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><A
NAME="RULES-PRIVILEGES"
>37.4. Rules and Privileges</A
></H1
><P
> Due to rewriting of queries by the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
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.</P
><P
> 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 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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.</P
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
Nobody except him (and the database superusers) can access the
<TT
CLASS="LITERAL"
>phone_data</TT
> table. But because of the <TT
CLASS="COMMAND"
>GRANT</TT
>,
the secretary can run a <TT
CLASS="COMMAND"
>SELECT</TT
> on the
<TT
CLASS="LITERAL"
>phone_number</TT
> view. The rule system will rewrite the
<TT
CLASS="COMMAND"
>SELECT</TT
> from <TT
CLASS="LITERAL"
>phone_number</TT
> into a
<TT
CLASS="COMMAND"
>SELECT</TT
> from <TT
CLASS="LITERAL"
>phone_data</TT
>.
Since the user is the owner of
<TT
CLASS="LITERAL"
>phone_number</TT
> and therefore the owner of the rule, the
read access to <TT
CLASS="LITERAL"
>phone_data</TT
> is now checked against his
privileges and the query is permitted. The check for accessing
<TT
CLASS="LITERAL"
>phone_number</TT
> is also performed, but this is done
against the invoking user, so nobody but the user and the
secretary can use it.</P
><P
> 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 <TT
CLASS="LITERAL"
>phone_number</TT
> data through the secretary's view.
What the secretary cannot do is to create a view that directly
accesses <TT
CLASS="LITERAL"
>phone_data</TT
>. (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 <TT
CLASS="LITERAL"
>phone_number</TT
> view, he can revoke his access. Immediately, any
access to the secretary's view would fail.</P
><P
> 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 <TT
CLASS="LITERAL"
>phone_number</TT
> 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
<TT
CLASS="COMMAND"
>GRANT</TT
> command means, <SPAN
CLASS="QUOTE"
>"I trust you"</SPAN
>.
If someone you trust does the thing above, it's time to
think it over and then use <TT
CLASS="COMMAND"
>REVOKE</TT
>.</P
><P
> 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 unless the
<TT
CLASS="LITERAL"
>security_barrier</TT
> flag has been set. For example,
the following view is insecure:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE VIEW phone_number AS
SELECT person, phone FROM phone_data WHERE phone NOT LIKE '412%';</PRE
><P>
This view might seem secure, since the rule system will rewrite any
<TT
CLASS="COMMAND"
>SELECT</TT
> from <TT
CLASS="LITERAL"
>phone_number</TT
> into a
<TT
CLASS="COMMAND"
>SELECT</TT
> from <TT
CLASS="LITERAL"
>phone_data</TT
> and add the
qualification that only entries where <TT
CLASS="LITERAL"
>phone</TT
> 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 <CODE
CLASS="FUNCTION"
>NOT LIKE</CODE
> expression.
For example:
</P><PRE
CLASS="PROGRAMLISTING"
>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);</PRE
><P>
Every person and phone number in the <TT
CLASS="LITERAL"
>phone_data</TT
> table will be
printed as a <TT
CLASS="LITERAL"
>NOTICE</TT
>, because the planner will choose to
execute the inexpensive <CODE
CLASS="FUNCTION"
>tricky</CODE
> function before the
more expensive <CODE
CLASS="FUNCTION"
>NOT LIKE</CODE
>. Even if the user is
prevented from defining new functions, built-in functions can be used in
similar attacks. (For example, most casting functions include their
input values in the error messages they produce.)</P
><P
> 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 <TT
CLASS="LITERAL"
>SELECT</TT
>,
<TT
CLASS="LITERAL"
>INSERT</TT
>, <TT
CLASS="LITERAL"
>UPDATE</TT
>, and <TT
CLASS="LITERAL"
>DELETE</TT
> on
the <TT
CLASS="LITERAL"
>shoelace</TT
> view to someone else, but only
<TT
CLASS="LITERAL"
>SELECT</TT
> on <TT
CLASS="LITERAL"
>shoelace_log</TT
>. 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 <TT
CLASS="LITERAL"
>shoelace_log</TT
> is an unqualified
<TT
CLASS="LITERAL"
>INSERT</TT
>. This might not be true in more complex scenarios.</P
><P
> When it is necessary for a view to provide row-level security, the
<TT
CLASS="LITERAL"
>security_barrier</TT
> attribute should be applied to
the view. This prevents maliciously-chosen functions and operators from
being invoked on rows until after the view has done its work. For
example, if the view shown above had been created like this, it would
be secure:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE VIEW phone_number WITH (security_barrier) AS
SELECT person, phone FROM phone_data WHERE phone NOT LIKE '412%';</PRE
><P>
Views created with the <TT
CLASS="LITERAL"
>security_barrier</TT
> may perform
far worse than views created without this option. In general, there is
no way to avoid this: the fastest possible plan must be rejected
if it may compromise security. For this reason, this option is not
enabled by default.</P
><P
> The query planner has more flexibility when dealing with functions that
have no side effects. Such functions are referred to as <TT
CLASS="LITERAL"
>LEAKPROOF</TT
>, and
include many simple, commonly used operators, such as many equality
operators. The query planner can safely allow such functions to be evaluated
at any point in the query execution process, since invoking them on rows
invisible to the user will not leak any information about the unseen rows.
In contrast, a function that might throw an error depending on the values
received as arguments (such as one that throws an error in the event of
overflow or division by zero) are not leak-proof, and could provide
significant information about the unseen rows if applied before the security
view's row filters.</P
><P
> It is important to understand that even a view created with the
<TT
CLASS="LITERAL"
>security_barrier</TT
> option is intended to be secure only
in the limited sense that the contents of the invisible tuples will not be
passed to possibly-insecure functions. The user may well have other means
of making inferences about the unseen data; for example, they can see the
query plan using <TT
CLASS="COMMAND"
>EXPLAIN</TT
>, or measure the run time of
queries against the view. A malicious attacker might be able to infer
something about the amount of unseen data, or even gain some information
about the data distribution or most common values (since these things may
affect the run time of the plan; or even, since they are also reflected in
the optimizer statistics, the choice of plan). If these types of "covert
channel" attacks are of concern, it is probably unwise to grant any access
to the data at all.</P
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