The function ip6_get_prevhdr() did return a pointer into a mbuf.

It was not guaranteed that the mbuf data was not somewhere else in
the chain.  So return an offset and do a proper mbuf pulldown.
found by Maxime Villard; from NetBSD; with markus@; OK deraadt@

1 files changed, 19 insertions, 25 deletions

diff --git a/sys/netinet6/ip6_input.c b/sys/netinet6/ip6_input.c
index 6f07b36d9d3..b7dc6287e5a 100644
--- a/sys/netinet6/ip6_input.c
+++ b/sys/netinet6/ip6_input.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: ip6_input.c,v 1.210 2017/11/23 13:45:46 mpi Exp $	*/
+/*	$OpenBSD: ip6_input.c,v 1.211 2018/02/01 21:11:33 bluhm Exp $	*/
 /*	$KAME: ip6_input.c,v 1.188 2001/03/29 05:34:31 itojun Exp $	*/
 
 /*
@@ -1188,50 +1188,44 @@ ip6_pullexthdr(struct mbuf *m, size_t off, int nxt)
 }
 
 /*
- * Get pointer to the previous header followed by the header
+ * Get offset to the previous header followed by the header
  * currently processed.
- * XXX: This function supposes that
- *	M includes all headers,
- *	the next header field and the header length field of each header
- *	are valid, and
- *	the sum of each header length equals to OFF.
- * Because of these assumptions, this function must be called very
- * carefully. Moreover, it will not be used in the near future when
- * we develop `neater' mechanism to process extension headers.
  */
-u_int8_t *
+int
 ip6_get_prevhdr(struct mbuf *m, int off)
 {
 	struct ip6_hdr *ip6 = mtod(m, struct ip6_hdr *);
 
-	if (off == sizeof(struct ip6_hdr))
-		return (&ip6->ip6_nxt);
-	else {
-		int len, nxt;
-		struct ip6_ext *ip6e = NULL;
+	if (off == sizeof(struct ip6_hdr)) {
+		return offsetof(struct ip6_hdr, ip6_nxt);
+	} else if (off < sizeof(struct ip6_hdr)) {
+		panic("%s: off < sizeof(struct ip6_hdr)", __func__);
+	} else {
+		int len, nlen, nxt;
+		struct ip6_ext ip6e;
 
 		nxt = ip6->ip6_nxt;
 		len = sizeof(struct ip6_hdr);
+		nlen = 0;
 		while (len < off) {
-			ip6e = (struct ip6_ext *)(mtod(m, caddr_t) + len);
+			m_copydata(m, len, sizeof(ip6e), (caddr_t)&ip6e);
 
 			switch (nxt) {
 			case IPPROTO_FRAGMENT:
-				len += sizeof(struct ip6_frag);
+				nlen = sizeof(struct ip6_frag);
 				break;
 			case IPPROTO_AH:
-				len += (ip6e->ip6e_len + 2) << 2;
+				nlen = (ip6e.ip6e_len + 2) << 2;
 				break;
 			default:
-				len += (ip6e->ip6e_len + 1) << 3;
+				nlen = (ip6e.ip6e_len + 1) << 3;
 				break;
 			}
-			nxt = ip6e->ip6e_nxt;
+			len += nlen;
+			nxt = ip6e.ip6e_nxt;
 		}
-		if (ip6e)
-			return (&ip6e->ip6e_nxt);
-		else
-			return NULL;
+
+		return (len - nlen);
 	}
 }