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Superuser can permit passwordless connections on postgres_fdw

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Currently postgres_fdw doesn't permit a non-superuser to connect to a
foreign server without specifying a password, or to use an
authentication mechanism that doesn't use the password. This is to avoid
using the settings and identity of the user running Postgres.

However, this doesn't make sense for all authentication methods. We
therefore allow a superuser to set "password_required 'false'" for user
mappings for the postgres_fdw. The superuser must ensure that the
foreign server won't try to rely solely on the server identity (e.g.
trust, peer, ident) or use an authentication mechanism that relies on the
password settings (e.g. md5, scram-sha-256).

This feature is a prelude to better support for sslcert and sslkey
settings in user mappings.

Author: Craig Ringer.
Discussion: https://postgr.es/m/075135da-545c-f958-fed0-5dcb462d6dae@2ndQuadrant.com
This commit is contained in:
Andrew Dunstan
2019-12-20 16:23:34 +10:30
parent 16a4e4aecd
commit 6136e94dcb
5 changed files with 257 additions and 8 deletions
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24
doc/src/sgml/postgres-fdw.sgml
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@ -136,6 +136,30 @@
authentication, so always specify the <literal>password</literal> option
for user mappings belonging to non-superusers.
</para>
<para>
A superuser may override this check on a per-user-mapping basis by setting
the user mapping option <literal>password_required 'false'</literal>, e.g.
<programlisting>
ALTER USER MAPPING FOR some_non_superuser SERVER loopback_nopw
OPTIONS (ADD password_required 'false');
</programlisting>
To prevent unprivileged users from exploiting the authentication rights
of the unix user the postgres server is running as to escalate to superuser
rights, only the superuser may set this option on a user mapping.
</para>
<para>
Care is required to ensure that this does not allow the mapped
user the ability to connect as superuser to the mapped database per
CVE-2007-3278 and CVE-2007-6601. Don't set
<literal>password_required=false</literal>
on the <literal>public</literal> role. Keep in mind that the mapped
user can potentially use any client certificates,
<filename>.pgpass</filename>,
<filename>.pg_service.conf</filename> etc in the unix home directory of the
system user the postgres server runs as. They can also use any trust
relationship granted by authentication modes like <literal>peer</literal>
or <literal>ident</literal> authentication.
</para>
</sect3>
<sect3>