untrusted comment: signature from openbsd 6.2 base secret key
RWRVWzAMgtyg7jzKvfbcXaqeYPPxfpMy3RH0sRZCSvMgeLPlqvO4UP3hFnxYwqV3LcZZ4ZB35FHGyswKMW9dMFTaXRvBC5KttAo=

OpenBSD 6.2 errata 013, May 8, 2018:

Incorrect handling of fragmented IPsec packets could result in a
system crash.

Apply by doing:
    signify -Vep /etc/signify/openbsd-62-base.pub -x 013_ipseclen.patch.sig \
        -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
    KK=`sysctl -n kern.osversion | cut -d# -f1`
    cd /usr/src/sys/arch/`machine`/compile/$KK
    make obj
    make config
    make
    make install

Index: sys/netinet/ip_ah.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_ah.c,v
retrieving revision 1.131.4.2
diff -u -p -r1.131.4.2 ip_ah.c
--- sys/netinet/ip_ah.c	15 Mar 2018 12:34:33 -0000	1.131.4.2
+++ sys/netinet/ip_ah.c	2 May 2018 20:35:18 -0000
@@ -846,10 +846,16 @@ ah_input_cb(struct cryptop *crp)
 		 * the mbuf.
 		 */
 		m_adj(m1, rplen + ahx->authsize);
-		if (!(m1->m_flags & M_PKTHDR))
+		/*
+		 * If m1 is the first mbuf, it has set M_PKTHDR and m_adj()
+		 * has already adjusted the packet header length for us.
+		 */
+		if (m1 != m)
 			m->m_pkthdr.len -= rplen + ahx->authsize;
 	} else
 		if (roff + rplen + ahx->authsize >= m1->m_len) {
+			int adjlen;
+
 			/*
 			 * Part or all of the AH header is at the end
 			 * of this mbuf, so first let's remove the
@@ -858,16 +864,16 @@ ah_input_cb(struct cryptop *crp)
 			 * chain, if any.
 			 */
 			if (roff + rplen + ahx->authsize > m1->m_len) {
+				adjlen = roff + rplen + ahx->authsize -
+				    m1->m_len;
 				/* Adjust the next mbuf by the remainder. */
-				m_adj(m1->m_next, roff + rplen +
-				    ahx->authsize - m1->m_len);
+				m_adj(m1->m_next, adjlen);
 
 				/*
 				 * The second mbuf is guaranteed not
 				 * to have a pkthdr...
 				 */
-				m->m_pkthdr.len -=
-				    (roff + rplen + ahx->authsize - m1->m_len);
+				m->m_pkthdr.len -= adjlen;
 			}
 
 			/* Now, let's unlink the mbuf chain for a second... */
@@ -878,9 +884,14 @@ ah_input_cb(struct cryptop *crp)
 			 * ...and trim the end of the first part of
 			 * the chain...sick
 			 */
-			m_adj(m1, -(m1->m_len - roff));
-			if (!(m1->m_flags & M_PKTHDR))
-				m->m_pkthdr.len -= (m1->m_len - roff);
+			adjlen = m1->m_len - roff;
+			m_adj(m1, -adjlen);
+			/*
+			 * If m1 is the first mbuf, it has set M_PKTHDR and
+			 * m_adj() has already adjusted the packet header len.
+			 */
+			if (m1 != m)
+				m->m_pkthdr.len -= adjlen;
 
 			/* Finally, let's relink. */
 			m1->m_next = m0;
Index: sys/netinet/ip_esp.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_esp.c,v
retrieving revision 1.150
diff -u -p -r1.150 ip_esp.c
--- sys/netinet/ip_esp.c	11 Aug 2017 21:24:20 -0000	1.150
+++ sys/netinet/ip_esp.c	2 May 2018 20:35:18 -0000
@@ -674,20 +674,28 @@ esp_input_cb(struct cryptop *crp)
 	if (roff == 0) {
 		/* The ESP header was conveniently at the beginning of the mbuf */
 		m_adj(m1, hlen);
-		if (!(m1->m_flags & M_PKTHDR))
+		/*
+		 * If m1 is the first mbuf, it has set M_PKTHDR and m_adj()
+		 * has already adjusted the packet header length for us.
+		 */
+		if (m1 != m)
 			m->m_pkthdr.len -= hlen;
 	} else if (roff + hlen >= m1->m_len) {
+		int adjlen;
+
 		/*
 		 * Part or all of the ESP header is at the end of this mbuf, so
 		 * first let's remove the remainder of the ESP header from the
 		 * beginning of the remainder of the mbuf chain, if any.
 		 */
 		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...*/
@@ -695,9 +703,14 @@ esp_input_cb(struct cryptop *crp)
 		m1->m_next = NULL;
 
 		/* ...and trim the end of the first part of the chain...sick */
-		m_adj(m1, -(m1->m_len - roff));
-		if (!(m1->m_flags & M_PKTHDR))
-			m->m_pkthdr.len -= (m1->m_len - roff);
+		adjlen = m1->m_len - roff;
+		m_adj(m1, -adjlen);
+		/*
+		 * If m1 is the first mbuf, it has set M_PKTHDR and m_adj()
+		 * has already adjusted the packet header length for us.
+		 */
+		if (m1 != m)
+			m->m_pkthdr.len -= adjlen;
 
 		/* Finally, let's relink */
 		m1->m_next = mo;
Index: sys/netinet/ip_ipcomp.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_ipcomp.c,v
retrieving revision 1.57
diff -u -p -r1.57 ip_ipcomp.c
--- sys/netinet/ip_ipcomp.c	11 Aug 2017 21:24:20 -0000	1.57
+++ sys/netinet/ip_ipcomp.c	2 May 2018 20:35:18 -0000
@@ -292,27 +292,40 @@ ipcomp_input_cb(struct cryptop *crp)
 	if (roff == 0) {
 		/* The IPCOMP header is at the beginning of m1 */
 		m_adj(m1, hlen);
-		if (!(m1->m_flags & M_PKTHDR))
+		/*
+		 * If m1 is the first mbuf, it has set M_PKTHDR and m_adj()
+		 * has already adjusted the packet header length for us.
+		 */
+		if (m1 != m)
 			m->m_pkthdr.len -= hlen;
 	} else if (roff + hlen >= m1->m_len) {
+		int adjlen;
+
 		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... */
 		mo = m1->m_next;
 		m1->m_next = NULL;
 
 		/* ...and trim the end of the first part of the chain...sick */
-		m_adj(m1, -(m1->m_len - roff));
-		if (!(m1->m_flags & M_PKTHDR))
-			m->m_pkthdr.len -= (m1->m_len - roff);
+		adjlen = m1->m_len - roff;
+		m_adj(m1, -adjlen);
+		/*
+		 * If m1 is the first mbuf, it has set M_PKTHDR and m_adj()
+		 * has already adjusted the packet header length for us.
+		 */
+		if (m1 != m)
+			m->m_pkthdr.len -= adjlen;
 
 		/* Finally, let's relink */
 		m1->m_next = mo;