ntpd plugin: Fix a possible buffer overflow.

diff --git a/src/ntpd.c b/src/ntpd.c
index b65a9e8..08c0c9f 100644 (file)
--- a/src/ntpd.c
+++ b/src/ntpd.c
@@ -631,6 +631,14 @@ static int ntpd_receive_response (int req_code, int *res_items, int *res_size,
                        continue;
                }
 
+               if (pkt_item_len > res_item_size)
+               {
+                       syslog (LOG_ERR, "ntpd plugin: (pkt_item_len = %i) "
+                                       ">= (res_item_size = %i)",
+                                       pkt_item_len, res_item_size);
+                       continue;
+               }
+
                /* If this is the first packet (time wise, not sequence wise),
                 * set `res_size'. If it's not the first packet check if the
                 * items have the same size. Discard invalid packets. */
@@ -647,9 +655,16 @@ static int ntpd_receive_response (int req_code, int *res_items, int *res_size,
                        continue;
                }
 
+               /*
+                * Because the items in the packet may be smaller than the
+                * items requested, the following holds true:
+                */
+               assert ((*res_size == pkt_item_len)
+                               && (pkt_item_len <= res_item_size));
+
                /* Calculate the padding. No idea why there might be any padding.. */
                pkt_padding = 0;
-               if (res_item_size > pkt_item_len)
+               if (pkt_item_len < res_item_size)
                        pkt_padding = res_item_size - pkt_item_len;
                DBG ("res_item_size = %i; pkt_padding = %i;",
                                res_item_size, pkt_padding);
@@ -694,18 +709,26 @@ static int ntpd_receive_response (int req_code, int *res_items, int *res_size,
                                (items_num + pkt_item_num) * res_item_size);
                items = realloc ((void *) *res_data,
                                (items_num + pkt_item_num) * res_item_size);
-               items_num += pkt_item_num;
                if (items == NULL)
                {
                        items = *res_data;
                        syslog (LOG_ERR, "ntpd plugin: realloc failed.");
                        continue;
                }
+               items_num += pkt_item_num;
                *res_data = items;
 
                for (i = 0; i < pkt_item_num; i++)
                {
+                       /* dst: There are already `*res_items' items with
+                        *      res_item_size bytes each in in `*res_data'. Set
+                        *      dst to the first byte after that. */
                        void *dst = (void *) (*res_data + ((*res_items) * res_item_size));
+                       /* src: We use `pkt_item_len' to calculate the offset
+                        *      from the beginning of the packet, because the
+                        *      items in the packet may be smaller than the
+                        *      items that were requested. We skip `i' such
+                        *      items. */
                        void *src = (void *) (((char *) res.data) + (i * pkt_item_len));
 
                        /* Set the padding to zeros */
@@ -713,8 +736,10 @@ static int ntpd_receive_response (int req_code, int *res_items, int *res_size,
                                memset (dst, '\0', res_item_size);
                        memcpy (dst, src, (size_t) pkt_item_len);
 
+                       /* Increment `*res_items' by one, so `dst' will end up
+                        * one further in the next round. */
                        (*res_items)++;
-               }
+               } /* for (pkt_item_num) */
 
                pkt_recvd[pkt_sequence] = (char) 1;
                pkt_recvd_num++;
@@ -724,7 +749,7 @@ static int ntpd_receive_response (int req_code, int *res_items, int *res_size,
        } /* while (done == 0) */
 
        return (0);
-}
+} /* int ntpd_receive_response */
 
 /* For a description of the arguments see `ntpd_do_query' below. */
 static int ntpd_send_request (int req_code, int req_items, int req_size, char *req_data)