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

1 files changed, 93 insertions, 10 deletions

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);