sslutil: abort when unable to verify peer connection (BC)

Previously, when we connected to a server and were unable to verify
its certificate against a trusted certificate authority we would
issue a warning and continue to connect. This is obviously not
great behavior because the x509 certificate model is based upon
trust of specific CAs. Failure to enforce that trust erodes security.
This behavior was defined several years ago when Python did not
support loading the system trusted CA store (Python 2.7.9's
backports of Python 3's improvements to the "ssl" module enabled
this).

This commit changes behavior when connecting to abort if the peer
certificate can't be validated. With an empty/default Mercurial
configuration, the peer certificate can be validated if Python is
able to load the system trusted CA store. Environments able to load
the system trusted CA store include:

* Python 2.7.9+ on most platforms and installations
* Python 2.7 distributions with a modern ssl module (e.g. RHEL7's
  patched 2.7.5 package)
* Python shipped on OS X

Environments unable to load the system trusted CA store include:

* Python 2.6
* Python 2.7 on many existing Linux installs (because they don't
  ship 2.7.9+ or haven't backported modern ssl module)
* Python 2.7.9+ on some installs where Python is unable to locate
  the system CA store (this is hopefully rare)

Users of these Pythongs will need to configure Mercurial to load the
system CA store using web.cacerts. This should ideally be performed
by packagers (by setting web.cacerts in the global/system hgrc file).
Where Mercurial packagers aren't setting this, the linked URL in the
new abort message can contain instructions for users.

In the future, we may want to add more code for finding the system
CA store. For example, many Linux distributions have the CA store
at well-known locations (such as /etc/ssl/certs/ca-certificates.crt
in the case of Ubuntu). This will enable CA loading to "just work"
on more Python configurations and will be best for our users since
they won't have to change anything after upgrading to a Mercurial
with this patch.

We may also want to consider distributing a trusted CA store with
Mercurial. Although we should think long and hard about that because
most systems have a global CA store and Mercurial should almost
certainly use the same store used by everything else on the system.

mercurial/sslutil.py

@@ -207,8 +207,9 b' def _hostsettings(ui, hostname):'
         else:
             # At this point we don't have a fingerprint, aren't being
             # explicitly insecure, and can't load CA certs. Connecting
-            # at this point is insecure. But we do it for BC reasons.
-            # TODO abort here to make secure by default.
+            # is insecure. We allow the connection and abort during
+            # validation (once we have the fingerprint to print to the
+            # user).
             s['verifymode'] = ssl.CERT_NONE
 
     assert s['verifymode'] is not None
@@ -413,11 +414,16 b' def validatesocket(sock):'
                             'fingerprint %s') % (host, nice),
                           hint=_('check %s configuration') % section)
 
+    # Security is enabled but no CAs are loaded. We can't establish trust
+    # for the cert so abort.
     if not sock._hgstate['caloaded']:
-        ui.warn(_('warning: certificate for %s not verified '
-                  '(set hostsecurity.%s:certfingerprints=%s or web.cacerts '
-                  'config settings)\n') % (host, host, nicefingerprint))
-        return
+        raise error.Abort(
+            _('unable to verify security of %s (no loaded CA certificates); '
+              'refusing to connect') % host,
+            hint=_('see https://mercurial-scm.org/wiki/SecureConnections for '
+                   'how to configure Mercurial to avoid this error or set '
+                   'hostsecurity.%s:fingerprints=%s to trust this server') %
+                   (host, nicefingerprint))
 
     msg = _verifycert(peercert2, host)
     if msg:

tests/test-https.t

@@ -103,10 +103,15 b' Defining both per-host certificate and a'
 
   $ DISABLECACERTS="--config devel.disableloaddefaultcerts=true"
 
-clone via pull
+Inability to verify peer certificate will result in abort
 
   $ hg clone https://localhost:$HGPORT/ copy-pull $DISABLECACERTS
-  warning: certificate for localhost not verified (set hostsecurity.localhost:certfingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 or web.cacerts config settings)
+  abort: unable to verify security of localhost (no loaded CA certificates); refusing to connect
+  (see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error or set hostsecurity.localhost:fingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 to trust this server)
+  [255]
+
+  $ hg clone --insecure https://localhost:$HGPORT/ copy-pull
+  warning: connection security to localhost is disabled per current settings; communication is susceptible to eavesdropping and tampering
   requesting all changes
   adding changesets
   adding manifests
@@ -133,7 +138,13 b' pull without cacert'
   $ echo "changegroup = printenv.py changegroup" >> .hg/hgrc
   $ hg pull $DISABLECACERTS
   pulling from https://localhost:$HGPORT/
-  warning: certificate for localhost not verified (set hostsecurity.localhost:certfingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 or web.cacerts config settings)
+  abort: unable to verify security of localhost (no loaded CA certificates); refusing to connect
+  (see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error or set hostsecurity.localhost:fingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 to trust this server)
+  [255]
+
+  $ hg pull --insecure
+  pulling from https://localhost:$HGPORT/
+  warning: connection security to localhost is disabled per current settings; communication is susceptible to eavesdropping and tampering
   searching for changes
   adding changesets
   adding manifests

tests/test-patchbomb-tls.t

@@ -63,8 +63,9 b' Without certificates:'
   (using smtps)
   sending mail: smtp host localhost, port * (glob)
   (verifying remote certificate)
-  warning: certificate for localhost not verified (set hostsecurity.localhost:certfingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 or web.cacerts config settings)
-  sending [PATCH] a ...
+  abort: unable to verify security of localhost (no loaded CA certificates); refusing to connect
+  (see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error or set hostsecurity.localhost:fingerprints=sha256:62:09:97:2f:97:60:e3:65:8f:12:5d:78:9e:35:a1:36:7a:65:4b:0e:9f:ac:db:c3:bc:6e:b6:a3:c0:16:e0:30 to trust this server)
+  [255]
 
 With global certificates: