Config option for verifying federation certificates (MSC 1711) (#4967)

8 files changed, 158 insertions, 17 deletions

diff --git a/changelog.d/4967.feature b/changelog.d/4967.feature
new file mode 100644
index 0000000000..7f9f81f849
--- /dev/null
+++ b/changelog.d/4967.feature
@@ -0,0 +1 @@
+Implementation of [MSC1711](https://github.com/matrix-org/matrix-doc/pull/1711) including config options for requiring valid TLS certificates for federation traffic, the ability to disable TLS validation for specific domains, and the ability to specify your own list of CA certificates.
diff --git a/docs/MSC1711_certificates_FAQ.md b/docs/MSC1711_certificates_FAQ.md
index 8eb22656db..ebfb20f5c8 100644
--- a/docs/MSC1711_certificates_FAQ.md
+++ b/docs/MSC1711_certificates_FAQ.md
@@ -177,7 +177,6 @@ You can do this with a `.well-known` file as follows:
        on `customer.example.net:8000` it correctly handles HTTP requests with
        Host header set to `customer.example.net:8000`.
 
-
 ## FAQ
 
 ### Synapse 0.99.0 has just been released, what do I need to do right now?
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index ab02e8f20e..a7f6bf31ac 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -260,6 +260,40 @@ listeners:
 #
 #tls_private_key_path: "CONFDIR/SERVERNAME.tls.key"
 
+# Whether to verify TLS certificates when sending federation traffic.
+#
+# This currently defaults to `false`, however this will change in
+# Synapse 1.0 when valid federation certificates will be required.
+#
+#federation_verify_certificates: true
+
+# Skip federation certificate verification on the following whitelist
+# of domains.
+#
+# This setting should only be used in very specific cases, such as
+# federation over Tor hidden services and similar. For private networks
+# of homeservers, you likely want to use a private CA instead.
+#
+# Only effective if federation_verify_certicates is `true`.
+#
+#federation_certificate_verification_whitelist:
+#  - lon.example.com
+#  - *.domain.com
+#  - *.onion
+
+# List of custom certificate authorities for federation traffic.
+#
+# This setting should only normally be used within a private network of
+# homeservers.
+#
+# Note that this list will replace those that are provided by your
+# operating environment. Certificates must be in PEM format.
+#
+#federation_custom_ca_list:
+#  - myCA1.pem
+#  - myCA2.pem
+#  - myCA3.pem
+
 # ACME support: This will configure Synapse to request a valid TLS certificate
 # for your configured `server_name` via Let's Encrypt.
 #
diff --git a/synapse/config/server.py b/synapse/config/server.py
index c5e5679d52..cdf1e4d286 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -114,11 +114,13 @@ class ServerConfig(Config):
         # FIXME: federation_domain_whitelist needs sytests
         self.federation_domain_whitelist = None
         federation_domain_whitelist = config.get(
-            "federation_domain_whitelist", None
+            "federation_domain_whitelist", None,
         )
-        # turn the whitelist into a hash for speed of lookup
+
         if federation_domain_whitelist is not None:
+            # turn the whitelist into a hash for speed of lookup
             self.federation_domain_whitelist = {}
+
             for domain in federation_domain_whitelist:
                 self.federation_domain_whitelist[domain] = True
 
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index f0014902da..72dd5926f9 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -24,8 +24,10 @@ import six
 from unpaddedbase64 import encode_base64
 
 from OpenSSL import crypto
+from twisted.internet._sslverify import Certificate, trustRootFromCertificates
 
 from synapse.config._base import Config, ConfigError
+from synapse.util import glob_to_regex
 
 logger = logging.getLogger(__name__)
 
@@ -70,6 +72,53 @@ class TlsConfig(Config):
 
         self.tls_fingerprints = list(self._original_tls_fingerprints)
 
