daemon: introduce pam extension mechanism

This abuses PAM_BINARY_PROMPT for our own nefarious purposes.
The way it works is GDM advertises what "extensions" it supports
with the environment variable, GDM_SUPPORTED_PAM_EXTENSIONS (a space
separated list of reverse dns notation names). PAM services that
support this protocol, will read the environment variable, and
check for extension strings they support. They then know that sending
PAM_BINARY_PROMPT won't blow up, and know what format to use for the
binary data.  The type field of the structure is the index of the
string from the environment variable.

This commit is just foundation work. It doesn't actually add any
extensions.

https://bugzilla.gnome.org/show_bug.cgi?id=788851

10 files changed, 325 insertions, 14 deletions

diff --git a/Makefile.am b/Makefile.am
index a6409e1b..25d679f6 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,6 +3,7 @@ NULL =
 SUBDIRS = 			\
 	data			\
 	common 			\
+	pam-extensions		\
 	daemon 			\
 	libgdm 			\
 	utils 			\
diff --git a/configure.ac b/configure.ac
index c9f69cf4..3c91520b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -539,6 +539,15 @@ if test "x$have_pam" = "xyes"; then
                         )],
        [AC_MSG_RESULT(["not found - assume const, Linux-type PAM"])]
        )
+   AC_CHECK_DECL(PAM_BINARY_PROMPT,
+                 [supports_pam_extensions=yes],
+                 [supports_pam_extensions=no],
+                 #include <security/pam_appl.h>
+                )
+fi
+if test "x$supports_pam_extensions" = "xyes" ; then
+	AM_CONDITIONAL(SUPPORTS_PAM_EXTENSIONS, true)
+	AC_DEFINE(SUPPORTS_PAM_EXTENSIONS, 1, [Define if PAM supports GDMs custom extensions])
 fi
 
 AC_CHECK_LIB(keyutils, keyctl_read, [
@@ -1548,6 +1557,8 @@ AC_DEFINE_UNQUOTED(X_XNEST_UNSCALED_FONTPATH,"$X_XNEST_UNSCALED_FONTPATH",[])
 
 AC_CONFIG_FILES([
 Makefile
+pam-extensions/Makefile
+pam-extensions/gdm-pam-extensions.pc
 daemon/Makefile
 docs/Makefile
 chooser/Makefile
diff --git a/daemon/Makefile.am b/daemon/Makefile.am
index bf55cb67..5e9eb5e0 100644
--- a/daemon/Makefile.am
+++ b/daemon/Makefile.am
@@ -4,6 +4,7 @@ AM_CPPFLAGS = \
 	-I.						\
 	-I..						\
 	-I$(top_srcdir)/common				\
+	-I$(top_srcdir)/pam-extensions			\
 	-I$(top_builddir)/common			\
 	-DBINDIR=\"$(bindir)\"				\
 	-DDATADIR=\"$(datadir)\"			\
@@ -133,6 +134,10 @@ gdm_session_worker_SOURCES = 			\
 	gdm-dbus-util.h				\
 	$(NULL)
 
+if SUPPORTS_PAM_EXTENSIONS
+gdm_session_worker_SOURCES += $(top_srcdir)/pam-extensions/gdm-pam-extensions.h
+endif
+
 nodist_gdm_session_worker_SOURCES =		\
 	gdm-session-glue.h			\
 	gdm-session-glue.c			\
diff --git a/daemon/gdm-session-worker.c b/daemon/gdm-session-worker.c
index 66d9e6d1..aafc8880 100644
--- a/daemon/gdm-session-worker.c
+++ b/daemon/gdm-session-worker.c
@@ -61,6 +61,11 @@
 
