net: fix out-of-bounds write in IP fragment reassembly

__net_defragment() reassembles IP fragments into the static buffer
pkt_buff[CONFIG_NET_MAXDEFRAG].  The bounds check

	if (start + len > IP_MAXUDP)
		return NULL;

only covers the fragment data copy.  The split-hole and move-hole
branches additionally write an 8-byte struct hole via "*newh = *h" at
newh = thisfrag + len / 8, which can land up to sizeof(struct hole)
bytes past the end of pkt_buff.  A single fragment with a non-zero
fragment offset and the More-Fragments flag set reaches this path, so
a crafted fragment received during netboot overflows the buffer.

Reject any fragment whose trailing hole descriptor would fall outside
pkt_buff.

Signed-off-by: Shahriyar Jalayeri <shahriyar@byteray.co.uk>
Acked-by: Jerome Forissier <jerome.forissier@arm.com>

net/net.c

@@ -1076,6 +1076,8 @@ static struct ip_udp_hdr *__net_defragment(struct ip_udp_hdr *ip, int *lenp)
 	} else if (h >= thisfrag) {
 		/* overlaps with initial part of the hole: move this hole */
 		newh = thisfrag + (len / 8);
+		if ((uchar *)(newh + 1) > pkt_buff + IP_PKTSIZE)
+			return NULL;	/* hole descriptor would overflow pkt_buff */
 		*newh = *h;
 		h = newh;
 		if (h->next_hole)
@@ -1088,6 +1090,8 @@ static struct ip_udp_hdr *__net_defragment(struct ip_udp_hdr *ip, int *lenp)
 	} else {
 		/* fragment sits in the middle: split the hole */
 		newh = thisfrag + (len / 8);
+		if ((uchar *)(newh + 1) > pkt_buff + IP_PKTSIZE)
+			return NULL;	/* hole descriptor would overflow pkt_buff */
 		*newh = *h;
 		h->last_byte = start;
 		h->next_hole = (newh - payload);