Re: Improving RLS planning

On Tue, Oct 25, 2016 at 5:58 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> The alternative I'm now thinking about pursuing is to get rid of the
> conversion of RLS quals to subqueries.  Instead, we can label individual
> qual clauses with security precedence markings.  Concretely, suppose we
> add an "int security_level" field to struct RestrictInfo.  The semantics
> of this would be that a qual with a lower security_level value must be
> evaluated before a qual with a higher security_level value, unless the
> latter qual is leakproof.  (It would likely also behoove us to add a
> "leakproof" bool field to struct RestrictInfo, to avoid duplicate
> leakproof-ness checks on quals.  But that's just an optimization.)
>
> In the initial implementation, quals coming from a RangeTblEntry's
> securityQuals field would have security_level 0, quals coming from
> anywhere else would have security_level 1; except that if we know
> there are no security quals anywhere (ie not Query->hasRowSecurity),
> we could give all quals security_level 0.  (I think this exception
> may be worth making because there's no need to test leakproofness
> for a qual with security level 0; it could never be a candidate
> for security delay anyway.)
>
> Having done that much, I think all we need in order to get rid of
> RLS subqueries, and just stick RLS quals into their relation's
> baserestrictinfo list, are two rules:
>
> 1. When selecting potential indexquals, a RestrictInfo can be considered
> for indexqual use only if it is leakproof or has security_level <= the
> minimum among the table's baserestrictinfo clauses.
>
> 2. In order_qual_clauses, sort first by security_level and second by cost.

This might work for RLS policies, if they can only reference a single
table, but I can't see how it's going to work for security barrier
views.  For example, consider CREATE VIEW v WITH (security_barrier) AS
SELECT * FROM x, y WHERE x.a = y.a followed by SELECT * FROM v WHERE
leak(somefield).  somefield is necessarily coming from either x or y,
and you can't let it be passed to leak() except for rows where the
join qual has been satisfied.

-- 
Robert Haas
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