Index: sys/kern/uipc_mbuf.c
===================================================================
--- sys/kern/uipc_mbuf.c	(revision 209948)
+++ sys/kern/uipc_mbuf.c	(working copy)
@@ -302,6 +302,7 @@
 	n->m_ext.ref_cnt = m->m_ext.ref_cnt;
 	n->m_ext.ext_type = m->m_ext.ext_type;
 	n->m_flags |= M_EXT;
+	n->m_flags |= m->m_flags & M_RDONLY;
 }
 
 /*