 #include "gdm-common.h"
 #include "gdm-log.h"
+
+#ifdef SUPPORTS_PAM_EXTENSIONS
+#include "gdm-pam-extensions.h"
+#endif
+
 #include "gdm-session-worker.h"
 #include "gdm-session-glue.h"
 #include "gdm-session.h"
@@ -149,6 +154,7 @@ struct GdmSessionWorkerPrivate
         uid_t             uid;
         gid_t             gid;
         gboolean          password_is_required;
+        char            **extensions;
 
         int               cred_flags;
         int               login_vt;
@@ -177,6 +183,15 @@ struct GdmSessionWorkerPrivate
         GDBusMethodInvocation *pending_invocation;
 };
 
+#ifdef SUPPORTS_PAM_EXTENSIONS
+static char gdm_pam_extension_environment_block[_POSIX_ARG_MAX];
+
+static const char * const
+gdm_supported_pam_extensions[] = {
+        NULL
+};
+#endif
+
 enum {
         PROP_0,
         PROP_SERVER_ADDRESS,
@@ -519,6 +534,32 @@ gdm_session_worker_report_problem (GdmSessionWorker *worker,
                                                           NULL);
 }
 
+#ifdef SUPPORTS_PAM_EXTENSIONS
+static gboolean
+gdm_session_worker_process_extended_pam_message (GdmSessionWorker          *worker,
+                                                 const struct pam_message  *query,
+                                                 char                     **response)
+{
+        GdmPamExtensionMessage *extended_message;
+        gboolean res;
+
+        extended_message = GDM_PAM_EXTENSION_MESSAGE_FROM_PAM_MESSAGE (query);
+
+        if (GDM_PAM_EXTENSION_MESSAGE_TRUNCATED (extended_message)) {
+                g_warning ("PAM service requested binary response for truncated query");
+                return FALSE;
+        }
+
+        if (GDM_PAM_EXTENSION_MESSAGE_INVALID_TYPE (extended_message)) {
+                g_warning ("PAM service requested binary response for unadvertised query type");
+                return FALSE;
+        }
+
+        g_debug ("GdmSessionWorker: received extended pam message of unknown type %u", (unsigned int) extended_message->type);
+        return FALSE;
+}
+#endif
+
 static char *
 convert_to_utf8 (const char *str)
 {
@@ -563,6 +604,11 @@ gdm_session_worker_process_pam_message (GdmSessionWorker          *worker,
 
         gdm_session_worker_update_username (worker);
 
+#ifdef SUPPORTS_PAM_EXTENSIONS
+        if (query->msg_style == PAM_BINARY_PROMPT)
+                return gdm_session_worker_process_extended_pam_message (worker, query, response);
+#endif
+
         g_debug ("GdmSessionWorker: received pam message of type %u with payload '%s'",
                  query->msg_style, query->msg);
 
@@ -991,16 +1037,17 @@ out:
 }
 
 static gboolean
-gdm_session_worker_initialize_pam (GdmSessionWorker *worker,
-                                   const char       *service,
-                                   const char       *username,
-                                   const char       *hostname,
-                                   gboolean          display_is_local,
-                                   const char       *x11_display_name,
-                                   const char       *x11_authority_file,
-                                   const char       *display_device,
-                                   const char       *seat_id,
-                                   GError          **error)
+gdm_session_worker_initialize_pam (GdmSessionWorker   *worker,
+                                   const char         *service,
+                                   const char * const *extensions,
+                                   const char         *username,
+                                   const char         *hostname,
+                                   gboolean            display_is_local,
+                                   const char         *x11_display_name,
+                                   const char         *x11_authority_file,
+                                   const char         *display_device,
+                                   const char         *seat_id,
+                                   GError            **error)
 {
         struct pam_conv        pam_conversation;
         int                    error_code;
@@ -1013,6 +1060,12 @@ gdm_session_worker_initialize_pam (GdmSessionWorker *worker,
                  username ? username : "(null)",
                  seat_id ? seat_id : "(null)");
 
