SECURITY: Ignore apparent answers before first RR we found the first time

This way the second answer scan finds the same RRs at the first.
Otherwise, adns can be confused by interleaving answers for the CNAME
target, with the CNAME itself.

In that case the answer data structure (on the heap) can be overrun.

With this change, we prefer to look only at the answer RRs which come
after the CNAME, which is at least arguably correct.

Found by AFL 2.35b.  CVE-2017-9109.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/src/reply.c b/src/reply.c
index bd2d0e8efc6f99ec19a54a3d4641caaf31498550..0379ca441ff52f90feefdce6cf3dc333c7b808a7 100644 (file)
--- a/src/reply.c
+++ b/src/reply.c
@@ -35,7 +35,7 @@ void adns__procdgram(adns_state ads, const byte *dgram, int dglen,
   int flg_ra, flg_rd, flg_tc, flg_qr, opcode;
   int rrtype, rrclass, rdlength, rdstart;
   int anstart, nsstart;
-  int ownermatched, l, nrrs;
+  int ownermatched, l, nrrs, restartfrom;
   unsigned long ttl, soattl;
   const typeinfo *typei;
   adns_query qu, nqu;
@@ -162,6 +162,7 @@ void adns__procdgram(adns_state ads, const byte *dgram, int dglen,
    * If it has any CNAMEs we stuff them in the answer.
    */
   wantedrrs= 0;
+  restartfrom= -1;
   cbyte= anstart;
   for (rri= 0; rri<ancount; rri++) {
     rrstart= cbyte;
@@ -229,6 +230,7 @@ void adns__procdgram(adns_state ads, const byte *dgram, int dglen,
         */
       }
     } else if (rrtype == (qu->answer->type & adns_rrt_typemask)) {
+      if (restartfrom==-1) restartfrom= rri;
       wantedrrs++;
     } else {
       adns__debug(ads,serv,qu,"ignoring answer RR"
@@ -331,12 +333,14 @@ void adns__procdgram(adns_state ads, const byte *dgram, int dglen,
   pai.arcount= arcount;
   pai.now= now;
 
+  assert(restartfrom>=0);
   for (rri=0, nrrs=0; rri<ancount; rri++) {
     st= adns__findrr(qu,serv, dgram,dglen,&cbyte,
                     &rrtype,&rrclass,&ttl, &rdlength,&rdstart,
                     &ownermatched);
     assert(!st); assert(rrtype != -1);
-    if (rrclass != DNS_CLASS_IN ||
+    if (rri < restartfrom ||
+       rrclass != DNS_CLASS_IN ||
        rrtype != (qu->answer->type & adns_rrt_typemask) ||
        !ownermatched)
       continue;