pgsql: Reject GSSEncRequest after direct SSL connection

Reject GSSEncRequest after direct SSL connection

When a direct SSL connection was established, ProcessStartupPacket()
still accepted GSSEncRequest messages. The GSSAPI negotiation would
then use raw writes and reads, bypassing the TLS encryption layer.
After the GSS encryption was established, the connection continued to
use TLS. This could betray the HBA rules so as the backend does
protocol exchanges inconsistent with the connection policies in place,
with TLS taking priority over GSS in the backend.

The SSL negotiation path already guarded against attempts to request
SSL after a direct SSL request has been processed. The GSS path is now
guarded the same way when receiving a startup packet.

Reported-by: p4p3r <kbfanta(at)naver(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Jacob Champion <champion(dot)p(at)gmail(dot)com>
Backpatch-through: 17
Security: CVE-2026-14681

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/203a48209aa4179d2419fa3243b07998f0373d0a
Author: Michael Paquier <michael(at)paquier(dot)xyz>

Modified Files
--------------
src/backend/tcop/backend_startup.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)