+#ifdef SUPPORTS_PAM_EXTENSIONS
+        if (extensions != NULL) {
+                GDM_PAM_EXTENSION_ADVERTISE_SUPPORTED_EXTENSIONS (gdm_pam_extension_environment_block, extensions);
+        }
+#endif
+
         pam_conversation.conv = (GdmSessionWorkerPamNewMessagesFunc) gdm_session_worker_pam_new_messages_handler;
         pam_conversation.appdata_ptr = worker;
 
@@ -2451,6 +2504,7 @@ do_setup (GdmSessionWorker *worker)
         error = NULL;
         res = gdm_session_worker_initialize_pam (worker,
                                                  worker->priv->service,
+                                                 (const char **) worker->priv->extensions,
                                                  worker->priv->username,
                                                  worker->priv->hostname,
                                                  worker->priv->display_is_local,
@@ -2817,6 +2871,32 @@ gdm_session_worker_handle_open (GdmDBusWorker         *object,
         return TRUE;
 }
 
+static char **
+filter_extensions (const char * const *extensions)
+{
+        size_t i, j;
+        GPtrArray *array = NULL;
+        char **filtered_extensions = NULL;
+
+        array = g_ptr_array_new ();
+
+        for (i = 0; extensions[i] != NULL; i++) {
+                for (j = 0; gdm_supported_pam_extensions[j] != NULL; j++) {
+                        if (g_strcmp0 (extensions[i], gdm_supported_pam_extensions[j]) == 0) {
+                                g_ptr_array_add (array, g_strdup (gdm_supported_pam_extensions[j]));
+                                break;
+                        }
+                }
+        }
+        g_ptr_array_add (array, NULL);
+
+        filtered_extensions = g_strdupv ((char **) array->pdata);
+
+        g_ptr_array_free (array, TRUE);
+
+        return filtered_extensions;
+}
+
 static gboolean
 gdm_session_worker_handle_initialize (GdmDBusWorker         *object,
                                       GDBusMethodInvocation *invocation,
@@ -2833,6 +2913,8 @@ gdm_session_worker_handle_initialize (GdmDBusWorker         *object,
         while (g_variant_iter_loop (&iter, "{sv}", &key, &value)) {
                 if (g_strcmp0 (key, "service") == 0) {
                         worker->priv->service = g_strdup (g_variant_get_string (value, NULL));
+                } else if (g_strcmp0 (key, "extensions") == 0) {
+                        worker->priv->extensions = filter_extensions (g_variant_get_strv (value, NULL));
                 } else if (g_strcmp0 (key, "username") == 0) {
                         worker->priv->username = g_strdup (g_variant_get_string (value, NULL));
                 } else if (g_strcmp0 (key, "is-program-session") == 0) {
@@ -3365,6 +3447,7 @@ gdm_session_worker_finalize (GObject *object)
         g_free (worker->priv->username);
         g_free (worker->priv->server_address);
         g_strfreev (worker->priv->arguments);
+        g_strfreev (worker->priv->extensions);
 
         g_hash_table_unref (worker->priv->reauthentication_requests);
 
diff --git a/daemon/gdm-session-worker.h b/daemon/gdm-session-worker.h
index cba3a4ec..5603e80e 100644
--- a/daemon/gdm-session-worker.h
+++ b/daemon/gdm-session-worker.h
@@ -52,7 +52,5 @@ GType              gdm_session_worker_get_type                 (void);
 
 GdmSessionWorker * gdm_session_worker_new                      (const char *server_address,
                                                                 gboolean    is_for_reauth) G_GNUC_MALLOC;
-
 G_END_DECLS
-
 #endif /* GDM_SESSION_WORKER_H */
diff --git a/daemon/gdm-session.c b/daemon/gdm-session.c
index 812c7be0..aa437e56 100644
--- a/daemon/gdm-session.c
+++ b/daemon/gdm-session.c
@@ -101,6 +101,7 @@ struct _GdmSessionPrivate
         char                 **conversation_environment;
 