+        # Whether to verify certificates on outbound federation traffic
+        self.federation_verify_certificates = config.get(
+            "federation_verify_certificates", False,
+        )
+
+        # Whitelist of domains to not verify certificates for
+        fed_whitelist_entries = config.get(
+            "federation_certificate_verification_whitelist", [],
+        )
+
+        # Support globs (*) in whitelist values
+        self.federation_certificate_verification_whitelist = []
+        for entry in fed_whitelist_entries:
+            # Convert globs to regex
+            entry_regex = glob_to_regex(entry)
+            self.federation_certificate_verification_whitelist.append(entry_regex)
+
+        # List of custom certificate authorities for federation traffic validation
+        custom_ca_list = config.get(
+            "federation_custom_ca_list", None,
+        )
+
+        # Read in and parse custom CA certificates
+        self.federation_ca_trust_root = None
+        if custom_ca_list is not None:
+            if len(custom_ca_list) == 0:
+                # A trustroot cannot be generated without any CA certificates.
+                # Raise an error if this option has been specified without any
+                # corresponding certificates.
+                raise ConfigError("federation_custom_ca_list specified without "
+                                  "any certificate files")
+
+            certs = []
+            for ca_file in custom_ca_list:
+                logger.debug("Reading custom CA certificate file: %s", ca_file)
+                content = self.read_file(ca_file)
+
+                # Parse the CA certificates
+                try:
+                    cert_base = Certificate.loadPEM(content)
+                    certs.append(cert_base)
+                except Exception as e:
+                    raise ConfigError("Error parsing custom CA certificate file %s: %s"
+                                      % (ca_file, e))
+
+            self.federation_ca_trust_root = trustRootFromCertificates(certs)
+
         # This config option applies to non-federation HTTP clients
         # (e.g. for talking to recaptcha, identity servers, and such)
         # It should never be used in production, and is intended for
@@ -99,15 +148,15 @@ class TlsConfig(Config):
         try:
             with open(self.tls_certificate_file, 'rb') as f:
                 cert_pem = f.read()
-        except Exception:
-            logger.exception("Failed to read existing certificate off disk!")
-            raise
+        except Exception as e:
+            raise ConfigError("Failed to read existing certificate file %s: %s"
+                              % (self.tls_certificate_file, e))
 
         try:
             tls_certificate = crypto.load_certificate(crypto.FILETYPE_PEM, cert_pem)
-        except Exception:
-            logger.exception("Failed to parse existing certificate off disk!")
-            raise
+        except Exception as e:
+            raise ConfigError("Failed to parse existing certificate file %s: %s"
+                              % (self.tls_certificate_file, e))
 
         if not allow_self_signed:
             if tls_certificate.get_subject() == tls_certificate.get_issuer():
@@ -192,6 +241,40 @@ class TlsConfig(Config):
         #
         #tls_private_key_path: "%(tls_private_key_path)s"
 
+        # Whether to verify TLS certificates when sending federation traffic.
+        #
+        # This currently defaults to `false`, however this will change in
+        # Synapse 1.0 when valid federation certificates will be required.
+        #
+        #federation_verify_certificates: true
+
+        # Skip federation certificate verification on the following whitelist
+        # of domains.
+        #
+        # This setting should only be used in very specific cases, such as
+        # federation over Tor hidden services and similar. For private networks
+        # of homeservers, you likely want to use a private CA instead.
+        #
+        # Only effective if federation_verify_certicates is `true`.
+        #
+        #federation_certificate_verification_whitelist:
+        #  - lon.example.com
+        #  - *.domain.com
+        #  - *.onion
+
+        # List of custom certificate authorities for federation traffic.
+        #
+        # This setting should only normally be used within a private network of
+        # homeservers.
+        #
+        # Note that this list will replace those that are provided by your
+        # operating environment. Certificates must be in PEM format.
+        #
+        #federation_custom_ca_list:
+        #  - myCA1.pem
+        #  - myCA2.pem
+        #  - myCA3.pem
+
         # ACME support: This will configure Synapse to request a valid TLS certificate
         # for your configured `server_name` via Let's Encrypt.
         #
diff --git a/synapse/crypto/context_factory.py b/synapse/crypto/context_factory.py
index 49cbc7098f..59ea087e66 100644
--- a/synapse/crypto/context_factory.py
+++ b/synapse/crypto/context_factory.py
@@ -18,10 +18,10 @@ import logging
 from zope.interface import implementer
 
