[tor-bugs] #22737 [Core Tor/Tor]: CID 1401591: errant memset in cell_pack().

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#22737: CID 1401591: errant memset in cell_pack().
-------------------------+-------------------------------------------------
     Reporter:  nickm    |      Owner:
         Type:  defect   |     Status:  new
     Priority:  High     |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  029-backport 030-backport
     Severity:  Normal   |  031-backport
Actual Points:           |  Parent ID:
       Points:           |   Reviewer:
      Sponsor:           |
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 There's a new issue that coverity found.  I believe it is harmless
 in practice, and I've had other network-team member sanity-check my
 analysis, but we should fix it anyway.

 The issue is in cell_pack, in connection_or.c.  Here is the function
 in question (comment is added):
 {{{
 void
 cell_pack(packed_cell_t *dst, const cell_t *src, int wide_circ_ids)
 {
   char *dest = dst->body;
   if (wide_circ_ids) {
     set_uint32(dest, htonl(src->circ_id));
     dest += 4;
   } else {
     set_uint16(dest, htons(src->circ_id));
     dest += 2;
     memset(dest+CELL_MAX_NETWORK_SIZE-2, 0, 2); // <--- PROBLEM memset()!
   }
   set_uint8(dest, src->command);
   memcpy(dest+1, src->payload, CELL_PAYLOAD_SIZE);
 }
 }}}
 Background: the format of a cell is like this:
 {{{
    xx circuit_id;
    u8 command;
    u8 payload[509];
 }}}
 For a long time, circuit_id was two bytes; now it's 4 bytes
 (whenever wide_circ_ids is true).  When we are using wide_circ_ids,
 then a packed cell is 514 bytes long; when we aren't, then a packed
 cell 512 bytes long.

 The packed_cell_t.body field is large enough to hold a packed cell
 of maximum size (514 bytes).  When we are sending a 512-byte cell,
 we want the final two bytes of the cell to be cleared. That's why we
 call the memset there.

 But as you can see with some arithmetic, the memset is wrong: it
 should come before "dest += 2", not after.  This mistake causes it
 to overwrite the two bytes _after_ packed_cell_t.body.

 This mistake causes two possible bugs.  I believe they are both
 harmless IRL.

 BUG 1: memory stomping

 When we call the memset, we are overwriting two 0 bytes past the end
 of packed_cell_t.body.  But I think that's harmless in practice,
 because the definition of packed_cell_t is:
 {{{
 #define CELL_MAX_NETWORK_SIZE 514
 // ...
 typedef struct packed_cell_t {
   TOR_SIMPLEQ_ENTRY(packed_cell_t) next;
   char body[CELL_MAX_NETWORK_SIZE];
   uint32_t inserted_time;
 } packed_cell_t;
 }}}
 So we will overwrite either two bytes of inserted_time, or two bytes
 of padding, depending on how the platform handles alignment.

 If we're overwriting padding, that's safe.

 If we are overwriting the inserted_time field, that's also safe: In
 every case where we call cell_pack() from connection_or.c, we ignore
 the inserted_time field.  When we call cell_pack() from relay.c, we
 don't set or use inserted_time until right after we have called
 cell_pack().  SO I believe we're safe in that case too.

 BUG 2: memory exposure

 The original reason for this memset was to avoid the possibility of
 accidentally leaking uninitialized ram to the network.  Now
 remember, if wide_circ_ids is false on a connection, we shouldn't
 actually be sending more than 512 bytes of packed_cell_t.body, so
 these two bytes can only leak to the network if there is another bug
 somewhere else in the code that sends more data than is correct.

 Fortunately, in relay.c, where we allocate packed_cell_t in
 packed_cell_new() , we allocate it with tor_malloc_zero(), which
 clears the RAM, right before we call cell_pack.  So those
 packed_cell_t.body bytes can't leak any information.

 That leaves the two calls to cell_pack() in connection_or.c, which
 use stack-alocated packed_cell_t instances.

 In or_handshake_state_record_cell(), we pass the cell's contents to
 crypto_digest_add_bytes().  When we do so, we get the number of
 bytes to pass using the same setting of wide_circ_ids as we passed
 to cell_pack().  So I believe that's safe.

 In connection_or_write_cell_to_buf(), we also use the same setting
 of wide_circ_ids in both calls.  So I believe that's safe too.

 CONCLUSION:

 I think this bug can't actually leak any data or overwrite anything
 of value.  Still we should fix it.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22737>
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