CVE-2015-7547: getaddrinfo() stack-based buffer overflow (Bug 18665).

* A stack-based buffer overflow was found in libresolv when invoked from
  libnss_dns, allowing specially crafted DNS responses to seize control
  of execution flow in the DNS client.  The buffer overflow occurs in
  the functions send_dg (send datagram) and send_vc (send TCP) for the
  NSS module libnss_dns.so.2 when calling getaddrinfo with AF_UNSPEC
  family.  The use of AF_UNSPEC triggers the low-level resolver code to
  send out two parallel queries for A and AAAA.  A mismanagement of the
  buffers used for those queries could result in the response of a query
  writing beyond the alloca allocated buffer created by
  _nss_dns_gethostbyname4_r.  Buffer management is simplified to remove
  the overflow.  Thanks to the Google Security Team and Red Hat for
  reporting the security impact of this issue, and Robert Holiday of
  Ciena for reporting the related bug 18665. (CVE-2015-7547)

See also:
https://sourceware.org/ml/libc-alpha/2016-02/msg00416.html
https://sourceware.org/ml/libc-alpha/2016-02/msg00418.html

ChangeLog

@@ -1,4 +1,19 @@
-2016-02-14  Carlos O'Donelll  <carlos@redhat.com>
+2016-02-15  Carlos O'Donell  <carlos@redhat.com>
+
+	[BZ #18665]
+	* resolv/nss_dns/dns-host.c (gaih_getanswer_slice): Always set
+	*herrno_p.
+	(gaih_getanswer): Document functional behviour. Return tryagain
+	if any result is tryagain.
+	* resolv/res_query.c (__libc_res_nsearch): Set buffer size to zero
+	when freed.
+	* resolv/res_send.c: Add copyright text.
+	(__libc_res_nsend): Document that MAXPACKET is expected.
+	(send_vc): Document. Remove buffer reuse.
+	(send_dg): Document. Remove buffer reuse. Set *thisanssizp to set the
+	size of the buffer. Add Dprint for truncated UDP buffer.
+
+2016-02-14  Carlos O'Donell  <carlos@redhat.com>
 
 	* manual/install.texi: Latest tested is GCC 5.3, texinfo 6.0, gawk
 	4.1.3, and sed 4.2.2. Remove po2test.sed comments.

NEWS

@@ -79,6 +79,20 @@ Security related changes:
   depending on the length of the string passed as an argument to the
   functions.  Reported by Joseph Myers.
 
+* A stack-based buffer overflow was found in libresolv when invoked from
+  libnss_dns, allowing specially crafted DNS responses to seize control
+  of execution flow in the DNS client.  The buffer overflow occurs in
+  the functions send_dg (send datagram) and send_vc (send TCP) for the
+  NSS module libnss_dns.so.2 when calling getaddrinfo with AF_UNSPEC
+  family.  The use of AF_UNSPEC triggers the low-level resolver code to
+  send out two parallel queries for A and AAAA.  A mismanagement of the
+  buffers used for those queries could result in the response of a query
+  writing beyond the alloca allocated buffer created by
+  _nss_dns_gethostbyname4_r.  Buffer management is simplified to remove
+  the overflow.  Thanks to the Google Security Team and Red Hat for
+  reporting the security impact of this issue, and Robert Holiday of
+  Ciena for reporting the related bug 18665. (CVE-2015-7547)
+
 * The following bugs are resolved with this release:
 
   [The release manager will add the list generated by

resolv/nss_dns/dns-host.c

@@ -1031,7 +1031,10 @@ gaih_getanswer_slice (const querybuf *answer, int anslen, const char *qname,
   int h_namelen = 0;
 
   if (ancount == 0)
-    return NSS_STATUS_NOTFOUND;
+    {
+      *h_errnop = HOST_NOT_FOUND;
+      return NSS_STATUS_NOTFOUND;
+    }
 
   while (ancount-- > 0 && cp < end_of_message && had_error == 0)
     {
@@ -1208,7 +1211,14 @@ gaih_getanswer_slice (const querybuf *answer, int anslen, const char *qname,
   /* Special case here: if the resolver sent a result but it only
      contains a CNAME while we are looking for a T_A or T_AAAA record,
      we fail with NOTFOUND instead of TRYAGAIN.  */
-  return canon == NULL ? NSS_STATUS_TRYAGAIN : NSS_STATUS_NOTFOUND;
+  if (canon != NULL)
+    {
+      *h_errnop = HOST_NOT_FOUND;
+      return NSS_STATUS_NOTFOUND;
+    }
+
+  *h_errnop = NETDB_INTERNAL;
+  return NSS_STATUS_TRYAGAIN;
 }
 
