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Doc: word-smith the discussion of secure schema usage patterns.

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Rearrange the discussion of user-private schemas so that details
applying only to upgraded-from-pre-v15 databases are in a follow-on
paragraph, not in the main description of how to set up this pattern.
This seems a little clearer even today, and it'll get more so as
pre-v15 systems fade into the sunset.

Wording contributions from Robert Haas, Tom Lane, Noah Misch.

Discussion: https://postgr.es/m/CA+TgmoYUHsfp90inEMAP0yNr7Y_L6EphPH1YOon1JKtBztXHyQ@mail.gmail.com
This commit is contained in:
Tom Lane 2022-12-01 12:10:25 -05:00
parent 1dd6700f44
commit ef2d7c6f0b
1 changed files with 28 additions and 25 deletions
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doc/src/sgml/ddl.sgml
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@ -3211,40 +3211,43 @@ REVOKE CREATE ON SCHEMA public FROM PUBLIC;
query that database would take protective action at the beginning of each
session. Specifically, they would begin each session by
setting <varname>search_path</varname> to the empty string or otherwise
removing non-superuser-writable schemas
removing schemas that are writable by non-superusers
from <varname>search_path</varname>. There are a few usage patterns
easily supported by the default configuration:
<itemizedlist>
<listitem>
<!-- "DROP SCHEMA public" is inferior to this REVOKE, because pg_dump
doesn't preserve that DROP.
A database owner can attack the database's users via "CREATE SCHEMA
<para>
Constrain ordinary users to user-private schemas.
To implement this pattern, first ensure that no schemas have
public <literal>CREATE</literal> privileges. Then, for every user
needing to create non-temporary objects, create a schema with the
same name as that user, for example
<literal>CREATE SCHEMA alice AUTHORIZATION alice</literal>.
(Recall that the default search path starts
with <literal>$user</literal>, which resolves to the user
name. Therefore, if each user has a separate schema, they access
their own schemas by default.) This pattern is a secure schema
usage pattern unless an untrusted user is the database owner or
holds the <literal>CREATEROLE</literal> privilege, in which case no
secure schema usage pattern exists.
</para>
<!-- A database owner can attack the database's users via "CREATE SCHEMA
trojan; ALTER DATABASE $mydb SET search_path = trojan, public;". A
CREATEROLE user can issue "GRANT $dbowner TO $me" and then use the
database owner attack. -->
<para>
Constrain ordinary users to user-private schemas. To implement this,
first issue <literal>REVOKE CREATE ON SCHEMA public FROM
PUBLIC</literal>. Then, for every user needing to create non-temporary
objects, create a schema with the same name as that user. Recall that
the default search path starts with <literal>$user</literal>, which
resolves to the user name. Therefore, if each user has a separate
schema, they access their own schemas by default. After adopting this
pattern in a database where untrusted users had already logged in,
consider auditing the public schema for objects named like objects in
schema <literal>pg_catalog</literal>. This pattern is a secure schema
usage pattern unless an untrusted user is the database owner or holds
the <literal>CREATEROLE</literal> privilege, in which case no secure
schema usage pattern exists.
</para>
<para>
If the database originated in an upgrade
from <productname>PostgreSQL</productname> 14 or earlier,
the <literal>REVOKE</literal> is essential. Otherwise, the default
configuration follows this pattern; ordinary users can create only
temporary objects until a privileged user furnishes a schema.
In <productname>PostgreSQL</productname> 15 and later, the default
configuration supports this usage pattern. In prior versions, or
when using a database that has been upgraded from a prior version,
you will need to remove the public <literal>CREATE</literal>
privilege from the <literal>public</literal> schema (issue
<literal>REVOKE CREATE ON SCHEMA public FROM PUBLIC</literal>).
Then consider auditing the <literal>public</literal> schema for
objects named like objects in schema <literal>pg_catalog</literal>.
</para>
<!-- "DROP SCHEMA public" is inferior to this REVOKE, because pg_dump
doesn't preserve that DROP. -->
</listitem>
<listitem>