libpq: Bail out during SSL/GSS negotiation errors

This commit changes libpq so that errors reported by the backend during
the protocol negotiation for SSL and GSS are discarded by the client, as
these may include bytes that could be consumed by the client and write
arbitrary bytes to a client's terminal.

A failure with the SSL negotiation now leads to an error immediately
reported, without a retry on any other methods allowed, like a fallback
to a plaintext connection.

A failure with GSS discards the error message received, and we allow a
fallback as it may be possible that the error is caused by a connection
attempt with a pre-11 server, GSS encryption having been introduced in
v12.  This was a problem only with v17 and newer versions; older
versions discard the error message already in this case, assuming a
failure caused by a lack of support for GSS encryption.

Author: Jacob Champion
Reviewed-by: Peter Eisentraut, Heikki Linnakangas, Michael Paquier
Security: CVE-2024-10977
Backpatch-through: 12

Author: michaelpq

doc/src/sgml/protocol.sgml

@@ -1526,10 +1526,10 @@ SELCT 1/0;<!-- this typo is intentional -->
 
    <para>
     The frontend should also be prepared to handle an ErrorMessage
-    response to SSLRequest from the server.  This would only occur if
-    the server predates the addition of <acronym>SSL</acronym> support
-    to <productname>PostgreSQL</productname>.  (Such servers are now very ancient,
-    and likely do not exist in the wild anymore.)
+    response to SSLRequest from the server. The frontend should not display
+    this error message to the user/application, since the server has not been
+    authenticated
+    (<ulink url="https://www.postgresql.org/support/security/CVE-2024-10977/">CVE-2024-10977</ulink>).
     In this case the connection must
     be closed, but the frontend might choose to open a fresh connection
     and proceed without requesting <acronym>SSL</acronym>.
@@ -1603,12 +1603,13 @@ SELCT 1/0;<!-- this typo is intentional -->
 
    <para>
     The frontend should also be prepared to handle an ErrorMessage
-    response to GSSENCRequest from the server.  This would only occur if
-    the server predates the addition of <acronym>GSSAPI</acronym> encryption
-    support to <productname>PostgreSQL</productname>.  In this case the
-    connection must be closed, but the frontend might choose to open a fresh
-    connection and proceed without requesting <acronym>GSSAPI</acronym>
-    encryption.
+    response to GSSENCRequest from the server.  The frontend should not display
+    this error message to the user/application, since the server has not been
+    authenticated
+    (<ulink url="https://www.postgresql.org/support/security/CVE-2024-10977/">CVE-2024-10977</ulink>).
+    In this case the connection must be closed, but the frontend might choose
+    to open a fresh connection and proceed without requesting
+    <acronym>GSSAPI</acronym> encryption.
    </para>
 
    <para>

src/interfaces/libpq/fe-connect.c

@@ -3168,16 +3168,13 @@ PQconnectPoll(PGconn *conn)
 					{
 						/*
 						 * Server failure of some sort, such as failure to
-						 * fork a backend process.  We need to process and
-						 * report the error message, which might be formatted
-						 * according to either protocol 2 or protocol 3.
-						 * Rather than duplicate the code for that, we flip
-						 * into AWAITING_RESPONSE state and let the code there
-						 * deal with it.  Note we have *not* consumed the "E"
-						 * byte here.
+						 * fork a backend process.  Don't bother retrieving
+						 * the error message; we should not trust it as the
+						 * server has not been authenticated yet.
 						 */
-						conn->status = CONNECTION_AWAITING_RESPONSE;
-						goto keep_going;
+						appendPQExpBuffer(&conn->errorMessage,
+										  libpq_gettext("server sent an error response during SSL exchange\n"));
+						goto error_return;
 					}
 					else
 					{