 
@@ -1222,11 +1232,101 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 
   enum nss_status status = NSS_STATUS_NOTFOUND;
 
+  /* Combining the NSS status of two distinct queries requires some
+     compromise and attention to symmetry (A or AAAA queries can be
+     returned in any order).  What follows is a breakdown of how this
+     code is expected to work and why. We discuss only SUCCESS,
+     TRYAGAIN, NOTFOUND and UNAVAIL, since they are the only returns
+     that apply (though RETURN and MERGE exist).  We make a distinction
+     between TRYAGAIN (recoverable) and TRYAGAIN' (not-recoverable).
+     A recoverable TRYAGAIN is almost always due to buffer size issues
+     and returns ERANGE in errno and the caller is expected to retry
+     with a larger buffer.
+
+     Lastly, you may be tempted to make significant changes to the
+     conditions in this code to bring about symmetry between responses.
+     Please don't change anything without due consideration for
+     expected application behaviour.  Some of the synthesized responses
+     aren't very well thought out and sometimes appear to imply that
+     IPv4 responses are always answer 1, and IPv6 responses are always
+     answer 2, but that's not true (see the implementation of send_dg
+     and send_vc to see response can arrive in any order, particularly
+     for UDP). However, we expect it holds roughly enough of the time
+     that this code works, but certainly needs to be fixed to make this
+     a more robust implementation.
+
+     ----------------------------------------------
+     | Answer 1 Status /   | Synthesized | Reason |
+     | Answer 2 Status     | Status      |        |
+     |--------------------------------------------|
+     | SUCCESS/SUCCESS     | SUCCESS     | [1]    |
+     | SUCCESS/TRYAGAIN    | TRYAGAIN    | [5]    |
+     | SUCCESS/TRYAGAIN'   | SUCCESS     | [1]    |
+     | SUCCESS/NOTFOUND    | SUCCESS     | [1]    |
+     | SUCCESS/UNAVAIL     | SUCCESS     | [1]    |
+     | TRYAGAIN/SUCCESS    | TRYAGAIN    | [2]    |
+     | TRYAGAIN/TRYAGAIN   | TRYAGAIN    | [2]    |
+     | TRYAGAIN/TRYAGAIN'  | TRYAGAIN    | [2]    |
+     | TRYAGAIN/NOTFOUND   | TRYAGAIN    | [2]    |
+     | TRYAGAIN/UNAVAIL    | TRYAGAIN    | [2]    |
+     | TRYAGAIN'/SUCCESS   | SUCCESS     | [3]    |
+     | TRYAGAIN'/TRYAGAIN  | TRYAGAIN    | [3]    |
+     | TRYAGAIN'/TRYAGAIN' | TRYAGAIN'   | [3]    |
+     | TRYAGAIN'/NOTFOUND  | TRYAGAIN'   | [3]    |
+     | TRYAGAIN'/UNAVAIL   | UNAVAIL     | [3]    |
+     | NOTFOUND/SUCCESS    | SUCCESS     | [3]    |
+     | NOTFOUND/TRYAGAIN   | TRYAGAIN    | [3]    |
+     | NOTFOUND/TRYAGAIN'  | TRYAGAIN'   | [3]    |
+     | NOTFOUND/NOTFOUND   | NOTFOUND    | [3]    |
+     | NOTFOUND/UNAVAIL    | UNAVAIL     | [3]    |
+     | UNAVAIL/SUCCESS     | UNAVAIL     | [4]    |
+     | UNAVAIL/TRYAGAIN    | UNAVAIL     | [4]    |
+     | UNAVAIL/TRYAGAIN'   | UNAVAIL     | [4]    |
+     | UNAVAIL/NOTFOUND    | UNAVAIL     | [4]    |
+     | UNAVAIL/UNAVAIL     | UNAVAIL     | [4]    |
+     ----------------------------------------------
+
+     [1] If the first response is a success we return success.
+	 This ignores the state of the second answer and in fact
