pgsql: libpq: reject extraneous data after SSL or GSS encryption handsh

libpq: reject extraneous data after SSL or GSS encryption handshake.

libpq collects up to a bufferload of data whenever it reads data from
the socket. When SSL or GSS encryption is requested during startup,
any additional data received with the server's yes-or-no reply
remained in the buffer, and would be treated as already-decrypted data
once the encryption handshake completed. Thus, a man-in-the-middle
with the ability to inject data into the TCP connection could stuff
some cleartext data into the start of a supposedly encryption-protected
database session.

This could probably be abused to inject faked responses to the
client's first few queries, although other details of libpq's behavior
make that harder than it sounds. A different line of attack is to
exfiltrate the client's password, or other sensitive data that might
be sent early in the session. That has been shown to be possible with
a server vulnerable to CVE-2021-23214.

To fix, throw a protocol-violation error if the internal buffer
is not empty after the encryption handshake.

Our thanks to Jacob Champion for reporting this problem.

Security: CVE-2021-23222

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/844b3169204c28cd086c1b4fae4a2cbdd0540640

Modified Files
--------------
doc/src/sgml/protocol.sgml | 28 ++++++++++++++++++++++++++++
src/interfaces/libpq/fe-connect.c | 26 ++++++++++++++++++++++++++
2 files changed, 54 insertions(+)