s4:heimdal Add hooks to check with the DB before we allow s4u2self

This allows us to resolve multiple forms of a name, allowing for
example machine$@REALM to get an S4U2Self ticket for
host/machine@REALM.

Andrew Bartlett

Signed-off-by: Love Hornquist Astrand <lha@h5l.org>

kdc/krb5tgs.c

@@ -494,7 +494,7 @@ check_tgs_flags(krb5_context context,
 }
 
 /*
- *
+ * Determine if constrained delegation is allowed from this client to this server
  */
 
 static krb5_error_code
@@ -536,6 +536,36 @@ check_constrained_delegation(krb5_context context,
     return ret;
 }
 
+/*
+ * Determine if s4u2self is allowed from this client to this server
+ *
+ * For example, regardless of the principal being impersonated, if the
+ * 'client' and 'server' are the same, then it's safe.
+ */
+
+static krb5_error_code
+check_s4u2self(krb5_context context,
+	       krb5_kdc_configuration *config,
+	       HDB *clientdb,
+	       hdb_entry_ex *client,
+	       krb5_const_principal server)
+{
+    krb5_error_code ret;
+
+    /* if client does a s4u2self to itself, that ok */
+    if (krb5_principal_compare(context, client->entry.principal, server) == TRUE)
+	return 0;
+
+    if (clientdb->hdb_check_s4u2self) {
+	ret = clientdb->hdb_check_s4u2self(context, clientdb, client, server);
+	if (ret == 0)
+	    return 0;
+    } else {
+	ret = KRB5KDC_ERR_BADOPTION;
+    }
+    return ret;
+}
+
 /*
  *
  */
@@ -1793,13 +1823,13 @@ server_lookup:
 	     * Check that service doing the impersonating is
 	     * requesting a ticket to it-self.
 	     */
-	    if (krb5_principal_compare(context, cp, sp) != TRUE) {
+	    ret = check_s4u2self(context, config, clientdb, client, sp);
+	    if (ret) {
 		kdc_log(context, config, 0, "S4U2Self: %s is not allowed "
-			"to impersonate some other user "
+			"to impersonate to service "
 			"(tried for user %s to service %s)",
 			cpn, selfcpn, spn);
 		free(selfcpn);
-		ret = KRB5KDC_ERR_BADOPTION; /* ? */
 		goto out;
 	    }
 

lib/hdb/hdb.h

@@ -235,9 +235,14 @@ typedef struct HDB{
      * Check if this name is an alias for the supplied client for PKINIT userPrinicpalName logins
      */
     krb5_error_code (*hdb_check_pkinit_ms_upn_match)(krb5_context, struct HDB *, hdb_entry_ex *, krb5_const_principal);
+
+    /**
+     * Check if s4u2self is allowed from this client to this server
+     */
+    krb5_error_code (*hdb_check_s4u2self)(krb5_context, struct HDB *, hdb_entry_ex *, krb5_const_principal);
 }HDB;
 
-#define HDB_INTERFACE_VERSION	6
+#define HDB_INTERFACE_VERSION	7
 
 struct hdb_so_method {
     int version;