+	 incorrectly sets errno and h_errno to that of the second
+	 answer.  However because the response is a success we ignore
+	 *errnop and *h_errnop (though that means you touched errno on
+	 success).  We are being conservative here and returning the
+	 likely IPv4 response in the first answer as a success.
+
+     [2] If the first response is a recoverable TRYAGAIN we return
+	 that instead of looking at the second response.  The
+	 expectation here is that we have failed to get an IPv4 response
+	 and should retry both queries.
+
+     [3] If the first response was not a SUCCESS and the second
+	 response is not NOTFOUND (had a SUCCESS, need to TRYAGAIN,
+	 or failed entirely e.g. TRYAGAIN' and UNAVAIL) then use the
+	 result from the second response, otherwise the first responses
+	 status is used.  Again we have some odd side-effects when the
+	 second response is NOTFOUND because we overwrite *errnop and
+	 *h_errnop that means that a first answer of NOTFOUND might see
+	 its *errnop and *h_errnop values altered.  Whether it matters
+	 in practice that a first response NOTFOUND has the wrong
+	 *errnop and *h_errnop is undecided.
+
+     [4] If the first response is UNAVAIL we return that instead of
+	 looking at the second response.  The expectation here is that
+	 it will have failed similarly e.g. configuration failure.
+
+     [5] Testing this code is complicated by the fact that truncated
+	 second response buffers might be returned as SUCCESS if the
+	 first answer is a SUCCESS.  To fix this we add symmetry to
+	 TRYAGAIN with the second response.  If the second response
+	 is a recoverable error we now return TRYAGIN even if the first
+	 response was SUCCESS.  */
+
   if (anslen1 > 0)
     status = gaih_getanswer_slice(answer1, anslen1, qname,
 				  &pat, &buffer, &buflen,
 				  errnop, h_errnop, ttlp,
 				  &first);
+
   if ((status == NSS_STATUS_SUCCESS || status == NSS_STATUS_NOTFOUND
        || (status == NSS_STATUS_TRYAGAIN
 	   /* We want to look at the second answer in case of an
@@ -1242,8 +1342,15 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 						     &pat, &buffer, &buflen,
 						     errnop, h_errnop, ttlp,
 						     &first);
+      /* Use the second response status in some cases.  */
       if (status != NSS_STATUS_SUCCESS && status2 != NSS_STATUS_NOTFOUND)
 	status = status2;
+      /* Do not return a truncated second response (unless it was
+	 unavoidable e.g. unrecoverable TRYAGAIN).  */
+      if (status == NSS_STATUS_SUCCESS
+	  && (status2 == NSS_STATUS_TRYAGAIN
+	      && *errnop == ERANGE && *h_errnop != NO_RECOVERY))
+	status = NSS_STATUS_TRYAGAIN;
     }
 
   return status;

resolv/res_query.c

@@ -396,6 +396,7 @@ __libc_res_nsearch(res_state statp,
 		  {
 		    free (*answerp2);
 		    *answerp2 = NULL;
+		    *nanswerp2 = 0;
 		    *answerp2_malloced = 0;
 		  }
 	}
@@ -447,6 +448,7 @@ __libc_res_nsearch(res_state statp,
 			  {
 			    free (*answerp2);
 			    *answerp2 = NULL;
+			    *nanswerp2 = 0;
 			    *answerp2_malloced = 0;
 			  }
 
@@ -521,6 +523,7 @@ __libc_res_nsearch(res_state statp,
 	  {
 	    free (*answerp2);
 	    *answerp2 = NULL;
+	    *nanswerp2 = 0;
 	    *answerp2_malloced = 0;
 	  }
 	if (saved_herrno != -1)