must be comma-separated.
@@ -1116,6 +1120,7 @@ Available keywords are
 .Cm IPQoS ,
 .Cm KbdInteractiveAuthentication ,
 .Cm KerberosAuthentication ,
+.Cm KerberosUseKuserok ,
 .Cm MaxAuthTries ,
 .Cm MaxSessions ,
 .Cm PasswordAuthentication ,
diff -up openssh-7.0p1/sshd_config.kuserok openssh-7.0p1/sshd_config
--- openssh-7.0p1/sshd_config.kuserok	2015-08-12 11:26:21.867536138 +0200
+++ openssh-7.0p1/sshd_config	2015-08-12 11:26:21.876536124 +0200
@@ -87,6 +87,7 @@ ChallengeResponseAuthentication no
 #KerberosOrLocalPasswd yes
 #KerberosTicketCleanup yes
 #KerberosGetAFSToken no
+#KerberosUseKuserok yes
 
 # GSSAPI options
 GSSAPIAuthentication yes