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 @@ -116,6 +116,7 @@ initialize_server_options(ServerOptions options->gss_strict_acceptor = -1; options->gss_store_rekey = -1; options->gss_kex_algorithms = NULL; + options->use_kuserok = -1; options->password_authentication = -1; options->kbd_interactive_authentication = -1; options->challenge_response_authentication = -1; @@ -278,6 +279,8 @@ fill_default_server_options(ServerOption if (options->gss_kex_algorithms == NULL) options->gss_kex_algorithms = strdup(GSS_KEX_DEFAULT_KEX); #endif + if (options->use_kuserok == -1) + options->use_kuserok = 1; if (options->password_authentication == -1) options->password_authentication = 1; if (options->kbd_interactive_authentication == -1) @@ -399,7 +402,7 @@ typedef enum { sPermitRootLogin, sLogFacility, sLogLevel, sRhostsRSAAuthentication, sRSAAuthentication, sKerberosAuthentication, sKerberosOrLocalPasswd, sKerberosTicketCleanup, - sKerberosGetAFSToken, sKerberosUniqueCCache, + sKerberosGetAFSToken, sKerberosUniqueCCache, sKerberosUseKuserok, sChallengeResponseAuthentication, sPasswordAuthentication, sKbdInteractiveAuthentication, sListenAddress, sAddressFamily, @@ -478,12 +481,14 @@ static struct { { "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL }, #endif { "kerberosuniqueccache", sKerberosUniqueCCache, SSHCFG_GLOBAL }, + { "kerberosusekuserok", sKerberosUseKuserok, SSHCFG_ALL }, #else { "kerberosauthentication", sUnsupported, SSHCFG_ALL }, { "kerberosorlocalpasswd", sUnsupported, SSHCFG_GLOBAL }, { "kerberosticketcleanup", sUnsupported, SSHCFG_GLOBAL }, { "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL }, { "kerberosuniqueccache", 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 = (inc_flags & SSHCFG_NEVERMATCH) ? 0 : value; break; + case sKerberosUseKuserok: + intptr = &options->use_kuserok; + goto parse_flag; + case sPermitListen: case sPermitOpen: if (opcode == sPermitListen) { @@ -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); M_CP_INTOPT(log_level); @@ -2309,6 +2319,7 @@ dump_config(ServerOptions *o) dump_cfg_fmtint(sKerberosGetAFSToken, o->kerberos_get_afs_token); # endif dump_cfg_fmtint(sKerberosUniqueCCache, o->kerberos_unique_ccache); + dump_cfg_fmtint(sKerberosUseKuserok, o->use_kuserok); #endif #ifdef GSSAPI dump_cfg_fmtint(sGssAuthentication, o->gss_authentication); 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 @@ -118,6 +118,7 @@ typedef struct { * authenticated with Kerberos. */ int kerberos_unique_ccache; /* If true, the acquired ticket will * be stored in per-session ccache */ + int use_kuserok; int gss_authentication; /* If true, permit GSSAPI authentication */ int gss_keyex; /* If true, permit GSSAPI key exchange */ int gss_cleanup_creds; /* If true, destroy cred cache on logout */ 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 .Cm no can lead to overwriting previous tickets by subseqent connections to the same user account. +.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 LogLevel , .Cm MaxAuthTries , .Cm MaxSessions , 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 no