src: Fix a pretty bad mistake, that has always been there.

[maxv <maxv%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/359fa12d8367
branches:  trunk
changeset: 318272:359fa12d8367
user:      maxv <maxv%NetBSD.org@localhost>
date:      Tue Apr 17 09:06:33 2018 +0000
description:
Fix a pretty bad mistake, that has always been there.

                m_adj(m1, -(m1->m_len - roff));
                if (m1 != m)
                        m->m_pkthdr.len -= (m1->m_len - roff);

This is wrong: m_adj will modify m1->m_len, so we're using a wrong value
when manually adjusting m->m_pkthdr.len.

Because of that, it is possible to exploit the attack I described in
uipc_mbuf.c::rev1.182. The exploit is more complicated, but works 100%
reliably.

diffstat:

 sys/netipsec/ipsec_mbuf.c |  16 ++++++++++------
 1 files changed, 10 insertions(+), 6 deletions(-)

diffs (55 lines):

diff -r fd6276e923ee -r 359fa12d8367 sys/netipsec/ipsec_mbuf.c
--- a/sys/netipsec/ipsec_mbuf.c Tue Apr 17 08:54:35 2018 +0000
+++ b/sys/netipsec/ipsec_mbuf.c Tue Apr 17 09:06:33 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: ipsec_mbuf.c,v 1.23 2018/04/17 06:23:30 maxv Exp $     */
+/*     $NetBSD: ipsec_mbuf.c,v 1.24 2018/04/17 09:06:33 maxv Exp $     */
 
 /*
  * Copyright (c) 2002, 2003 Sam Leffler, Errno Consulting
@@ -29,7 +29,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: ipsec_mbuf.c,v 1.23 2018/04/17 06:23:30 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: ipsec_mbuf.c,v 1.24 2018/04/17 09:06:33 maxv Exp $");
 
 /*
  * IPsec-specific mbuf routines.
@@ -404,6 +404,7 @@
                        m->m_pkthdr.len -= hlen;
        } else if (roff + hlen >= m1->m_len) {
                struct mbuf *mo;
+               int adjlen;
 
                /*
                 * Part or all of the header is at the end of this mbuf,
@@ -412,11 +413,13 @@
                 */
                IPSEC_STATINC(IPSEC_STAT_INPUT_END);
                if (roff + hlen > m1->m_len) {
+                       adjlen = roff + hlen - m1->m_len;
+
                        /* Adjust the next mbuf by the remainder */
-                       m_adj(m1->m_next, roff + hlen - m1->m_len);
+                       m_adj(m1->m_next, adjlen);
 
                        /* The second mbuf is guaranteed not to have a pkthdr... */
-                       m->m_pkthdr.len -= (roff + hlen - m1->m_len);
+                       m->m_pkthdr.len -= adjlen;
                }
 
                /* Now, let's unlink the mbuf chain for a second...*/
@@ -424,9 +427,10 @@
                m1->m_next = NULL;
 
                /* ...and trim the end of the first part of the chain...sick */
-               m_adj(m1, -(m1->m_len - roff));
+               adjlen = m1->m_len - roff;
+               m_adj(m1, -adjlen);
                if (m1 != m)
-                       m->m_pkthdr.len -= (m1->m_len - roff);
+                       m->m_pkthdr.len -= adjlen;
 
                /* Finally, let's relink */
                m1->m_next = mo;