gh-108310: Fix CVE-2023-40217: Check for & avoid the ssl pre-close flaw (#108315)

Instances of `ssl.SSLSocket` were vulnerable to a bypass of the TLS handshake
and included protections (like certificate verification) and treating sent
unencrypted data as if it were post-handshake TLS encrypted data.

The vulnerability is caused when a socket is connected, data is sent by the
malicious peer and stored in a buffer, and then the malicious peer closes the
socket within a small timing window before the other peers’ TLS handshake can
begin. After this sequence of events the closed socket will not immediately
attempt a TLS handshake due to not being connected but will also allow the
buffered data to be read as if a successful TLS handshake had occurred.

Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org>

Lib/ssl.py

@@ -975,7 +975,7 @@ class SSLSocket(socket):
         )
         self = cls.__new__(cls, **kwargs)
         super(SSLSocket, self).__init__(**kwargs)
-        self.settimeout(sock.gettimeout())
+        sock_timeout = sock.gettimeout()
         sock.detach()
 
         self._context = context
@@ -994,9 +994,38 @@ class SSLSocket(socket):
             if e.errno != errno.ENOTCONN:
                 raise
             connected = False
+            blocking = self.getblocking()
+            self.setblocking(False)
+            try:
+                # We are not connected so this is not supposed to block, but
+                # testing revealed otherwise on macOS and Windows so we do
+                # the non-blocking dance regardless. Our raise when any data
+                # is found means consuming the data is harmless.
+                notconn_pre_handshake_data = self.recv(1)
+            except OSError as e:
+                # EINVAL occurs for recv(1) on non-connected on unix sockets.
+                if e.errno not in (errno.ENOTCONN, errno.EINVAL):
+                    raise
+                notconn_pre_handshake_data = b''
+            self.setblocking(blocking)
+            if notconn_pre_handshake_data:
+                # This prevents pending data sent to the socket before it was
+                # closed from escaping to the caller who could otherwise
+                # presume it came through a successful TLS connection.
+                reason = "Closed before TLS handshake with data in recv buffer."
+                notconn_pre_handshake_data_error = SSLError(e.errno, reason)
+                # Add the SSLError attributes that _ssl.c always adds.
+                notconn_pre_handshake_data_error.reason = reason
+                notconn_pre_handshake_data_error.library = None
+                try:
+                    self.close()
+                except OSError:
+                    pass
+                raise notconn_pre_handshake_data_error
         else:
             connected = True
 
+        self.settimeout(sock_timeout)  # Must come after setblocking() calls.
         self._connected = connected
         if connected:
             # create the SSL object