         GdmDBusUserVerifier   *user_verifier_interface;
+        GHashTable            *user_verifier_extensions;
         GdmDBusGreeter        *greeter_interface;
         GdmDBusRemoteGreeter  *remote_greeter_interface;
         GdmDBusChooser        *chooser_interface;
@@ -1219,6 +1220,20 @@ begin_verification_conversation (GdmSession            *self,
 }
 
 static gboolean
+gdm_session_handle_client_enable_extensions (GdmDBusUserVerifier    *user_verifier_interface,
+                                             GDBusMethodInvocation  *invocation,
+                                             const char * const *    extensions,
+                                             GDBusConnection        *connection)
+{
+        GdmSession *self = g_object_get_data (G_OBJECT (connection), "gdm-session");
+
+        g_hash_table_remove_all (self->priv->user_verifier_extensions);
+
+        gdm_dbus_user_verifier_complete_enable_extensions (user_verifier_interface, invocation);
+
+        return TRUE;
+}
+static gboolean
 gdm_session_handle_client_begin_verification (GdmDBusUserVerifier    *user_verifier_interface,
                                               GDBusMethodInvocation  *invocation,
                                               const char             *service_name,
@@ -1372,6 +1387,13 @@ export_user_verifier_interface (GdmSession      *self,
 {
         GdmDBusUserVerifier   *user_verifier_interface;
         user_verifier_interface = GDM_DBUS_USER_VERIFIER (gdm_dbus_user_verifier_skeleton_new ());
+
+        g_object_set_data (G_OBJECT (connection), "gdm-session", self);
+
+        g_signal_connect (user_verifier_interface,
+                          "handle-enable-extensions",
+                          G_CALLBACK (gdm_session_handle_client_enable_extensions),
+                          connection);
         g_signal_connect (user_verifier_interface,
                           "handle-begin-verification",
                           G_CALLBACK (gdm_session_handle_client_begin_verification),
@@ -1811,6 +1833,15 @@ next_line:
 }
 
 static void
+unexport_and_free_user_verifier_extension (GDBusInterfaceSkeleton *interface)
+{
+        g_dbus_interface_skeleton_unexport (interface);
+
+        g_object_run_dispose (G_OBJECT (interface));
+        g_object_unref (interface);
+}
+
+static void
 gdm_session_init (GdmSession *self)
 {
         self->priv = G_TYPE_INSTANCE_GET_PRIVATE (self,
@@ -1826,6 +1857,11 @@ gdm_session_init (GdmSession *self)
                                                          g_str_equal,
                                                          (GDestroyNotify) g_free,
                                                          (GDestroyNotify) g_free);
+        self->priv->user_verifier_extensions = g_hash_table_new_full (g_str_hash,
+                                                                      g_str_equal,
+                                                                      NULL,
+                                                                      (GDestroyNotify)
+                                                                      unexport_and_free_user_verifier_extension);
 
         load_lang_config_file (self);
         setup_worker_server (self);
@@ -2099,14 +2135,19 @@ initialize (GdmSession *self,
             const char *username,
             const char *log_file)
 {
-        GVariantBuilder details;
-        GdmSessionConversation *conversation;
+        GVariantBuilder          details;
+        const char             **extensions;
+        GdmSessionConversation  *conversation;
 
         g_assert (service_name != NULL);
 
         g_variant_builder_init (&details, G_VARIANT_TYPE ("a{sv}"));
 
         g_variant_builder_add_parsed (&details, "{'service', <%s>}", service_name);
+        extensions = (const char **) g_hash_table_get_keys_as_array (self->priv->user_verifier_extensions, NULL);
+
+        g_variant_builder_add_parsed (&details, "{'extensions', <%^as>}", extensions);
+
         if (username != NULL)
                 g_variant_builder_add_parsed (&details, "{'username', <%s>}", username);
 
