Allow "KEYINFO --list"

Currently, this command is blocked.  GnuPG detects that the agent
connection is restricted and doesn't try to use it, while Sequoia
Chameleon does not implement the fallback and is unable to list secret
keys or decrypt messages.  Furthermore, gpg prints
"gpg: problem with fast path key listing: Forbidden - ignored", which
Mutt interprets as a prompt the user must respond to.  This causes the
user to need to press enter twice to send a signed email.

Fix these problems by allowing this request.  The request does not work
over a restricted connection, so an unrestricted connection must be
used.  However, the filtering done by split-gpg2 is far stronger than
the access checks in gpg-agent so there is no loss of security.

Fixes: QubesOS/qubes-issues#9529

splitgpg2/__init__.py

@@ -454,10 +454,14 @@ async def connect_agent(self) -> None:
 
         dirs = subprocess.check_output(
             ['gpgconf', *self.homedir_opts(), '--list-dirs', '-o/dev/stdout'])
-        if self.allow_keygen:
-            socket_field = b'agent-socket:'
-        else:
-            socket_field = b'agent-extra-socket:'
+        # Do not use the restricted socket.
+        # Sequoia Chameleon is unable to list secret keys or decrypt messages,
+        # and gpg prints "gpg: problem with fast path key listing: Forbidden - ignored",
+        # which causes Mutt to require the user to press "Enter" again before sending
+        # a message.
+        # The filtering done by split-gpg2 is far stronger than anything the agent does
+        # internally.
+        socket_field = b'agent-socket:'
         # search for agent-socket:/run/user/1000/gnupg/S.gpg-agent
         agent_socket_path = [d.split(b':', 1)[1] for d in dirs.splitlines()
                              if d.startswith(socket_field)][0]
@@ -641,14 +645,24 @@ def fake_respond(self, response: bytes) -> None:
 
     @staticmethod
     def verify_keygrip_arguments(min_count: int, max_count: int,
-            untrusted_args: Optional[bytes]) -> bytes:
+                                 untrusted_args: Optional[bytes],
+                                 allow_list: bool) -> bytes:
         if untrusted_args is None:
             raise Filtered
-        args_regex = re.compile(rb'\A[0-9A-F]{40}( [0-9A-F]{40}){%d,%d}\Z' %
-                                (min_count-1, max_count-1))
+        if allow_list and untrusted_args.startswith(b'--list'):
+            if untrusted_args == b'--list':
+                pass
+            elif untrusted_args[6] == 61: # ASCII '='
+                # 1000 is the default value used by gpg2
+                sanitize_int(untrusted_args[len(b'--list='):], 1, 1000)
+            else:
+                raise Filtered
+        else:
+            args_regex = re.compile(rb'\A[0-9A-F]{40}( [0-9A-F]{40}){%d,%d}\Z' %
+                                    (min_count-1, max_count-1))
 
-        if args_regex.match(untrusted_args) is None:
-            raise Filtered
+            if args_regex.match(untrusted_args) is None:
+                raise Filtered
         return untrusted_args
 
     def sanitize_key_desc(self, untrusted_args: bytes) -> bytes:
@@ -731,21 +745,11 @@ async def command_HAVEKEY(self, untrusted_args: Optional[bytes]) -> None:
         if untrusted_args is None:
             raise Filtered
         # upper keygrip limit is arbitary
-        if untrusted_args.startswith(b'--list'):
-            if b'=' in untrusted_args:
-                # 1000 is the default value used by gpg2
-                limit = sanitize_int(untrusted_args[len(b'--list='):], 1, 1000)
-                args = b'--list=%d' % limit
-            else:
-                if untrusted_args != b'--list':
-                    raise Filtered
-                args = b'--list'
-        else:
-            args = self.verify_keygrip_arguments(1, 200, untrusted_args)
+        args = self.verify_keygrip_arguments(1, 200, untrusted_args, True)
         await self.send_agent_command(b'HAVEKEY', args)
 
     async def command_KEYINFO(self, untrusted_args: Optional[bytes]) -> None:
-        args = self.verify_keygrip_arguments(1, 1, untrusted_args)
+        args = self.verify_keygrip_arguments(1, 1, untrusted_args, True)
         await self.send_agent_command(b'KEYINFO', args)
 
     async def command_GENKEY(self, untrusted_args: Optional[bytes]) -> None:
@@ -785,12 +789,12 @@ async def command_GENKEY(self, untrusted_args: Optional[bytes]) -> None:
         await self.send_agent_command(b'GENKEY', b' '.join(args))
 
     async def command_SIGKEY(self, untrusted_args: Optional[bytes]) -> None:
-        args = self.verify_keygrip_arguments(1, 1, untrusted_args)
+        args = self.verify_keygrip_arguments(1, 1, untrusted_args, False)
         await self.send_agent_command(b'SIGKEY', args)
         await self.setkeydesc(args)
 
     async def command_SETKEY(self, untrusted_args: Optional[bytes]) -> None:
-        args = self.verify_keygrip_arguments(1, 1, untrusted_args)
+        args = self.verify_keygrip_arguments(1, 1, untrusted_args, False)
         await self.send_agent_command(b'SETKEY', args)
         await self.setkeydesc(args)
 
@@ -996,7 +1000,7 @@ async def command_READKEY(self, untrusted_args: Optional[bytes]) -> None:
             raise Filtered
         if untrusted_args.startswith(b'-- '):
             untrusted_args = untrusted_args[3:]
-        args = self.verify_keygrip_arguments(1, 1, untrusted_args)
+        args = self.verify_keygrip_arguments(1, 1, untrusted_args, False)
 
         await self.send_agent_command(b'READKEY', b'-- ' + args)