mod_saslauth, mod_c2s: Disable tls-server-end-point channel binding by default

This channel binding method is now enabled when a hash is manually set in the
config, or it attempts to discover the hash automatically if the value is the
special string "auto".

A related change to mod_c2s prevents complicated certificate lookups in the
client connection hot path - this work now happens only when this channel
binding method is used. I'm not aware of anything else that uses ssl_cfg (vs
ssl_ctx).

Rationale for disabling by default:

- Minor performance impact in automatic cert detection
- This method is weak against a leaked/stolen private key (other methods such
  as 'tls-exporter' would not be compromised in such a case)

Rationale for keeping the implementation:

- For some deployments, this may be the only method available (e.g. due to
  TLS offloading in another process/server).

2 files changed, 23 insertions, 14 deletions

diff --git a/plugins/mod_c2s.lua b/plugins/mod_c2s.lua
index d6c8a7b8..6587aa68 100644
--- a/plugins/mod_c2s.lua
+++ b/plugins/mod_c2s.lua
@@ -11,11 +11,9 @@ module:set_global();
 local add_task = require "prosody.util.timer".add_task;
 local new_xmpp_stream = require "prosody.util.xmppstream".new;
 local nameprep = require "prosody.util.encodings".stringprep.nameprep;
-local certmanager = require "prosody.core.certmanager";
 local sessionmanager = require "prosody.core.sessionmanager";
 local statsmanager = require "prosody.core.statsmanager";
 local st = require "prosody.util.stanza";
-local pm_get_tls_config_at = require "core.portmanager".get_tls_config_at;
 local sm_new_session, sm_destroy_session = sessionmanager.new_session, sessionmanager.destroy_session;
 local uuid_generate = require "prosody.util.uuid".generate;
 local async = require "prosody.util.async";
@@ -309,11 +307,6 @@ function listener.onconnect(conn)
 	if conn:ssl() then
 		session.secure = true;
 		session.encrypted = true;
-
-		local server = conn:server();
-		local tls_config = pm_get_tls_config_at(server:ip(), server:serverport());
-		local autocert = certmanager.find_host_cert(session.conn:socket():getsniname());
-		session.ssl_cfg = autocert or tls_config;
 		session.ssl_ctx = conn:sslctx();
 
 		-- Check if TLS compression is used
diff --git a/plugins/mod_saslauth.lua b/plugins/mod_saslauth.lua
index 4c0a5c1c..4a5b744d 100644
--- a/plugins/mod_saslauth.lua
+++ b/plugins/mod_saslauth.lua
@@ -19,6 +19,8 @@ local pem2der = require"util.x509".pem2der;
 local hashes = require"util.hashes";
 local ssl = require "ssl"; -- FIXME Isolate LuaSec from the rest of the code
 
+local certmanager = require "core.certmanager";
+local pm_get_tls_config_at = require "prosody.core.portmanager".get_tls_config_at;
 local usermanager_get_sasl_handler = require "prosody.core.usermanager".get_sasl_handler;
 
 local secure_auth_only = module:get_option_boolean("c2s_require_encryption", module:get_option_boolean("require_encryption", true));
@@ -330,17 +332,31 @@ module:hook("stream-features", function(event)
 				else
 					log("debug", "Channel binding 'tls-unique' not supported (by LuaSec?)");
 				end
-				local certfile = origin.ssl_cfg and origin.ssl_cfg.certificate;
-				-- FIXME .ssl_cfg is set by mod_tls and thus only available with starttls
-				if tls_server_end_point_hash then
+
+				local certfile;
+				if tls_server_end_point_hash == "auto" then
+					local ssl_cfg = origin.ssl_cfg;
+					if not ssl_cfg then
+						local server = origin.conn:server();
+						local tls_config = pm_get_tls_config_at(server:ip(), server:serverport());
+						local autocert = certmanager.find_host_cert(origin.conn:socket():getsniname());
+						ssl_cfg = autocert or tls_config;
+					end
+
+					certfile = ssl_cfg and ssl_cfg.certificate;
+					if certfile then
+						log("debug", "Channel binding 'tls-server-end-point' can be offered based on the certificate used");
+						sasl_handler:add_cb_handler("tls-server-end-point", tls_server_end_point);
+						channel_bindings:add("tls-server-end-point");
+					else
+						log("debug", "Channel binding 'tls-server-end-point' set to 'auto' but cannot determine cert");
+					end
+				elseif tls_server_end_point_hash then
 					log("debug", "Channel binding 'tls-server-end-point' can be offered with the configured certificate hash");
 					sasl_handler:add_cb_handler("tls-server-end-point", tls_server_end_point);
 					channel_bindings:add("tls-server-end-point");
-				elseif certfile then
-					log("debug", "Channel binding 'tls-server-end-point' can be offered based on the certificate used");
-					sasl_handler:add_cb_handler("tls-server-end-point", tls_server_end_point);
-					channel_bindings:add("tls-server-end-point");
 				end
+
 				sasl_handler["userdata"] = {
 					["tls-unique"] = origin.conn;
 					["tls-exporter"] = origin.conn;