… completion

MariaDB Connector/C does not distinguish [application-layer error
packets](https://mariadb.com/kb/en/err_packet) that it receives prior to TLS
handshake completion from those that it receives immediately after.

(A trivially modified server built from
dlenski/mariadb-server@demonstration_of_CONC-648_vulnerability
can easily be used to demonstrate this.)

Pre-TLS error packet received from this trivially modified server. This packet
should NOT be trusted to actually originate from the server:

    $ mariadb --ssl --ssl-verify-server-cert -uUsername -pVerySecretPassword -h CONC-648.vuln.demo.server.com
    ERROR 1815 (HY000): Internal error: Client will accept this error as genuine even if running with --ssl --ssl-verify-server-cert, and even though this error is sent in plaintext PRIOR TO TLS HANDSHAKE.

Post-(TLS handshake) error packet received from a normal MariaDB server upon
an attempt to connect with incorrect credentials.  This error packet CAN be
trusted to actually originate from the server, assuming transitive trust in
the TLS protocol implementation and PKI-based certificate validation:

    $ mariadb --ssl --ssl-verify-server-cert -uUsername -pWrongPassword -h $NORMAL_MARIADB10.6.14_SERVER
    ERROR 1045 (28000): Access denied for user 'Username'@'A.B.C.D' (using password: YES)

This client behavior opens up MariaDB Connector/C clients to an extremely
straightforward [downgrade attack](https://en.wikipedia.org/wiki/Downgrade_attack).

An on-path or pervasive attacker can inject errors into MariaDB
client→server connections that are intended to be protected by TLS, and the
client has no clear mechanism to distinguish such errors from errors that
actually come from the server.

An attacker could easily use this to DOS a client, or even influence its
behavior.  For example, consider a client application which is configured…

1. To use TLS with server certificate validation
   (`--ssl --ssl-verify-server-cert`), and
2. To wait for a back-off period and then *retry* connection attempts if the server
   responds with `ER_CON_COUNT_ERROR` ("Too many connections") from the
   server, and
3. To give up and shut down if its connection attempts fail with
   `ER_ACCESS_DENIED_ERROR` ("Access denied for user"), on the assumption
   that this is due to an incorrect or expired password, and cannot be
   resolved without human intervention.

An attacker could completely disable the retry mechanism of this application
by intercepting connection attempts and replying with
`ER_ACCESS_DENIED_ERROR` packets.

This patch modifies MariaDB Connector/C so that if the client is configured
to use TLS, error packets received prior to the completion of the TLS
handshake are untrusted, and are changed to a generic `CR_CONNECTION_ERROR`.

    $ mariadb --ssl --ssl-verify-server-cert -uUsername -pVerySecretPassword -h CONC-648.vuln.demo.server.com
    ERROR 2002 (HY000): Received error packet before completion of TLS handshake. Suppressing its details so that the client cannot vary its behavior based on this UNTRUSTED input.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

…s requested

The 2015 commit
mariadb-corporation@23895fbd4#diff-4339ae6506ef1fb201f6f836085257e72c191d2b4498df507d499fc30d891005
was described as "Fixed gnutls support", which is an extremely incomplete
description of what it actually does.

In that commit, the Connector/C client authentication plugin was modified to
abort following the initial server greeting packet if the client has
requested a secure transport (`mariadb --ssl`), but the server does not
advertise support for SSL/TLS.

However, there's a crucial caveat here:

If the client has requested secure transport (`mariadb --ssl`) but has not
requested verification of the server's TLS certificate
(`mariadb --ssl --ssl-verify-server-cert`), then the client will silently
accept an unencrypted connection, without even printing a diagnostic message.

Thus, any such client is susceptible to a trivial downgrade to an
unencrypted connection by an on-path attacker who simply flips the TLS/SSL
capability bit in the advertised server capabilities in the greeting packet.

The entire design of MariaDB's TLS support is severely flawed in terms of
user expectations surrounding secure-by-default connections: the `--ssl`
option SHOULD imply `--ssl-verify-server-cert` BY DEFAULT; if a client
actually wants a TLS connection that's susceptible to a trivial MITM'ed
by any pervasive or on-path attacker, that should be an exceptional case
(e.g. `--insecure-ssl-without-server-cert-verification`) rather than the
default.

Even without resolving the issue of no default verification of server
certificates, the issue of silent downgrade to unencrypted connections can
and should be resolved.

Backwards compatibility: This is an INTENTIONAL backwards-incompatible
change to prevent clients from being silently downgraded to unencrypted
connections, when they've specified `--ssl` and thus clearly indicated that
they do not want unencrypted connections.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

…_connect

Two reasons:

1. Reduction of attack surface

   As soon as the client receives the server's capability flags, it knows
   whether the server supports TLS/SSL.

   If the server does not support TLS/SSL, but the client expects and
   requires it, the client should immediately abort at this point in order
   to truncate any code paths by which it could inadvertently continue to
   communicate without TLS/SSL.

2. Separation of concerns

   Whether or not the server supports TLS/SSL encryption at the transport
   layer (TLS stands for TRANSPORT-layer security) is a logically separate
   issue from what APPLICATION-layer authentication modes the client and
   server support or should use.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

If the server responds to our initial connection with an error packet having
error number 4196 (`ER_SERVER_REDIRECT`), and an error string formatted as
`|Human-readable message|host[:port]`, then redirect accordingly.

This redirection is performed in `mthd_my_real_connect`, which appears to be
the most appropriate location given that it already contains code for
retrying/repeating a new server connection.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

This adds a new  boolean option, `follow-server-redirects` (ENABLED by
default), to allow the client to disable server redirection and simply
return an error message.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

Set the limit to 8 for now. This will prevent the client from getting stuck
in redirect loops.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.