[tor-bugs] #10536 [Tor]: extend_cell_parse: don't try to parse payload if zero length

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#10536: extend_cell_parse: don't try to parse payload if zero length
-------------------------+---------------------
 Reporter:  cypherpunks  |          Owner:
     Type:  defect       |         Status:  new
 Priority:  normal       |      Milestone:
Component:  Tor          |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
-------------------------+---------------------
 If payload length for EXTEND2 cell is zero then extend_cell_parse() still
 tries to parse it by:
 {{{
 uint8_t n_specs = *payload
 }}}
 This bug should be harmless as
 {{{
  if (eop - payload < 2)
 }}}
 still true.

 Fixed code should be looking like:
 {{{
 uint8_t n_specs = 0;
 ...
 if (eop - payload > 0)
   ++payload;
 ...
 }}}
 or
 {{{
 uint8_t n_specs;
 ...
 if (eop - payload < 1)
   return -1;
 cell_out->cell_type = RELAY_COMMAND_EXTEND2;
 ++payload;
 }}}
 or like dropping cell for any cell type if zero length.
 {{{
 int
 extend_cell_parse(extend_cell_t *cell_out, const uint8_t command,
                   const uint8_t *payload, size_t payload_length)
 {
   const uint8_t *eop;

   memset(cell_out, 0, sizeof(*cell_out));
   if (payload_length > RELAY_PAYLOAD_SIZE || 0 == payload_length)
     return -1;
 }}}
 or something.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/10536>
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