openssl/ssl/statem
Benjamin Kaduk e3743355e8 Don't modify resumed session objects
If s->hit is set, s->session corresponds to a session created on
a previous connection, and is a data structure that is potentially
shared across other SSL objects.  As such, there are thread-safety
issues with modifying the structure without taking its lock (and
of course all corresponding read accesses would also need to take
the lock as well), which have been observed to cause double-frees.

Regardless of thread-safety, the resumed session object is intended
to reflect parameters of the connection that created the session,
and modifying it to reflect the parameters from the current connection
is confusing.  So, modifications to the session object during
ClientHello processing should only be performed on new connections,
i.e., those where s->hit is not set.

The code mostly got this right, providing such checks when processing
SNI and EC point formats, but the supported groups (formerly
supported curves) extension was missing it, which is fixed by this commit.

However, TLS 1.3 makes the suppported_groups extension mandatory
(when using (EC)DHE, which is the normal case), checking for the group
list in the key_share extension processing.  But, TLS 1.3 only [0] supports
session tickets for session resumption, so the session object in question
is the output of d2i_SSL_SESSION(), and will not be shared across SSL
objects.  Thus, it is safe to modify s->session for TLS 1.3 connections.

[0] A psk_find_session callback can also be used, but the restriction that
each callback execution must produce a distinct SSL_SESSION structure
can be documented when the psk_find_session callback documentation is
completed.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4123)
2017-08-09 14:54:47 -05:00
..
extensions.c Fix SSL_set_tlsext_debug_callback/-tlsextdebug 2017-07-31 13:38:05 -04:00
extensions_clnt.c Move ossl_assert 2017-08-03 10:48:00 +01:00
extensions_cust.c Move ossl_assert 2017-08-03 10:48:00 +01:00
extensions_srvr.c Don't modify resumed session objects 2017-08-09 14:54:47 -05:00
README Add a state machine README 2015-10-30 08:38:18 +00:00
statem.c Switch from ossl_rand to DRBG rand 2017-08-03 09:23:28 -04:00
statem.h Remove special case code for SCTP reneg handling 2017-04-25 11:13:39 +01:00
statem_clnt.c Add a DRBG to each SSL object 2017-08-03 10:24:03 -04:00
statem_dtls.c Move ossl_assert 2017-08-03 10:48:00 +01:00
statem_lib.c Move ossl_assert 2017-08-03 10:48:00 +01:00
statem_locl.h Add TLSv1.3 server side external PSK support 2017-06-21 14:45:35 +01:00
statem_srvr.c Add a DRBG to each SSL object 2017-08-03 10:24:03 -04:00

State Machine Design
====================

This file provides some guidance on the thinking behind the design of the
state machine code to aid future maintenance.

The state machine code replaces an older state machine present in OpenSSL
versions 1.0.2 and below. The new state machine has the following objectives:
    - Remove duplication of state code between client and server
    - Remove duplication of state code between TLS and DTLS
    - Simplify transitions and bring the logic together in a single location
      so that it is easier to validate
    - Remove duplication of code between each of the message handling functions
    - Receive a message first and then work out whether that is a valid
      transition - not the other way around (the other way causes lots of issues
      where we are expecting one type of message next but actually get something
      else)
    - Separate message flow state from handshake state (in order to better
      understand each)
      - message flow state = when to flush buffers; handling restarts in the
        event of NBIO events; handling the common flow of steps for reading a
        message and the common flow of steps for writing a message etc
      - handshake state = what handshake message are we working on now
    - Control complexity: only the state machine can change state: keep all
      the state changes local to the state machine component

The message flow state machine is divided into a reading sub-state machine and a
writing sub-state machine. See the source comments in statem.c for a more
detailed description of the various states and transitions possible.

Conceptually the state machine component is designed as follows:

                        libssl
                           |
---------------------------|-----statem.h--------------------------------------
                           |
                    _______V____________________
                   |                            |
                   |    statem.c                |
                   |                            |
                   |    Core state machine code |
                   |____________________________|
        statem_locl.h     ^          ^
                 _________|          |_______
                |                            |
   _____________|____________   _____________|____________
  |                          | |                          |
  | statem_clnt.c            | | statem_srvr.c            |
  |                          | |                          |
  | TLS/DTLS client specific | | TLS/DTLS server specific |
  | state machine code       | | state machine code       |
  |__________________________| |__________________________|
               |        |_______________|__       |
               |        ________________|  |      |
               |       |                   |      |
   ____________V_______V________   ________V______V_______________
  |                             | |                               |
  | statem_both.c               | | statem_dtls.c                 |
  |                             | |                               |
  | Non core functions common   | | Non core functions common to  |
  | to both servers and clients | | both DTLS servers and clients |
  |_____________________________| |_______________________________|