openssl/ssl/statem
Matt Caswell e9cd6e763c Fix SSL_get_servername() and SNI behaviour
The SNI behaviour for TLSv1.3 and the behaviour of SSL_get_servername()
was not quite right, and not entirely consistent with the RFC.

The TLSv1.3 RFC explicitly says that SNI is negotiated on each handshake
and the server is not required to associate it with the session. This was
not quite reflected in the code so we fix that.

Additionally there were some additional checks around early_data checking
that the SNI between the original session and this session were
consistent. In fact the RFC does not require any such checks, so they are
removed.

Finally the behaviour of SSL_get_servername() was not quite right. The
behaviour was not consistent between resumption and normal handshakes,
and also not quite consistent with historical behaviour. We clarify the
behaviour in various scenarios and also attempt to make it match historical
behaviour as closely as possible.

Fixes #8822

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/10018)

(cherry picked from commit 7955c1f16e72dc944677fd1dbf4b1300e75f1c84)
2020-01-30 16:07:12 +00:00
..
extensions.c Fix SSL_get_servername() and SNI behaviour 2020-01-30 16:07:12 +00:00
extensions_clnt.c Reorganize local header files 2019-09-27 23:58:06 +02:00
extensions_cust.c Reorganize local header files 2019-09-27 23:58:06 +02:00
extensions_srvr.c Fix SSL_get_servername() and SNI behaviour 2020-01-30 16:07:12 +00:00
README Reorganize local header files 2019-09-27 23:58:06 +02:00
statem.c Reorganize local header files 2019-09-27 23:58:06 +02:00
statem.h Tolerate encrypted or plaintext alerts 2018-08-08 10:16:58 +01:00
statem_clnt.c Reorganize local header files 2019-09-27 23:58:06 +02:00
statem_dtls.c Reorganize local header files 2019-09-27 23:58:06 +02:00
statem_lib.c ssl/statem/statem_lib.c: make servercontext/clientcontext arrays of chars instead of char pointers to fix EBCDIC builds. 2019-10-10 16:23:31 +02:00
statem_local.h Reorganize local header files 2019-09-27 23:58:06 +02:00
statem_srvr.c Parse large GOST ClientKeyExchange messages 2019-12-14 23:08:09 +03:00

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_local.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 |
  |_____________________________| |_______________________________|