auth: add pinentry support fido2 plugin

This CL adds pinentry support to ask for security key PIN.

Security key PINs may be required (by the manufacturer, or configured by
the user) to perform FIDO2 assertions.

PIN entry is done by calling pinentry command (or overridden by
LUCI_AUTH_PINENTRY environment variable), which we'll ask users to
install during onboarding.

Bug: 448235795
Change-Id: Ie87389330668dc5eaf8214699defec094757ca9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7004844
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Commit-Queue: Jiewei Qian <qjw@chromium.org>
Reviewed-by: Chenlin Fan <fancl@chromium.org>

luci_auth_fido2_plugin.py

@@ -29,10 +29,13 @@ from contextlib import contextmanager
 import dataclasses
 import json
 import logging
+import os
 import signal
+import subprocess
 import sys
 from threading import Event
-from typing import BinaryIO
+import traceback
+from typing import BinaryIO, Optional
 
 from fido2.client import DefaultClientDataCollector
 from fido2.client import Fido2Client, UserInteraction, WebAuthnClient
@@ -52,7 +55,7 @@ _PLUGIN_HEADER_SIZE = 4
 _EXIT_NO_FIDO2_DEVICES = 11
 _EXIT_ALL_ASSERTIONS_FAILED = 12
 _EXIT_NO_MATCHING_CRED = 13
-
+_EXIT_PINENTRY_FAILED = 14
 
 def read_full(r: BinaryIO, size: int) -> bytes:
     """Read an exact amount of data.
@@ -152,7 +155,72 @@ def encode_plugin_response(a: AuthenticationResponse) -> bytes:
     }).encode('utf-8')
 
 
-class DiscardInteraction(UserInteraction):
+def request_pin_pinentry() -> Optional[str]:
+    """Requests a PIN entry with pinentry program."""
+    try:
+        # Using pinentry to ask for PIN.
+        # https://www.gnupg.org/documentation/manuals/assuan/Client-requests.html
+        pinentry_path = os.environ.get('LUCI_AUTH_PINENTRY', 'pinentry')
+        proc = subprocess.Popen([pinentry_path],
+                                stdin=subprocess.PIPE,
+                                stdout=subprocess.PIPE,
+                                stderr=subprocess.PIPE,
+                                text=True)
+    except FileNotFoundError as e:
+        traceback.print_exception(e, file=sys.stderr)
+        logging.error("PIN requested, but can't find a pinentry program.")
+        logging.error(
+            "Please install a suitable pinentry program for your operating system."
+        )
+        sys.exit(_EXIT_PINENTRY_FAILED)
+
+    pinentry_input = """
+OPTION ttyname=/dev/tty
+SETTITLE Chromium Infra Auth
+SETDESC Enter the FIDO2 PIN for your security key, then touch your security key to continue.
+SETPROMPT PIN:
+GETPIN
+"""
+    stdout, stderr = proc.communicate(pinentry_input)
+
+    if proc.returncode != 0:
+        logging.error('pinentry failed: %s', stderr)
+        sys.exit(_EXIT_PINENTRY_FAILED)
+
+    for line in stdout.splitlines():
+        if line.startswith('D '):
+            return line[2:].strip()
+
+    logging.warning(
+        'An empty PIN was entered. Security key assertion may fail.')
+    return None
+
+
+def request_pin_mac() -> Optional[str]:
+    """Request a PIN entry with macOS's built-in `osascript` utility."""
+    osascript_command = (
+        'text returned of ('
+        'display'
+        '  dialog "Enter security key PIN.\\n\\nThen touch your security key to continue."'
+        '  default answer ""'
+        '  with hidden answer'
+        '  with title "Chromium Infra Auth"'
+        ')')
+
+    result = subprocess.run(
+        ['osascript', '-e', osascript_command],
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode != 0:
+        logging.error("PIN entry failed: %s", result.stderr)
+        return None
+
+    return result.stdout.strip()
+
+
+class PinEntryInteraction(UserInteraction):
     """Handler when user interaction is required.
 
     This plugin's stdin/stdout talks with git-credential-luci, so we fail
@@ -164,14 +232,18 @@ class DiscardInteraction(UserInteraction):
         sys.stderr.write("\nTouch your blinking security key to continue.\n\n")
 
     def request_pin(self, permissions, rp_id):
-        # This plugin shouldn't set assertion flags that will require
-        # PIN entry.
-        return None
+        """Ask for PIN entry with a GUI dialog by using a system tool.
+
+           We only handle Linux and MacOS here. We use Windows WebAuthn API
+           directly, which handles PIN entry if necessary.
+        """
+        if sys.platform == 'darwin':
+            return request_pin_mac()
+        return request_pin_pinentry()
 
     def request_uv(self, permissions, rp_id):
-        # Don't allow user verification (UV), because we don't allow PIN
-        # entry, UV will fail.
-        return False
+        # Allows PIN entry.
+        return True
 
 
 def get_clients(origin: str) -> list[tuple[WebAuthnClient, str]]:
@@ -190,7 +262,7 @@ def get_clients(origin: str) -> list[tuple[WebAuthnClient, str]]:
         logging.debug("Using WindowsClient")
         return [(WindowsClient(client_data_collector), "WindowsWebAuthn")]
 
-    user_interaction = DiscardInteraction()
+    user_interaction = PinEntryInteraction()
     clients = []
     for dev in CtapHidDevice.list_devices():
         desc = dev.descriptor