Rules and Privileges

PostgreSQL 9.0.13 Documentation
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# 37.4. 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, CASE WHEN NOT private THEN phone END AS phone
        FROM phone_data;
    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`. 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`.

Note that while views can be used to hide the contents of certain columns using the technique shown above, they cannot be used to reliably conceal the data in unseen rows. For example, the following view is insecure:

    CREATE VIEW phone_number AS
        SELECT person, phone FROM phone_data WHERE phone NOT LIKE '412%';

This view might seem secure, since the rule system will rewrite any `SELECT` from `phone_number` into a `SELECT` from `phone_data` and add the qualification that only entries where `phone` does not begin with 412 are wanted. But if the user can create his or her own functions, it is not difficult to convince the planner to execute the user-defined function prior to the `NOT LIKE` expression.

    CREATE FUNCTION tricky(text, text) RETURNS bool AS $$
    BEGIN
        RAISE NOTICE '% => %', $1, $2;
        RETURN true;
    END
    $$ LANGUAGE plpgsql COST 0.0000000000000000000001;
    SELECT * FROM phone_number WHERE tricky(person, phone);

Every person and phone number in the `phone_data` table will be printed as a `NOTICE`, because the planner will choose to execute the inexpensive `tricky` function before the more expensive `NOT LIKE`. Even if the user is prevented from defining new functions, built-in functions can be used in similar attacks. (For example, casting functions include their inputs in the error messages they produce.)

Similar considerations apply to 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. In this case, there is no possibility of subverting the rules by convincing the planner to alter the order of operations, because the only rule which references `shoelace_log` is an unqualified `INSERT`. This might not be true in more complex scenarios.

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