gh-84559: multiprocessing detect of forkserver cannot work due to missing hmac-sha256

Default to the spawn start method in that scenario.

Author: gpshead

Lib/multiprocessing/connection.py

@@ -23,6 +23,7 @@
 from . import util
 
 from . import AuthenticationError, BufferTooShort
+from . import context
 from .context import reduction
 _ForkingPickler = reduction.ForkingPickler
 
@@ -899,9 +900,9 @@ def _create_response(authkey, message):
             return hmac.new(authkey, message, 'md5').digest()
         except ValueError:
             # HMAC-MD5 is not available (FIPS mode?), fall back to
-            # HMAC-SHA2-256 modern protocol. The legacy server probably
+            # our modern HMAC-SHA* protocol. The legacy server probably
             # doesn't support it and will reject us anyways. :shrug:
-            digest_name = 'sha256'
+            digest_name = context._DIGEST_FOR_CONNECTION_HMAC
     # Modern protocol, indicate the digest used in the reply.
     response = hmac.new(authkey, message, digest_name).digest()
     return b'{%s}%s' % (digest_name.encode('ascii'), response)
@@ -932,10 +933,12 @@ def _verify_challenge(authkey, message, response):
         raise AuthenticationError('digest received was wrong')
 
 
-def deliver_challenge(connection, authkey: bytes, digest_name='sha256'):
+def deliver_challenge(connection, authkey: bytes, digest_name: str = ''):
     if not isinstance(authkey, bytes):
         raise ValueError(
             "Authkey must be bytes, not {0!s}".format(type(authkey)))
+    if not digest_name:
+        digest_name = context._DIGEST_FOR_CONNECTION_HMAC
     assert MESSAGE_LENGTH > _MD5ONLY_MESSAGE_LENGTH, "protocol constraint"
     message = os.urandom(MESSAGE_LENGTH)
     message = b'{%s}%s' % (digest_name.encode('ascii'), message)

Lib/multiprocessing/context.py

@@ -23,6 +23,12 @@ class TimeoutError(ProcessError):
 class AuthenticationError(ProcessError):
     pass
 
+# The default digest for multiprocessing.connection to use for auth.
+# We configure it here so that it can be tested when choosing a
+# default context without a circular import.
+# Must be the str of a value seen in connection._ALLOWED_DIGESTS.
+_DIGEST_FOR_CONNECTION_HMAC = 'sha256'
+
 #
 # Base type for contexts. Bound methods of an instance of this type are included in __all__ of __init__.py
 #
@@ -313,6 +319,21 @@ class ForkServerContext(BaseContext):
         def _check_available(self):
             if not reduction.HAVE_SEND_HANDLE:
                 raise ValueError('forkserver start method not available')
+            if not _test_if_connection_can_work():
+                raise ValueError(f'forkserver start method not available due to missing hmac-{_DIGEST_FOR_CONNECTION_HMAC}')
+
+    def _test_if_connection_can_work() -> bool:
+        # Authenticated connections required for forkserver using hmac.
+        # If the algorithm is unavailable (poor FIPS mode config?) at
+        # import time, we cannot default to forkserver.  If a user
+        # changes the _DIGEST_FOR_CONNECTION_HMAC to one that works in
+        # their strange config, the forkserver context will still work.
+        import hmac
+        try:
+            hmac.new(b'test-key'*8, b'', _DIGEST_FOR_CONNECTION_HMAC)
+        except ValueError:
+            return False
+        return True
 
     _concrete_contexts = {
         'fork': ForkContext(),
@@ -322,7 +343,8 @@ def _check_available(self):
     # bpo-33725: running arbitrary code after fork() is no longer reliable
     # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
     # gh-84559: We changed everyones default to a thread safeish one in 3.14.
-    if reduction.HAVE_SEND_HANDLE and sys.platform != 'darwin':
+    if (reduction.HAVE_SEND_HANDLE and sys.platform != 'darwin' and
+        _test_if_connection_can_work()):
         _default_context = DefaultContext(_concrete_contexts['forkserver'])
     else:
         _default_context = DefaultContext(_concrete_contexts['spawn'])

Misc/NEWS.d/next/Library/2024-12-01-07-46-23.gh-issue-84559.nj2wpQ.rst

@@ -0,0 +1,4 @@
+On hosts without the hmac-sha256 algorithm available (rare),
+:mod:`multiprocessing` will default to the ``"spawn"`` start method instead
+of ``"forkserver"`` which requires the algorithm for control socket
+authentication.