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

1 files changed, 11 insertions, 10 deletions

diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index 1b27d0a5479..5a3d6fdb5a3 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -1506,10 +1506,10 @@ 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>.
@@ -1619,12 +1619,13 @@ 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>