Elsa/Oink/Cqual++: Open-Source Static Analysis for C++ Scott McPeak Daniel Wilkerson work with Rob Johnson CodeCon 2006

Goals Build extensible infrastructure to Find certain categories of bugs –Exhaustively, within some constraints At compile time In real-world C and C++ programs Using composable analyses

Components Elkhound: Generalized LR Parser Generator Elsa: C++ Parser Oink: Whole-program dataflow Cqual++: Type qualifier analysis

Elkhound: GLR Parser Generator GLR eliminates the pain of LALR(1) –Unbounded lookahead –Allows ambiguous grammars! 10x faster than other GLR implementations –Novel combination of GLR and LALR(1) User-defined disambiguation –Early: during parsing –Late: after generating AST w/ambiguities

Example: ‘>’ ambiguity new C + 4 > + 5 ; Expr Type Expr Type

Example: ‘>’ ambiguity new C + 4 > + 5 ; Expr Type Expr Type unparenthesized ‘ > ’ symbol Correct Incorrect

Example: Type vs. Variable In C & C++, sometimes hard to tell whether a name refers to a type or a variable (a) & (b) Expr TypeExpr or

Example: Type vs. Variable In C & C++, sometimes hard to tell whether a name refers to a type or a variable int a; // hidden class C { int f(int b) { return (a) & (b); } typedef int a; // visible };

Elsa: Extensible C++ Front-end Parses ANSI C++ with GNU extensions Uses GLR to handle the ambiguities Extensible components: –flex lexer –Elkhound parser –AST defined with custom tool –Type checker

The Elsa Block Diagram Lexer preproc’d source Parser token stream Type Checker possibly ambiguous AST Post Process annotated unambiguous AST final AST No lexer feedback hack!

Extending the Syntax ANSI or GNU? Both! –Declarative language –Extend simply by concatenating nonterm ConditionalExp { -> Exp {...} -> Exp "?" Exp ":" Exp {...} } ANSI Base: nonterm ConditionalExp { -> Exp "?" ":" Exp {...} } GNU Extension:

Declarative Abstract Syntax class Statement (SourceLoc loc) { -> S_compound(ASTList stmts); -> S_if(Condition cond, Statement thenBranch, Statement elseBranch); -> S_while(Condition cond, Statement body); //... } superclass name superclass ctor parameter subclass names subclass ctor parameter subclass ctor list parameter

Extending the Abstract Syntax ANSI or GNU? Both! –Declarative language –Extend simply by concatenating ANSI Base:GNU Extension: class Statement { -> S_decl(Declaration decl); -> S_expr(Expression expr); -> S_if(...); -> S_for(...);  } class Statement { -> S_function(Function f); } GNU nested functions

Semantic Analysis Disambiguate Compute types Resolve overloading Insert implicit conversions Instantiate templates

Disambiguation Ambiguous syntax example: return (x)(y); S_return E_cast TypeId x E_funCall E_variable y ambiguity link expr typefuncarg

Lowered Output: Simplified C++ Original or Lowered output can be printed Lowering always done: –Templates are instantiated –Implicit type conversions inserted Lowering optionally done: –Implicit member functions created –Implicit ctor/dtor calls inserted

C++ or XML, In and Out Elsa C++ XML C++ XML First pass renders to a canonical form. Serialization commutes with lowering.

Cqual++: Dataflow Dataflow Analysis on Type Qualifiers Successor to Cqual: Jeff Foster, Alex Aiken char $tainted *getenv(); void printf(char $untainted *fmt,...); int main() { char *x = getenv(“foo”)); printf(x); }

Feature: Polymorphic Dataflow int f(int x) {return x;} int main() { int $tainted t =...; int a = f(t); int $untainted u = f(3); }

Feature: “Funky Qualifiers”: Fake Function Bodies char $_1_2 *strcat(char $_1_2 *dest, const char $_1 *src); int main() { char $tainted *x; char $untainted *y; strcat(y, x); } {1} ½ {1,2}

Feature: Separate Compilation for Scalability “Compile” each file to a dataflow graph –only flow behavior between external symbols matters –compress by finding smaller graph with same flow behavior; typically saves factor of 12 “Link” each graph –AST is gone at linking so we save even more space

Non-Feature: Cqual++ Is Not Flow-Sensitive q = p;... time passes... p->s = read_from_network(); use_in_untrusting_way(p->s); // does p == q still?? q->s = "innocuous"; use_in_trusting_way(p->s); $tainted??

What Exactly Is ‘Data-Flow’? char *launderString(char *in) { int len = strlen(in); char *out = malloc(len+1); for (int i=0; i<len; ++i) { out[i] = 0; for (int j=0; j<8; ++j) if (in[i] & (1<<j)) out[i] |= (1<<j); } out[len] = '\0'; return out; }

Application: Finding Format- String Vulnerabilities Printf() is an interpreter the format string is a program –%n writes number of bytes written to memory pointed to by the arg –ex: printf(“stuff%n”, p) means *p = 5 if no argument p, printf() writes through some pointer on the stack –do not allow untrusted data in first arg to printf

Application: Finding User-Kernel Vulnerabilities Kernel must check user pointers are valid –must point to memory mapped into user process’s address space –otherwise could manipulate the kernel data This is also a dataflow/taint analysis

Rob’s Cqual Linux User-Kernel Results , full config, 7 bugs, 275 false pos , full config, 6 bugs, 264 false pos. including other trials on same kernels: –found 17 different security vulnerabilites –found bugs missed by other tools and manually –all but one bug confirmed exploitable –significant “bug churn” across kernel versions

Linus’s “Sparse” Tool for User-Kernel Vulnerabilities Linus also has a tool using type qualifiers –it requires manual annotation of every var In contrast, Cqual++ infers the qualifiers –only sources and sinks need be annotated –and any “sanitizer” functions: Linus says this “is not the C way” –ok, he can write all the annotations

Future Application: Finding Character-Set Confusions Microsoft confusing ASCII and UCS2 Mozilla has 20-ish differnt charcter sets they should only flow together through conversion functions if array sizes differ, confusions can be a security hole too

Oink Vision: Composable Analysis Tools Compilers refuse to compile bugs –well, some classes of bugs –and you may have to wait until tomorrow morning to find out Correctness analysis is expected as part of any compiler toolchain The analyses are composable and extensible