@@ -2148,6 +2189,8 @@ initialize (GdmSession *self,
                                                  (GAsyncReadyCallback) on_initialization_complete_cb,
                                                  conversation);
         }
+
+        g_free (extensions);
 }
 
 void
@@ -3294,6 +3337,8 @@ gdm_session_dispose (GObject *object)
                          g_hash_table_unref);
 
         g_clear_object (&self->priv->user_verifier_interface);
+        g_clear_pointer (&self->priv->user_verifier_extensions,
+                         g_hash_table_unref);
         g_clear_object (&self->priv->greeter_interface);
         g_clear_object (&self->priv->chooser_interface);
 
diff --git a/daemon/gdm-session.xml b/daemon/gdm-session.xml
index 9d44005b..af2976a4 100644
--- a/daemon/gdm-session.xml
+++ b/daemon/gdm-session.xml
@@ -24,6 +24,9 @@
     </method>
   </interface>
   <interface name="org.gnome.DisplayManager.UserVerifier">
+    <method name="EnableExtensions">
+      <arg name="extensions" direction="in" type="as"/>
+    </method>
     <method name="BeginVerification">
       <arg name="service_name" direction="in" type="s"/>
     </method>
diff --git a/pam-extensions/Makefile.am b/pam-extensions/Makefile.am
new file mode 100644
index 00000000..572494ac
--- /dev/null
+++ b/pam-extensions/Makefile.am
@@ -0,0 +1,23 @@
+NULL =
+
+AM_CPPFLAGS = \
+        -I$(srcdir)                                                     \
+        -I$(builddir)                                                   \
+        -I$(top_srcdir)                                                 \
+        -I$(top_builddir)                                               \
+        -DG_LOG_DOMAIN=\"Gdm\"                                          \
+        -DDMCONFDIR=\""$(dmconfdir)"\"                                  \
+        -DDATADIR=\""$(datadir)"\"                                      \
+        $(NULL)
+
+if SUPPORTS_PAM_EXTENSIONS
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = gdm-pam-extensions.pc
+
+pam_extensions_includedir = $(includedir)/gdm
+pam_extensions_include_HEADERS = gdm-pam-extensions.h
+endif
+
+EXTRA_DIST =                                                            \
+        gdm-pam-extensions.pc                                           \
+        $(NULL)
diff --git a/pam-extensions/gdm-pam-extensions.h b/pam-extensions/gdm-pam-extensions.h
new file mode 100644
index 00000000..eff2f40a
--- /dev/null
+++ b/pam-extensions/gdm-pam-extensions.h
@@ -0,0 +1,133 @@
+/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 8 -*-
+ *
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ */
+#ifndef GDM_PAM_EXTENSIONS_H
+#define GDM_PAM_EXTENSIONS_H
+
+#include <alloca.h>
+#include <endian.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <limits.h>
+
+#include <security/pam_appl.h>
+
+typedef struct {
+        uint32_t length;
+
+        unsigned char type;
+        unsigned char data[];
+} GdmPamExtensionMessage;
+
+#define GDM_PAM_EXTENSION_MESSAGE_FROM_PAM_MESSAGE(query) (GdmPamExtensionMessage *) (void *) query->msg
+#define GDM_PAM_EXTENSION_MESSAGE_TO_PAM_REPLY(msg) (char *) (void *) msg
+#define GDM_PAM_EXTENSION_MESSAGE_TO_BINARY_PROMPT_MESSAGE(extended_message, binary_message) \
+{ \
+        (binary_message)->msg_style = PAM_BINARY_PROMPT; \
+        (binary_message)->msg = (void *) extended_message; \
+}
+#define GDM_PAM_EXTENSION_MESSAGE_TRUNCATED(msg) be32toh(msg->length) < sizeof (GdmPamExtensionMessage)
+#define GDM_PAM_EXTENSION_MESSAGE_INVALID_TYPE(msg) \
+({ \
+        bool _invalid = true; \
+        int _n = -1; \
+        const char *_supported_extensions; \
+        _supported_extensions = getenv ("GDM_SUPPORTED_PAM_EXTENSIONS"); \
