sslutil: improve messaging around unsupported protocols (issue5303)

There are various causes for the inability to negotiate common SSL/TLS
protocol between client and server. Previously, we had a single, not
very actionable warning message for all of them.

As people encountered TLS 1.0 servers in real life, it was quickly
obvious that the existing messaging was inadequate to help users
rectify the situation.

This patch makes the warning messages much more verbose in hopes of
making them more actionable while simultaneously encouraging users
and servers to adopt better security practices.

This messaging flirts with the anti-pattern of "never blame the
user" by signaling out poorly-configured servers. But if we're going to
disallow TLS 1.0 by default, I think we need to say *something* or
people are just going to blame Mercurial for not being able to connect.
The messaging tries to exonerate Mercurial from being the at fault
party by pointing out the server is the entity that doesn't support
proper security (when appropriate, of course).

mercurial/sslutil.py

@@ -417,11 +417,57 b' def wrapsocket(sock, keyfile, certfile, '
                       'how to configure Mercurial to avoid this error)\n'))
         # Try to print more helpful error messages for known failures.
         if util.safehasattr(e, 'reason'):
+            # This error occurs when the client and server don't share a
+            # common/supported SSL/TLS protocol. We've disabled SSLv2 and SSLv3
+            # outright. Hopefully the reason for this error is that we require
+            # TLS 1.1+ and the server only supports TLS 1.0. Whatever the
+            # reason, try to emit an actionable warning.
             if e.reason == 'UNSUPPORTED_PROTOCOL':
-                ui.warn(_('(could not negotiate a common protocol; see '
-                          'https://mercurial-scm.org/wiki/SecureConnections '
-                          'for how to configure Mercurial to avoid this '
-                          'error)\n'))
+                # We attempted TLS 1.0+.
+                if settings['protocolui'] == 'tls1.0':
+                    # We support more than just TLS 1.0+. If this happens,
+                    # the likely scenario is either the client or the server
+                    # is really old. (e.g. server doesn't support TLS 1.0+ or
+                    # client doesn't support modern TLS versions introduced
+                    # several years from when this comment was written).
+                    if supportedprotocols != set(['tls1.0']):
+                        ui.warn(_(
+                            '(could not communicate with %s using security '
+                            'protocols %s; if you are using a modern Mercurial '
+                            'version, consider contacting the operator of this '
+                            'server; see '
+                            'https://mercurial-scm.org/wiki/SecureConnections '
+                            'for more info)\n') % (
+                                serverhostname,
+                                ', '.join(sorted(supportedprotocols))))
+                    else:
+                        ui.warn(_(
+                            '(could not communicate with %s using TLS 1.0; the '
+                            'likely cause of this is the server no longer '
+                            'supports TLS 1.0 because it has known security '
+                            'vulnerabilities; see '
+                            'https://mercurial-scm.org/wiki/SecureConnections '
+                            'for more info)\n') % serverhostname)
+                else:
+                    # We attempted TLS 1.1+. We can only get here if the client
+                    # supports the configured protocol. So the likely reason is
+                    # the client wants better security than the server can
+                    # offer.
+                    ui.warn(_(
+                        '(could not negotiate a common security protocol (%s+) '
+                        'with %s; the likely cause is Mercurial is configured '
+                        'to be more secure than the server can support)\n') % (
+                        settings['protocolui'], serverhostname))
+                    ui.warn(_('(consider contacting the operator of this '
+                              'server and ask them to support modern TLS '
+                              'protocol versions; or, set '
+                              'hostsecurity.%s:minimumprotocol=tls1.0 to allow '
+                              'use of legacy, less secure protocols when '
+                              'communicating with this server)\n') %
+                            serverhostname)
+                    ui.warn(_(
+                        '(see https://mercurial-scm.org/wiki/SecureConnections '
+                        'for more info)\n'))
         raise
 
     # check if wrap_socket failed silently because socket had been

tests/test-https.t

@@ -469,20 +469,28 b' Clients talking same TLS versions work'
 Clients requiring newer TLS version than what server supports fail
 
   $ P="$CERTSDIR" hg id https://localhost:$HGPORT/
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.1+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: *unsupported protocol* (glob)
   [255]
 
   $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.1 id https://localhost:$HGPORT/
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.1+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: *unsupported protocol* (glob)
   [255]
   $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.2 id https://localhost:$HGPORT/
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.2+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: *unsupported protocol* (glob)
   [255]
   $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.2 id https://localhost:$HGPORT1/
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.2+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: *unsupported protocol* (glob)
   [255]
 
@@ -503,7 +511,9 b' The per-host config option by itself wor'
 
   $ P="$CERTSDIR" hg id https://localhost:$HGPORT/ \
   > --config hostsecurity.localhost:minimumprotocol=tls1.2
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.2+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: *unsupported protocol* (glob)
   [255]
 
@@ -514,7 +524,9 b' The per-host config option by itself wor'
   > localhost:minimumprotocol=tls1.2
   > EOF
   $ P="$CERTSDIR" hg -R copy-pull id https://localhost:$HGPORT/
-  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  (could not negotiate a common security protocol (tls1.2+) with localhost; the likely cause is Mercurial is configured to be more secure than the server can support)
+  (consider contacting the operator of this server and ask them to support modern TLS protocol versions; or, set hostsecurity.localhost:minimumprotocol=tls1.0 to allow use of legacy, less secure protocols when communicating with this server)
+  (see https://mercurial-scm.org/wiki/SecureConnections for more info)
   abort: error: [SSL: UNSUPPORTED_PROTOCOL] unsupported protocol (_ssl.c:590)
   [255]