Rules and Privileges

PostgreSQL 7.4.18 Documentation
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# 34.4. Rules and Privileges

Due to rewriting of queries by the PostgreSQL rule system, other tables/views than those used in the original query get accessed. When update rules are used, this can include write access to tables.

Rewrite rules don't have a separate owner. The owner of a relation (table or view) is automatically the owner of the rewrite rules that are defined for it. The PostgreSQL rule system changes the behavior of the default access control system. Relations that are used due to rules get checked against the privileges of the rule owner, not the user invoking the rule. This means that a user only needs the required privileges for the tables/views that he names explicitly in his queries.

For example: A user has a list of phone numbers where some of them are private, the others are of interest for the secretary of the office. He can construct the following:

    CREATE TABLE phone_data (person text, phone text, private boolean);
    CREATE VIEW phone_number AS
        SELECT person, phone FROM phone_data WHERE NOT private;
    GRANT SELECT ON phone_number TO secretary;

Nobody except him (and the database superusers) can access the `phone_data` table. But because of the `GRANT`, the secretary can run a `SELECT` on the `phone_number` view. The rule system will rewrite the `SELECT` from `phone_number` into a `SELECT` from `phone_data` and add the qualification that only entries where `private` is false are wanted. Since the user is the owner of `phone_number` and therefore the owner of the rule, the read access to `phone_data` is now checked against his privileges and the query is permitted. The check for accessing `phone_number` is also performed, but this is done against the invoking user, so nobody but the user and the secretary can use it.

The privileges are checked rule by rule. So the secretary is for now the only one who can see the public phone numbers. But the secretary can setup another view and grant access to that to the public. Then, anyone can see the `phone_number` data through the secretary's view. What the secretary cannot do is to create a view that directly accesses `phone_data`. (Actually he can, but it will not work since every access will be denied during the permission checks.) And as soon as the user will notice, that the secretary opened his `phone_number` view, he can revoke his access. Immediately, any access to the secretary's view would fail.

One might think that this rule-by-rule checking is a security hole, but in fact it isn't. But if it did not work this way, the secretary could set up a table with the same columns as `phone_number` and copy the data to there once per day. Then it's his own data and he can grant access to everyone he wants. A `GRANT` command means, "I trust you". If someone you trust does the thing above, it's time to think it over and then use `REVOKE`.

This mechanism also works for update rules. In the examples of the previous section, the owner of the tables in the example database could grant the privileges `SELECT`, `INSERT`, `UPDATE`, and `DELETE` on the `shoelace` view to someone else, but only `SELECT` on `shoelace_log`. The rule action to write log entries will still be executed successfully, and that other user could see the log entries. But he cannot create fake entries, nor could he manipulate or remove existing ones.

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