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In particular, adhere to the rule that we must not modify any property of an SSL_SESSION object once it is (or might be) in a session cache. Such modifications are thread-unsafe and have been observed to cause crashes at runtime. To effect this change, standardize on the property that SSL_SESSION->ext.hostname is set only when that SNI value has been negotiated by both parties for use with that session. For session resumption this is trivially the case, so only new handshakes are affected. On the client, the new semantics are that the SSL->ext.hostname is for storing the value configured by the caller, and this value is used when constructing the ClientHello. On the server, SSL->ext.hostname is used to hold the value received from the client. Only if the SNI negotiation is successful will the hostname be stored into the session object; the server can do this after it sends the ServerHello, and the client after it has received and processed the ServerHello. This obviates the need to remove the hostname from the session object in case of failed negotiation (a change that was introduced in commit 9fb6cb810b769abbd60f11ef6e936a4e4456b19d in order to allow TLS 1.3 early data when SNI was present in the ClientHello but not the session being resumed), which was modifying cached sessions in certain cases. (In TLS 1.3 we always produce a new SSL_SESSION object for new connections, even in the case of resumption, so no TLS 1.3 handshakes were affected.) Reviewed-by: Matt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/6378)
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 |
|_____________________________| |_______________________________|