derivepassphrase.git

Work around double-response for "query" extension from ssh-agent (OpenSSH 10.3)
Parent ea78e13
Marco Ricci

Marco Ricci commited on 2026-06-13 18:52:55
Zeige 1 geänderte Dateien mit 89 Einfügungen und 1 Löschungen. 

Detect and work around `ssh-agent` (OpenSSH 10.3 only) responding twice
when answering a "query" extension request: once with the expected
"extension response" or "success" message, and once with an
unexpected/additional empty "success" message.  We detect the extra
empty "success" message in a generic and protocol compliant way, and
re-align request-response-pair boundaries for any future requests.  From
the agent's perspective, the work-around does a read-only operation and
should not change any permanent agent state, whether there are extra
empty "success" messages or not.

As written, the work-around tests specifically for output from OpenSSH
10.3, but is otherwise agnostic to the "extension response" message
contents and could be easily adapted to trigger for other content
patterns.
src/derivepassphrase/ssh_agent/__init__.py
Zeige Datei @ 20bcd8f
... ... 
@@ -32,6 +32,79 @@ __all__ = ("SSHAgentClient",)
32 32 
 # a 4-byte/32-bit unsigned integer at the beginning.
33 33 
 HEAD_LEN = 4
34 34
35 
+# Needed for handling OpenSSH bug 3967.  See
36 
+# _check_for_extra_success_response_after_query_extension_request.
37 
+_OPENSSH_AGENT_10_3_QUERY_EXTENSION_RESPONSE_PAYLOAD = bytes.fromhex("""
38 
+00 00 00 05 71 75 65 72 79
39 
+00 00 00 18 73 65 73 73 69 6f 6e 2d
40 
+62 69 6e 64 40 6f 70 65 6e 73 73 68 2e 63 6f 6d
41 
+""")
42 
+
43 
+
44 
+def _check_for_extra_empty_success_response_after_query_extension_request(
45 
+    client: SSHAgentClient, response_data: bytes | bytearray
46 
+) -> None:
47 
+    """Check for extra `SUCCESS` responses after a "query" extension request.
48 
+
49 
+    OpenSSH's `ssh-agent` version 10.3 erroneously answers a "query"
50 
+    extension request with two response messages: the expected
51 
+    [`EXTENSION_RESPONSE`][_types.SSH_AGENT.EXTENSION_RESPONSE] or
52 
+    [`SUCCESS`][_types.SSH_AGENT.SUCCESS] message, and the unexpected
53 
+    extra `SUCCESS` message.  This causes a misalignment of the request
54 
+    and response messages if further requests are later issued to the
55 
+    agent.
56 
+
57 
+    ([OpenSSH acknowledges this as Bug 3967][OPENSSH_BZ3967], and
58 
+    intends to fix this in version 10.4.)
59 
+
60 
+    This callback is intended to run immediately after issuing a "query"
61 
+    extension request to the agent, and after reading its response.  If
62 
+    we detect the aforementioned OpenSSH behavior, then we re-align the
63 
+    request and response messages queue.  Otherwise, we do nothing.  In
64 
+    particular, it is safe to call us (at the aforementioned time) even
65 
+    on different SSH agents, or on versions of OpenSSH's agent that do
66 
+    not exhibit this bug.
67 
+
68 
+    [OPENSSH_BZ3967]: https://bugzilla.mindrot.org/show_bug.cgi?id=3967
69 
+
70 
+    Args:
71 
+        client:
72 
+            The client connected to the SSH agent that may or may not
73 
+            exhibit this OpenSSH behavior.  We assume that the client
74 
+            has just issued a "query" extension request, that the client
75 
+            has read and parsed the (first) response of the agent, that
76 
+            the response message was an `EXTENSION_RESPONSE` or
77 
+            a `SUCCESS` message, and that the response message (except
78 
+            for the response code) is exactly the contents of the
79 
+            `response_data` argument.
80 
+        response_data:
81 
+            The payload of the response message to a "query" extension
82 
+            request issued to the SSH agent immediately prior.
83 
+
84 
+    """
85 
+    if response_data == _OPENSSH_AGENT_10_3_QUERY_EXTENSION_RESPONSE_PAYLOAD:
86 
+        # We cannot blindly read from the agent socket to check
87 
+        # for a possible extra SUCCESS response: that would
88 
+        # deadlock if the extra response is missing.  Instead,
89 
+        # we issue a second "query" request, and depending on
90 
+        # whether we next see the extra empty SUCCESS response or not,
91 
+        # we know whether to discard an additional two response
92 
+        # messages or not.
93 
+        #
94 
+        # (Principally any request that does *not* ellicit
95 
+        # a SUCCESS response and which does not modify the
96 
+        # agent's state would do for this.)
97 
+        response_code, response_data2 = client.request(
98 
+            _types.SSH_AGENTC.REQUEST_IDENTITIES, b"", response_code=None
99 
+        )
100 
+        if response_code == _types.SSH_AGENT.SUCCESS and not response_data2:
101 
+            response_code, response_data2 = client._get_response_pair()  # noqa: SLF001
102 
+        assert response_code == _types.SSH_AGENT.IDENTITIES_ANSWER, (
103 
+            "failed to re-synchronize with agent via REQUEST_IDENTITIES "
104 
+            'after "query" EXTENSION_REQUEST: want IDENTITIES_ANSWER, '
105 
+            f"got {response_code!r}, {response_data2!r}"
106 
+        )
107 
+
35 108
36 109 
 class TrailingDataError(RuntimeError):
37 110 
     """The result contained trailing data."""
... ... 
@@ -736,6 +809,17 @@ class SSHAgentClient:
736 809
737 810 
         """
738 811 
         try:
812 
+            # The draft-miller-ssh-agent-14 (2024-04-15) version of the
813 
+            # SSH agent protocol specification introduced the
814 
+            # SSH_AGENT_EXTENSION_RESPONSE protocol message, and
815 
+            # suggested the use of this message in the response for
816 
+            # SSH_AGENTC_EXTENSION requests.  Older versions of the spec
817 
+            # used the SSH_AGENT_SUCCESS response, and even the newer
818 
+            # spec versions contain wording that allows such usage
819 
+            # across all SSH_AGENTC_EXTENSION requests.  Both response
820 
+            # codes are used in the wild (`ssh-agent`/OpenSSH 10.3,
821 
+            # `pageant`/PuTTY 0.83), so both of them need to be
822 
+            # supported for compatibility reasons.
739 823 
             response_data = self.request(
740 824 
                 _types.SSH_AGENTC.EXTENSION,
741 825 
                 self.string(b"query"),
... ... 
@@ -744,9 +828,13 @@ class SSHAgentClient:
744 828 
                     _types.SSH_AGENT.SUCCESS,
745 829 
                 },
746 830 
             )
831 
+            _check_for_extra_empty_success_response_after_query_extension_request(
832 
+                self, response_data
833 
+            )
747 834 
         except SSHAgentFailedError:
748 835 
             # Cannot query extension support.  Assume no extensions.
749 
-            # This isn't necessarily true, e.g. for OpenSSH's ssh-agent.
836 
+            # This isn't necessarily true, e.g. for OpenSSH's ssh-agent
837 
+            # 10.2 and older.
750 838 
             return frozenset()
751 839 
         extensions: set[bytes] = set()
752 840 
         msg = "Malformed response from SSH agent"
753 841