cros shell: Disable known_hosts for USB connections.

known_hosts isn't necessary for the USB link since there's no real risk
of a man-in-the-middle attack. Additionally, our USB connection reuses
the same IP address for each device so it triggers a false warning
whenever switching devices.

This CL adds support to remote_access for indicating when a device is
attached over USB, and disables known_hosts automatically in cros shell
in this case.

BUG=brillo:1170
TEST=cbuildbot/run_tests
TEST=`brillo shell --debug` to confirm known_hosts isn't used
TEST=cros shell <ip>
     # modify ~/.ssh/known_hosts to corrupt identity
     cros shell <ip>   # still gives known_hosts warning as expected

Change-Id: If8c95ce2a25d829954bc4a68788d2d4b1511a4d1
Reviewed-on: https://chromium-review.googlesource.com/275355
Trybot-Ready: David Pursell <dpursell@chromium.org>
Reviewed-by: Yiming Chen <yimingc@chromium.org>
Commit-Queue: David Pursell <dpursell@chromium.org>
Tested-by: David Pursell <dpursell@chromium.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/cli/cros/cros_shell.py b/cli/cros/cros_shell.py
index ed2a3f0d5..b5c29f22e 100644
--- a/cli/cros/cros_shell.py
+++ b/cli/cros/cros_shell.py
@@ -81,7 +81,8 @@ Quoting can be tricky; the rules are the same as with ssh:
         help='SSH identify file (private key).')
     parser.add_argument(
         '--no-known-hosts', action='store_false', dest='known_hosts',
-        default=True, help='Do not use a known_hosts file.')
+        default=True, help='Do not use a known_hosts file; always set'
+        ' for USB connections.')
     parser.add_argument(
         'command', nargs=argparse.REMAINDER,
         help='(optional) Command to execute on the device.')
@@ -105,7 +106,10 @@ Quoting can be tricky; the rules are the same as with ssh:
   def _ConnectSettings(self):
     """Generates the correct SSH connect settings based on our state."""
     kwargs = {'NumberOfPasswordPrompts': 2}
-    if self.known_hosts:
+    # USB has no risk of a man-in-the-middle attack so we can turn off
+    # known_hosts for any USB connection.
+    if (self.known_hosts and
+        self.device.connection_type != remote_access.CONNECTION_TYPE_USB):
       # Use the default known_hosts and our current key check setting.
       kwargs['UserKnownHostsFile'] = None
       kwargs['StrictHostKeyChecking'] = self.host_key_checking