Automatically updatable security barrier views

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Automatically updatable security barrier views

As part of the Row-security work, and as a useful feature on its own, it is desirable to support automatically updatable security_barrier views. Ordinary views are updatable in 9.3, but security_barrier views aren't considered "simple" views and are not updatable.

Status in 9.3

"simple" views are made updatable in 9.3 by flattening the view quals into the outer query. So given:

 SELECT n AS id, 'secret'||n AS secret FROM generate_series(1,10) n;
 CREATE VIEW t_even AS SELECT * FROM t WHERE id % 2 = 0;
 CREATE VIEW t_even_sb WITH (security_barrier) AS SELECT * FROM t WHERE id % 2 = 0;

you can update the simple non-security-barrier view, but can also steal values:

   RAISE NOTICE 'Saw secret=%',$1;
   RETURN true;
 $$ LANGUAGE plpgsql COST 1;
 test=>   UPDATE t_even SET id = id WHERE f_leak(secret);
 NOTICE:  Saw secret=secret1
 NOTICE:  Saw secret=secret3
 NOTICE:  Saw secret=secret5
 NOTICE:  Saw secret=secret7
 NOTICE:  Saw secret=secret9
 NOTICE:  Saw secret=secret2
 NOTICE:  Saw secret=secret4
 NOTICE:  Saw secret=secret6
 NOTICE:  Saw secret=secret8
 NOTICE:  Saw secret=secret10

You can't update the security barrier view at all:

 test=> UPDATE t_even_sb SET id = id WHERE f_leak(secret);
 ERROR:  cannot update view "t_even_sb"
 DETAIL:  Security-barrier views are not automatically updatable.
 HINT:  To enable updating the view, provide an INSTEAD OF UPDATE trigger or an unconditional ON UPDATE DO INSTEAD rule.

so there's no secure way to give a user update on only some rows of table. That's something we need for row-security and it's a feature that would be useful for users in general.

Issue with security barrier support

The existing updatable view code relies on flattening a view's quals into the outer query. So with the above example:

 UPDATE t_even SET id = id WHERE f_leak(secret);

gets view-expanded into something logically like:

 UPDATE (SELECT * FROM t WHERE id % 2 = 0) SET id = id WHERE f_leak(secret)

but the executor doesn't know how to deal with that so it can't be passed through as-is. Instead the subquery gets flattened out, producing:

 UPDATE t SET id = id WHERE id % 2 = 0 AND f_leak(secret)

which the executor can deal with, since it's a simple update of a relation. You can see this in the plan:

 test=# explain  UPDATE t_even SET id = id WHERE f_leak(secret);
                        QUERY PLAN                        
  Update on t  (cost=0.00..31.53 rows=2 width=42)
    ->  Seq Scan on t  (cost=0.00..31.53 rows=2 width=42)
          Filter: (f_leak(secret) AND ((id % 2) = 0))
 (3 rows)

See how the f_leak(secret) qual and id % 2 = 0 are at the same level? The executor will tend to pick f_leak(secret) to run first because its COST is artificially low.

This won't work with security-barrier views, since we have to make sure that the predicate id % 2 = 0 filters rows before any are passed to the user-supplied predicate, in this case f_leak(secret). So the current code refuses to operate on security_barrier views.

How automatically updatable views work in 9.3

Automatically updatable views were introduced by Dean Rasheed's code, committed by Tom in a99c42f291421572aef2b0a9360294c7d89b8bc7.

This code extends src/backend/rewrite/rewriteHandler.c, adding functions to:

  • get_view_query: Get the _RETURN rule from a view
  • view_has_instead_trigger: Check whether there's an INSTEAD trigger that should supersede auto-upd view
  • view_is_auto_updatable: Check whether a view is "simple" enough to be auto-upd. Rejects sb views.
  • relation_is_updatable: Used by information_schema. Only interesting for real views.
  • adjust_view_column_set: Map column permissions from view to sub-table. Only interesting if it is a real view.
  • rewriteTargetView: The guts. Rewrite a view into the outer query. It:
    • Makes sure the view is auto-updatable with view_is_auto_updatable
    • Finds the RTE for the view in the outer query
    • Gets the query that defines the view from its _RETURN rule using get_view_query
    • Locks the base relation (might be another view)update resnos in the targetlist to refer to columns of the base relation
    • Pull up the view (should be merged with optimizer pull-up code if possible):
    • Create a new target RTE describing the base rel and add it to the outer query's range table
      • Copy the quals to the outer query's qual list, fixing up varnos to point to the new target
      • Deal with permissions where view owner != query caller
      • Deal with column permissions
    • Fix up vars in outer rel to point to vars in new base rel instead of view, using ReplaceVarsFromTargetList (TODO: can we re-use this approach for RLS?)
    • Fix up everything else that references the view to point to the new base rel using ChangeVarNodes
    • Update resnos from target list to point to cols of base rel (UPDATE / INSERT) only.
    • Add the rewritten update/insert/delete, either to the beginning of the list of rewritten queries + rules (for insert) or after (for update/delete).

Also changes fireRIRrules

Notably, this means that the updatable view code doesn't actually add support for updating a view. Instead, it adds support for rewriting simple views to pull their quals up into the outer query and flatten the subquery.

We can't do that for updatable security barrier views.

The path to updatable s.b. views

To support updatable security barrier views we have to support UPDATE directly on a subquery without flattening the query. That's because we must still enforce the order of qual execution imposed by the security_barrier flag on the subquery. Anything else would require implementing a different approach to security barriers and introduce its own problems.

Updatable security barrier views via updatable subqueries

One approach is to allow the rewriter to expand views that're subquery targets. The rather extensive history on this wiki page covers an attempt to implement that.

The two fundamental issues that've arisen out of that effort are:

resultRelation vs source relation

Parts of the parser and executor are used to the resultRelation being the same RTE as the relation we get rows to feed into ExecModifyTable from. If updating a subquery this is no longer true. Instead of, after target view flattening:

      SeqScan<baseRel> WHERE quals1 AND quals2 AND quals3
      -> heap_modify_tuple(baseRel)
      -> RETURNING (mix of refs to baserel and other expressions)

we get:

    (SELECT ctid, ... FROM (
      (SELECT ctid, ... FROM (
        (SELECT ctid, ... FROM SeqScan<baseRel> WHERE quals1)
      WHERE quals2) 
    WHERE quals3)
    -> resultRelation(baserel)
    -> RETURNING (mix of refs to baserel and other expressions)

The RETURNING list may contain expressions referencing the result relation, which must reflect the final rel after the effect of any BEFORE triggers etc. Expressions may also contain subqueries, may refer to expressions in the view queries that aren't based on the result relation, etc. So we must rewrite the tlist to replace references to the result relation while leaving everything else untouched.

Docs refs

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