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Current Techniques in Language-based Security David Walker COS 597B With slides stolen from: Steve Zdancewic University of Pennsylvania

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COS 597B2 Security Talks this Week My Dad's Computer, Microsoft, and the Future of Internet Security. Bill Cheswick, Lumeta, co-inventor of the Internet firewall Today at 4PM (We’ll stop class early) Extensible Semantics for XrML Vicky Weissman, Cornell Friday at 2:30, CS 402 XrML (the eXtensible rights Markup Language) is a popular language in which to write software licenses. See what it is all about and how to give it a semantics.

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COS 597B3 Types for Stack Inspection Want to do static checking of sec code Statically detect security failures. Eliminate redundant checks. Example of nonstandard type system for enforcing security properties. Type system based on work by Pottier, Skalka, and Smith: “A Systematic Approach to Static Access Control” Explain the type system by taking a detour through “security-passing” style. Wallach’s & Felten’s “Understanding Stack Inspection”

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COS 597B4 Security-passing Style Basic idea: Convert the “stack-crawling” form of stack inspection into a “permission-set passing style” Compute the set of current permissions at any point in the code. Make the set of permissions explicit as an extra parameter to functions (hence “security-passing style) Target language is a lambda calculus with a primitive datatype of sets.

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COS 597B5 Target Language: set Language syntax: e,f ::= expressions xvariable x.efunction e fapplication fail failure let x = e in f local decl. if p se then e else f member test seset expr. se ::= S perm. set se seunion se se intersection x

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COS 597B6 Translation: sec to set [[e]]R = “translation of e in domain R with s = current permissions” [[x]]R = x [[ x.e]]R = x. s.[[e]]R [[e f]]R = [[e]]R [[f]]R s [[let x = e in f]]R = let x = [[e]]R in [[f]R [[enable p in e]]R = let s = s ({p} R) in [[e]]R [[R’{e}]]R = let s = s R’ in [[e]]R’ [[check p e]]R= if p s then [[e]]R else fail [[test p then e1 else e2]]R= if p s then [[e1]]R else [[e2]]R Top level translation: [[e]] = [[e]]P{P/s}

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COS 597B7 Example Translation System = {“f1, “f2”, “f3”} Applet = {“f1”} h = System{enable “f1” in Applet{( x. System{check “f1” then write x}) “kwijibo”}}

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COS 597B8 Example Translation [[h]] = (* System *) let s = P {“f1”, “f2”, “f3”} in (* enable “f1” *) let s = s ({“f1”} {“f1”, “f2”, “f3”}) in (* Applet *) let s = s {“f1”} in ( x. s. (* System *) let s = s {“f1”, “f2”, “f3”} in if “f1” s then write x else fail) “kwijibo” s

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COS 597B9 Example Translation [[h]] = (* System *) let s = P {“f1”, “f2”, “f3”} in (* enable “f1” *) let s = s ({“f1”} {“f1”, “f2”, “f3”}) in (* Applet *) let s = s {“f1”} in ( x. s. (* System *) let s = s {“f1”, “f2”, “f3”} in if “f3” s then write x else fail) “kwijibo” s Change permission check

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COS 597B10 Stepping Back Have two formulations of stack inspection: “original” and “eager” Have a translation to a language that manipulates sets of permissions explicitly. Includes the “administrative” reductions that just compute sets of permissions. Similar computations can be done statically!

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COS 597B11 Typing Judgments R;S; |-- e : t Current protection domain Subset of current runtime perms Variable context Term Type

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COS 597B12 Form of types Only interesting (non administrative) change during compilation was for functions: [[ x.e]]R = x. s.[[e]]R Source type: t u Target type: t s u The 2 nd argument, is always a set, so we “specialize” the type to: t –{S} u

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COS 597B13 Types Types: t ::= types int, string, …base types t –{S} tfunctions

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COS 597B14 Simple Typing Rules R;S; |-- x : (x) R;S; |-- x.e : t1 –{S’} t2 R;S’; ,x:t1 |-- e : t2 Abstraction: Variables:

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COS 597B15 More Simple Typing Rules R;S; |-- e f : t’ R;S; |-- e : t –{S} t’ R;S; |-- f : t Application: R;S; |-- let x = e in f : t R;S; |-- e : u R;S; ,x:u |-- f : t Let:

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COS 597B16 Rule for Check R; S {p}; |-- check p then e : t R; S {p}; |-- e : t Note that this typing rule requires that the permission p is statically known to be available.

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COS 597B17 Typing Rules for Enable Enable fail: R;S; |-- enable p in e : t R;S; |-- e : t p R Enable succeed: R;S; |-- enable p in e : t R;S {p}; |-- e : t p R -- latter should be flagged as useless code

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COS 597B18 Rule for Test R;S; |-- test p then e else f: t R;S; |-- f : t R; S {p}; |-- e : t Check the first branch under assumption that p is present, check the else branch under assumption that p is absent.

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COS 597B19 Rule for Protection Domains R;S; |-- S’{e}: t S’;S S’; |-- e : t Intersect the permissions in the static protection domain with the current permission set.

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COS 597B20 Weakening (Subsumption) R;S; |-- e : t R;S’; |-- e : tS’ S It is always safe to “forget” permissions.

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COS 597B21 Type Safety Theorem: If P;P; |-- e : t then either e * v or e diverges. In particular: e never fails. (i.e. check always succeeds) Proof: Preservation & Progress.

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COS 597B22 Example: Good Code h = System{enable “f1” in Applet{( x. System{check “f1” then write x}) “kwijibo”}} Then P;S; |-- h : unit for any S

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COS 597B23 Example: Bad Code g = System{enable “f1” in Applet{( x. System{check “f2” then write x}) “kwijibo”}} Then R;S; |-- g : t is not derivable for any R,S, and t.

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COS 597B24 Static vs. Dynamic Checks ; ; |-- x.check p in x : int –{p} int Calling this function requires the static permission p: Only way to call it (assuming initial perms. are empty) is to put it in the scope of a dynamic test: test p then …can call it here… else …may not call it here…

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COS 597B25 Expressiveness This type system is very simple No subtyping No polymorphism Not algorithmic Hard to do inference Can add all of these features… See François Pottier’s paper for a nice example. Uses Didier Rémy’s row types to describe the sets of permission. Uses HM(X) – Hindley Milner with constraints Also shows how to derive a type system for the source language from the translation!

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COS 597B26 Conclusions Stack inspection is a complex security mechanism In practice, useful for preventing some attacks Formal reasoning useful for understanding what optimizations preserve semantics Type systems or program analysis have the potential to catch “obvious” security violations at compile time where they can easily be fixed

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