dnsd: check that we don't read past packet

function                                             old     new   delta
dnsd_main                                           1296    1304      +8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 12 insertions, 5 deletions

diff --git a/networking/dnsd.c b/networking/dnsd.c
index 0ff0290fb..a0f320c6c 100644
--- a/networking/dnsd.c
+++ b/networking/dnsd.c
@@ -379,7 +379,8 @@ Domain name in a message can be represented as either:
  */
 static int process_packet(struct dns_entry *conf_data,
                 uint32_t conf_ttl,
-                uint8_t *buf)
+                uint8_t *buf,
+                unsigned buflen)
 {
         struct dns_head *head;
         struct type_and_class *unaligned_type_class;
@@ -402,9 +403,6 @@ static int process_packet(struct dns_entry *conf_data,
                 bb_simple_error_msg("response packet, ignored");
                 return 0; /* don't reply */
         }
-        /* QR = 1 "response", RCODE = 4 "Not Implemented" */
-        outr_flags = htons(0x8000 | 4);
-        err_msg = NULL;
 
         /* start of query string */
         query_string = (void *)(head + 1);
@@ -416,6 +414,15 @@ static int process_packet(struct dns_entry *conf_data,
         /* where to append answer block */
         answb = (void *)(unaligned_type_class + 1);
 
+        if (buflen < answb - buf) {
+                bb_simple_error_msg("packet too short");
+                return 0; /* don't reply */
+        }
+
+        /* QR = 1 "response", RCODE = 4 "Not Implemented" */
+        outr_flags = htons(0x8000 | 4);
+        err_msg = NULL;
+
         /* OPCODE != 0 "standard query"? */
         if ((head->flags & htons(0x7800)) != 0) {
                 err_msg = "opcode != 0";
@@ -559,7 +566,7 @@ int dnsd_main(int argc UNUSED_PARAM, char **argv)
                 if (OPT_verbose)
                         bb_simple_info_msg("got UDP packet");
                 buf[r] = '\0'; /* paranoia */
-                r = process_packet(conf_data, conf_ttl, buf);
+                r = process_packet(conf_data, conf_ttl, buf, r);
                 if (r <= 0)
                         continue;
                 send_to_from(udps, buf, r, 0, &from->u.sa, &to->u.sa, lsa->len);