 from OpenSSL import SSL, crypto
-from twisted.internet._sslverify import _defaultCurveName
+from twisted.internet._sslverify import ClientTLSOptions, _defaultCurveName
 from twisted.internet.abstract import isIPAddress, isIPv6Address
 from twisted.internet.interfaces import IOpenSSLClientConnectionCreator
-from twisted.internet.ssl import CertificateOptions, ContextFactory
+from twisted.internet.ssl import CertificateOptions, ContextFactory, platformTrust
 from twisted.python.failure import Failure
 
 logger = logging.getLogger(__name__)
@@ -90,7 +90,7 @@ def _tolerateErrors(wrapped):
 
 
 @implementer(IOpenSSLClientConnectionCreator)
-class ClientTLSOptions(object):
+class ClientTLSOptionsNoVerify(object):
     """
     Client creator for TLS without certificate identity verification. This is a
     copy of twisted.internet._sslverify.ClientTLSOptions with the identity
@@ -127,9 +127,30 @@ class ClientTLSOptionsFactory(object):
     to remote servers for federation."""
 
     def __init__(self, config):
-        # We don't use config options yet
-        self._options = CertificateOptions(verify=False)
+        self._config = config
+        self._options_noverify = CertificateOptions()
+
+        # Check if we're using a custom list of a CA certificates
+        trust_root = config.federation_ca_trust_root
+        if trust_root is None:
+            # Use CA root certs provided by OpenSSL
+            trust_root = platformTrust()
+
+        self._options_verify = CertificateOptions(trustRoot=trust_root)
 
     def get_options(self, host):
         # Use _makeContext so that we get a fresh OpenSSL CTX each time.
-        return ClientTLSOptions(host, self._options._makeContext())
+
+        # Check if certificate verification has been enabled
+        should_verify = self._config.federation_verify_certificates
+
+        # Check if we've disabled certificate verification for this host
+        if should_verify:
+            for regex in self._config.federation_certificate_verification_whitelist:
+                if regex.match(host):
+                    should_verify = False
+                    break
+
+        if should_verify:
+            return ClientTLSOptions(host, self._options_verify._makeContext())
+        return ClientTLSOptionsNoVerify(host, self._options_noverify._makeContext())
diff --git a/synapse/http/federation/matrix_federation_agent.py b/synapse/http/federation/matrix_federation_agent.py
index 1334c630cc..b4cbe97b41 100644
--- a/synapse/http/federation/matrix_federation_agent.py
+++ b/synapse/http/federation/matrix_federation_agent.py
@@ -149,7 +149,7 @@ class MatrixFederationAgent(object):
             tls_options = None
         else:
             tls_options = self._tls_client_options_factory.get_options(
-                res.tls_server_name.decode("ascii")
+                res.tls_server_name.decode("ascii"),
             )
 
         # make sure that the Host header is set correctly
diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
index dcf184d3cf..e9eb662c4c 100644
--- a/tests/http/federation/test_matrix_federation_agent.py
+++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -39,6 +39,7 @@ from synapse.util.logcontext import LoggingContext
 from tests.http import ServerTLSContext
 from tests.server import FakeTransport, ThreadedMemoryReactorClock
 from tests.unittest import TestCase
+from tests.utils import default_config
 
 logger = logging.getLogger(__name__)
 
@@ -53,7 +54,7 @@ class MatrixFederationAgentTests(TestCase):
 
         self.agent = MatrixFederationAgent(
             reactor=self.reactor,
-            tls_client_options_factory=ClientTLSOptionsFactory(None),
+            tls_client_options_factory=ClientTLSOptionsFactory(default_config("test")),
             _well_known_tls_policy=TrustingTLSPolicyForHTTPS(),
             _srv_resolver=self.mock_resolver,
             _well_known_cache=self.well_known_cache,