diff --git a/sys/kern/subr_mbuf.c b/sys/kern/subr_mbuf.c
index 5d593e6f3026..5d624f6839c0 100644
--- a/sys/kern/subr_mbuf.c
+++ b/sys/kern/subr_mbuf.c
@@ -1380,6 +1380,11 @@ m_free(struct mbuf *mb)
 	int cchnum;
 	short persist = 0;
 
+#ifdef INVARIANTS
+	if (mb->m_flags & M_FREELIST)
+		panic("m_free detected a mbuf double-free");
+	mb->m_flags |= M_FREELIST;
+#endif
 	if ((mb->m_flags & M_PKTHDR) != 0)
 		m_tag_delete_chain(mb, NULL);
 	nb = mb->m_next;
@@ -1422,6 +1427,11 @@ m_freem(struct mbuf *mb)
 	short persist;
 
 	while (mb != NULL) {
+#ifdef INVARIANTS
+		if (mb->m_flags & M_FREELIST)
+			panic("m_freem detected a mbuf double-free");
+		mb->m_flags |= M_FREELIST;
+#endif
 		if ((mb->m_flags & M_PKTHDR) != 0)
 			m_tag_delete_chain(mb, NULL);
 		persist = 0;
diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
index a00534a420f1..8a27462913fd 100644
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -153,6 +153,7 @@ struct mbuf {
 #define	M_PROTO3	0x0040	/* protocol-specific */
 #define	M_PROTO4	0x0080	/* protocol-specific */
 #define	M_PROTO5	0x0100	/* protocol-specific */
+#define	M_FREELIST	0x4000	/* mbuf is on the free list */
 
 /*
  * mbuf pkthdr flags (also stored in m_flags).