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bgpd/security: invalid attr length sends NOTIFY with data overrun

Security issue: Quagga-2018-0543


* bgpd/bgp_attr.c: (bgp_attr_parse) An invalid attribute length is correctly
  checked, and a NOTIFY prepared.  The NOTIFY can include the incorrect
  received data with the NOTIFY, for debug purposes.  Commit
  c69698704806a9ac5 modified the code to do that just, and also send the
  malformed attr with the NOTIFY.  However, the invalid attribute length was
  used as the length of the data to send back.

  The result is a read past the end of data, which is then written to the
  NOTIFY message and sent to the peer.

  A configured BGP peer can use this bug to read up to 64 KiB of memory from
  the bgpd process, or crash the process if the invalid read is caught by
  some means (unmapped page and SEGV, or other mechanism) resulting in a DoS.

  This bug _ought_ /not/ be exploitable by anything other than the connected
  BGP peer, assuming the underlying TCP transport is secure.  For no BGP
  peer should send on an UPDATE with this attribute.  Quagga will not, as
  Quagga always validates the attr header length, regardless of type.

  However, it is possible that there are BGP implementations that do not
  check lengths on some attributes (e.g.  optional/transitive ones of a type
  they do not recognise), and might pass such malformed attrs on.  If such
  implementations exists and are common, then this bug might be triggerable
  by BGP speakers further hops away.  Those peers will not receive the
  NOTIFY (unless they sit on a shared medium), however they might then be
  able to trigger a DoS.

  Fix: use the valid bound to calculate the length.
Paul Jakma 4 years ago
1 changed files with 3 additions and 1 deletions
  1. 3 1

+ 3 - 1

@@ -2147,6 +2147,8 @@ bgp_attr_parse (struct peer *peer, struct attr *attr, bgp_size_t size,
   memset (seen, 0, BGP_ATTR_BITMAP_SIZE);
   /* End pointer of BGP attribute. */
+  assert (size <= stream_get_size (BGP_INPUT (peer)));
+  assert (size <= stream_get_endp (BGP_INPUT (peer)));
   endp = BGP_INPUT_PNT (peer) + size;
   /* Get attributes to the end of attribute length. */
@@ -2228,7 +2230,7 @@ bgp_attr_parse (struct peer *peer, struct attr *attr, bgp_size_t size,
           bgp_notify_send_with_data (peer,
-                                     startp, attr_endp - startp);
+                                     startp, endp - startp);