Add callback SASL_CB_SERVER_CHANNEL_BINDING

Provide a callback function to change the channel binding type
of servers (e.g. "tls-server-end-point" instead of "tls-unique")
during authentication of secure mechanisms like GS2-KRB5-PLUS or
SCRAM-SHA-256-PLUS.

The callback is used by the plugins SCRAM and GS2 when the desired
binding type of the client does not match the binding type set with
property SASL_CHANNEL_BINDING.
A server can check the requested type of channel binding and
overwrite the current channel binding data with the property
SASL_CHANNEL_BINDING before the authentication proceeds.

Issue cyrusimap#823

Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>

include/sasl.h

@@ -671,6 +671,27 @@ typedef int sasl_canon_user_t(sasl_conn_t *conn,
 
 #define SASL_CB_CANON_USER (0x8007)
 
+/* Callback for servers to change channel binding type
+ * (e.g. "tls-server-end-point" instead of "tls-unique" or "tls-exporter").
+ * The callback is used by plugins like SCRAM-SHA-*-PLUS or GS2-KRB5-PLUS
+ * when the desired binding type of the client does not match the binding
+ * type set with property SASL_CHANNEL_BINDING.
+ * A server should check the requested 'cbindingname' of the 'plugin' and
+ * overwrite the channel binding data with the property SASL_CHANNEL_BINDING
+ * within the callback function.
+ * Note that plugins only store a pointer of your channel binding data and
+ * the memory of your struct sasl_channel_binding_t must be valid during
+ * the complete authentication process.
+ *
+ *  plugin        -- name of plugin
+ *  cbindingname  -- name of desired channel binding type
+ */
+typedef int sasl_server_channel_binding_t(sasl_conn_t* conn,
+					  void* context,
+					  const char* plugin,
+					  const char* cbindingname);
+#define SASL_CB_SERVER_CHANNEL_BINDING (0x8008)
+
 /**********************************
  * Common Client/server functions *
  **********************************/

plugins/gs2.c

@@ -1025,6 +1025,29 @@ gs2_save_cbindings(context_t *text,
     return SASL_OK;
 }
 
+/*
+ * Request for (alternate) channel binding data
+ */
+static int
+gs2_server_get_channel_binding(sasl_server_params_t* sparams,
+                               const char *mech,
+                               const char *cbindingname)
+{
+    sasl_server_channel_binding_t* binding = NULL;
+    void* context = NULL;
+    /* try to get alternate channel binding data */
+    if (sparams->utils->callback(sparams->utils->conn,
+                                 SASL_CB_SERVER_CHANNEL_BINDING,
+                                 (sasl_callback_ft*)&binding,
+                                 &context) == SASL_OK) {
+        int ret = binding(sparams->utils->conn,
+                          context,
+                          mech, cbindingname);
+        return ret;
+    }
+    return SASL_BADPROT;
+}
+
 #define CHECK_REMAIN(n)     do { if (remain < (n)) return SASL_BADPROT; } while (0)
 
 /*
@@ -1073,6 +1096,13 @@ gs2_verify_initial_message(context_t *text,
         if (ret != SASL_OK)
             return ret;
 
+        if (sparams->cbinding == NULL ||
+            strcmp(sparams->cbinding->name, text->cbindingname) != 0) {
+            /* Try to get desired channel binding data */
+            gs2_server_get_channel_binding(sparams,
+                                           text->plug.server->mech_name,
+                                           text->cbindingname);
+        }
         text->gs2_flags |= GS2_CB_FLAG_P;
         break;
     case 'n':

plugins/scram.c

@@ -1155,7 +1155,27 @@ scram_server_mech_step1(server_context_t *text,
     }
     return result;
 }
-
+
+static int
+scram_server_get_channel_binding(sasl_server_params_t* sparams,
+				 const char *mech,
+				 const char *cbindingname)
+{
+    sasl_server_channel_binding_t* binding = NULL;
+    void* context = NULL;
+    /* try to get alternate channel binding data */
+    if (sparams->utils->callback(sparams->utils->conn,
+				 SASL_CB_SERVER_CHANNEL_BINDING,
+				 (sasl_callback_ft*)&binding,
+				 &context) == SASL_OK) {
+	int ret = binding(sparams->utils->conn,
+			  context,
+			  mech, cbindingname);
+	return ret;
+    }
+    return SASL_BADPROT;
+}
+
 static int
 scram_server_mech_step2(server_context_t *text,
 			sasl_server_params_t *sparams,
@@ -1281,6 +1301,14 @@ scram_server_mech_step2(server_context_t *text,
 	    goto cleanup;
 	}
 
+	if (sparams->cbinding == NULL ||
+	    strcmp(sparams->cbinding->name, text->cbindingname) != 0) {
+	    /* Try to get desired channel binding data */
+	    scram_server_get_channel_binding(sparams,
+					     scram_sasl_mech,
+					     text->cbindingname);
+	}
+
 	if (sparams->cbinding == NULL) {
 	    sparams->utils->seterror (sparams->utils->conn,
 				      0,