+        if (_supported_extensions != NULL) { \
+                const char *_p = _supported_extensions; \
+                while (*_p != '\0' && _n < UCHAR_MAX) { \
+                        size_t _length; \
+                        _length = strcspn (_p, " "); \
+                        if (_length > 0) \
+                                _n++; \
+                        _p += _length; \
+                        _length = strspn (_p, " "); \
+                        _p += _length; \
+                } \
+                if (_n >= msg->type) \
+                        _invalid = false; \
+        } \
+        _invalid; \
+})
+#define GDM_PAM_EXTENSION_MESSAGE_MATCH(msg, supported_extensions, name) (strcmp (supported_extensions[msg->type], name) == 0)
+
+/* environment block should be a statically allocated chunk of memory.  This is important because
+ * putenv() will leak otherwise (and setenv isn't thread safe)
+ */
+#define GDM_PAM_EXTENSION_ADVERTISE_SUPPORTED_EXTENSIONS(environment_block, supported_extensions) \
+{ \
+        size_t _size = 0; \
+        unsigned char _t, _num_chunks; \
+        char *_p; \
+        _p = environment_block; \
+        _p = stpncpy (_p, "GDM_SUPPORTED_PAM_EXTENSIONS", sizeof(environment_block)); \
+        *_p = '\0'; \
+        _size += strlen (_p); \
+        for (_t = 0; supported_extensions[_t] != NULL && _t < UCHAR_MAX; _t++) {\
+                size_t _next_chunk = strlen (supported_extensions[_t]) + strlen (" "); \
+                if (_size + _next_chunk >= sizeof (environment_block)) \
+                        break; \
+                _size += _next_chunk; \
+        }\
+        _num_chunks = _t; \
+        if (_t != 0) { \
+                _p = stpcpy (_p, "="); \
+                for (_t = 0; _t < _num_chunks; _t++) { \
+                        if (_t != 0) \
+                                _p = stpcpy (_p, " "); \
+                        _p = stpcpy (_p, supported_extensions[_t]); \
+                } \
+                *_p = '\0'; \
+                putenv (environment_block); \
+        } \
+}
+
+#define GDM_PAM_EXTENSION_LOOK_UP_TYPE(name, extension_type) \
+({ \
+        bool _supported = false; \
+        unsigned char _t = 0; \
+        const char *_supported_extensions; \
+        _supported_extensions = getenv ("GDM_SUPPORTED_PAM_EXTENSIONS"); \
+        if (_supported_extensions != NULL) { \
+                const char *_p = _supported_extensions; \
+                while (*_p != '\0') { \
+                        size_t _length; \
+                        _length = strcspn (_p, " "); \
+                        if (strncmp (_p, name, _length) == 0) { \
+                                _supported = true; \
+                                break; \
+                        } \
+                        _p += _length; \
+                        _length = strspn (_p, " "); \
+                        _p += _length; \
+                        if (_t >= UCHAR_MAX) { \
+                                break; \
+                        } \
+                        _t++; \
+                } \
+                if (_supported && extension_type != NULL) \
+                        *extension_type = _t; \
+        } \
+        _supported; \
+})
+
+#define GDM_PAM_EXTENSION_SUPPORTED(name) GDM_PAM_EXTENSION_LOOK_UP_TYPE(name, (unsigned char *) NULL)
+
+#endif
diff --git a/pam-extensions/gdm-pam-extensions.pc.in b/pam-extensions/gdm-pam-extensions.pc.in
new file mode 100644
index 00000000..5fc64b01
--- /dev/null
+++ b/pam-extensions/gdm-pam-extensions.pc.in
@@ -0,0 +1,9 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: GDM PAM Extensions
+Description: Macros for custom protocols over PAM
+Version: @VERSION@
+Cflags: -I${includedir}/gdm