pgsql: Restore PGREQUIRESSL recognition in libpq.

Restore PGREQUIRESSL recognition in libpq.

Commit 65c3bf19fd3e1f6a591618e92eb4c54d0b217564 moved handling of the,
already then, deprecated requiressl parameter into conninfo_storeval().
The default PGREQUIRESSL environment variable was however lost in the
change resulting in a potentially silent accept of a non-SSL connection
even when set. Its documentation remained. Restore its implementation.
Also amend the documentation to mark PGREQUIRESSL as deprecated for
those not following the link to requiressl. Back-patch to 9.3, where
commit 65c3bf1 first appeared.

Behavior has been more complex when the user provides both deprecated
and non-deprecated settings. Before commit 65c3bf1, libpq operated
according to the first of these found:

requiressl=1
PGREQUIRESSL=1
sslmode=*
PGSSLMODE=*

(Note requiressl=0 didn't override sslmode=*; it would only suppress
PGREQUIRESSL=1 or a previous requiressl=1. PGREQUIRESSL=0 had no effect
whatsoever.) Starting with commit 65c3bf1, libpq ignored PGREQUIRESSL,
and order of precedence changed to this:

last of requiressl=* or sslmode=*
PGSSLMODE=*

Starting now, adopt the following order of precedence:

last of requiressl=* or sslmode=*
PGSSLMODE=*
PGREQUIRESSL=1

This retains the 65c3bf1 behavior for connection strings that contain
both requiressl=* and sslmode=*. It retains the 65c3bf1 change that
either connection string option overrides both environment variables.
For the first time, PGSSLMODE has precedence over PGREQUIRESSL; this
avoids reducing security of "PGREQUIRESSL=1 PGSSLMODE=verify-full"
configurations originating under v9.3 and later.

Daniel Gustafsson

Security: CVE-2017-7485

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ed36c1fe172aec866d92d6e5071150a0ec901f8b

Modified Files
--------------
doc/src/sgml/libpq.sgml | 3 +++
src/interfaces/libpq/fe-connect.c | 24 ++++++++++++++++++++++++
2 files changed, 27 insertions(+)