net: check packet payload length

While computing IP checksum, 'net_checksum_calculate' reads
payload length from the packet. It could exceed the given 'data'
buffer size. Add a check to avoid it.
Reported-by: NLiu Ling <liuling-it@360.cn>
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: NJason Wang <jasowang@redhat.com>

net/checksum.c

@@ -60,6 +60,11 @@ void net_checksum_calculate(uint8_t *data, int length)
     int hlen, plen, proto, csum_offset;
     uint16_t csum;
 
+    /* Ensure data has complete L2 & L3 headers. */
+    if (length < 14 + 20) {
+        return;
+    }
+
     if ((data[14] & 0xf0) != 0x40)
 	return; /* not IPv4 */
     hlen  = (data[14] & 0x0f) * 4;
@@ -77,8 +82,9 @@ void net_checksum_calculate(uint8_t *data, int length)
 	return;
     }
 
-    if (plen < csum_offset+2)
+    if (plen < csum_offset + 2 || 14 + hlen + plen > length) {
         return;
+    }
 
     data[14+hlen+csum_offset]   = 0;
     data[14+hlen+csum_offset+1] = 0;