Safe, Closure-free, Serializable functions

Mark S. Miller erights at google.com
Wed Sep 25 17:50:56 PDT 2013


On Wed, Sep 25, 2013 at 5:32 PM, Mark S. Miller <erights at google.com> wrote:

> Hi François, your goals here have a tremendous overlap with SES. In what
> ways is SES not satisfactory for these purposes?
>
> The best short-but-accurate summary of SES, sufficient for this question,
> is <http://research.google.com/pubs/pub40673.html> section 2.3.
>


I just looked and this section is indeed short -- shorter than your
question ;). Here it is:

  2.3 SES: Securing JavaScript

In a memory-safe object language with unforgeable object references
(protected pointers) and encapsulated objects, an object reference grants
the right to invoke the public interface of the object it designates. A
message sent on a reference both exercises this right and grants to the
receiving object the right to invoke the passed arguments.

In an object-capability (ocap) language [7], an object can cause effects on
the world outside itself only by using the references it holds. Some
objects are transitively immutable or powerless [8], while others might
cause effects. An object must not be given any powerful references by
default; any references it has implicit access to, such as
language-provided global variables, must be powerless. Under these rules,
granted references are the sole representation of permission.

Secure EcmaScript (SES) is an ocap subset of ES5. SES is lexically scoped,
its functions are encapsulated, and only the global variables on its
whitelist (including all globals defined by ES5) are accessible. Those
globals are unassignable, and all objects transitively reachable from them
are immutable, rendering all implicit access powerless.

SES supports defensive consistency [7]. An object is defensively
consistent when
it can defend its own invariants and provide correct service to its well
behaved clients, despite arbitrary or malicious misbehavior by its other
clients. SES has a formal semantics supporting automated verification of
some security properties of SES code [9]. The code in this paper uses the
following functions from the SES library:


   - def(obj) def ines a def ensible object. To support defensive
   consistency, the def function makes the properties of its argument
   read-only, likewise for all objects transitively reachable from there by
   reading properties. As a result, this subgraph of objects is effectively
   tamper proof. A tamper-proof record of encapsulated functions hiding lexical
   variables is a defensible object. In SES, if makePoint called def on the
   points it returns by saying “return def({...})”, it would make
   defensively consistent points.
   - confine(exprSrc, endowments) enables safe mobile code. The
confine function
   takes the source code string for a SES expression and an endowments record.
   It evaluates the expression in a new global environment consisting of the
   SES whitelisted (powerless) global variables and the properties of this
   endowments record. For example, confine(‘x + y’, {x: 3, y: 6}) returns 9.
   - Nat(allegedNumber) tests whether allegedNumber is indeed a primitive
   number, and whether it is a non-negative integer (a natural number) within
   the contiguous range of exactly representable integers in JavaScript. If
   so, it returns allegedNumber. Otherwise it throws an error.
   - var m = WeakMap() assigns to m a new empty weak map. WeakMaps are an
   ES6 extension (emulated by SES on ES5 browsers) supporting rights
   amplification [10]. Ignoring space usage, m is simply an
   object-identity-keyed table.  m.set(obj,val) associates obj’s identity
   as key with val as value, so m.get(obj) returns val and
m.delete(obj) removes
   this entry. These methods use only obj’s identity without interacting
   with obj.




-- 
    Cheers,
    --MarkM
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