C Static Analyzer

The goal of this homework is to implement a static analyzer that detects potentially vulnerable data flows from source points to sink points (so called taint analysis) in C programs. Students will write the following functions in src/analyzer.ml based on the definitions in the lecture slides:

1. the function extract using extract_* (e.g., extract_cedge, extract_def, ..) that extract initial facts.
2. the functions derive_* (e.g., derive_kill, derive_path, ..) that derive inductive facts.
3. the function solve that computes the fixed point.

In short, replace all failwith "Not implemented" with your own implementation. You assume that input programs are always syntactically valid.

Notice that students are not permitted to change directory structures and types of the functions. All the functions you implement must have the same types as described in the module signatures. However, students are allowed to change let to let rec if needed.

Setup

This homework assumes that students set up the OCaml environment following the same instructions in the previous homework. After running make under the root directory of this repository, you can run the analyzer with the provided C programs:

# run the analyzer
$ ./analyzer test/example1.c

The command line argument specifies the input file path. Once you complete the implementation, the following output will be printed:

$ ./analyzer test/example1.c
Potential Error @ example1.c:4 (example1.c:5)

Each line reports a potentially vulnerable source and sink point at a certain source code location denoted by [file]:[source_line] ([file]:[sink_line]).

Instructions

1. Working with Control-flow Graph

The analyzer converts a C source file into a control-flow graph using function of_cfile in src/CFG.ml. In the graph representation of a program, each node has a unique ID (label) and is associated with a command (module Command in src/CFG.ml). Note that the IDs do not necessarily correspond to the line numbers in the C source code.

Here are the descriptions of each command (interchangably, node):

• Assign (l, v, e): The command l assigns an expression e to variable v.
• Source (l, v): The command l gets user input by calling the source function (declared in test/homework.h) and assign the value to variable v. Then, the variable v has a tainted value.
• Sanitizer (l, v, e): The command l sanitizes the expression e by calling the sanitizer function and assigns the sanitized value to variable v. Then, the value of v is not tainted anymore.
• Sink (l, e): The command l calls the sink function with an expression e as an argument. If e contains any tainted value, the analyzer reports this function call as a potentially vulnerable point.
• Branch l: The command l is the beginning of the branching out control-flows. For simplicity, we ignore the branch condition.
• Skip l: The command l is considered to be a no-op. All the other commands except for the above will be represented as Skip such as return.

Consider the following small C program with an if-then-else statement:

0:  x = source();
1:  if (cond) {
2:    y = 1;
    } else {
3:    y = sanitize(x);
    }
4:  sink(y);
5:  return 0;

The corresponding control-flow graph is as follows:

        ┌───────┐
        │   0   │
        └───┬───┘
        ┌───┴───┐
        │   1   │
        └───┬───┘
     ┌──────┴──────┐
 ┌───┴───┐     ┌───┴───┐
 │   2   │     │   3   │
 └───┬───┘     └───┬───┘
     └──────┬──────┘
        ┌───┴───┐
        │   4   │
        └───┬───┘
        ┌───┴───┐
        │   5   │
        └───────┘

The program will be translated to the following set of commands:

Source(0, x), Branch(1), Assign(2, y, 1), Sanitizer(3, y, x), Sink(4, y), and Skip(5).

After running the analyzer, the information will be shown in cfg.txt.

2. Extracting Basic Facts

Given a program, students will extract basic facts using the extract function. Function extract will extract the following basic facts:

• Relation CEdge denotes control-flow edges in a program. Control-flow edges specify the execution order. That is, tuple CEdge(a, b) represents the fact that b is executed right after command a. In the example, the extracted control-flow edges will be CEdge(0, 1), CEdge(1, 2), CEdge(1, 3), CEdge(2, 4), CEdge(3, 4)and CEdge(4, 5).

• Relation Source denotes the source commands. The fact Source(l) is directly derived from the command Source (l, v). In the example, Source(0) will be extracted.

• Relation Sanitizer denotes the sanitizer commands. The fact Sanitizer(l) is directly derived from the command Sanitizer (l, v, e). In the example, Sanitizer(4) will be extracted.

• Relation Sink denotes the sink commands. The fact Sink(l) is directly derived from the command Sink (l, e). In the example, Sink(6) will be extracted.

• Relation Kill denotes the definitions potentially killed at each point. The fact Kill(l1, l2) represents that the definition at l2 is killed by the definition at l1. In the example, Kill(2, 3) and Kill(3, 2) will be extracted.

• Relations Def and Use denote definitions and uses of values, respectively. The facts Def(l, v) and Use(l, v') are derived from the command Assign (l, v, e) where v' is a variable in e. Given an expression e, one can get all variables in e using function get_vars in the Command module. Here, Def(0, x), Def(2, y), Def(4, y), Use(4, x), and Use(6, y) will be extracted.

You will extract the facts by traversing control-flow graphs. The CFG module provides two functions:

• Function fold_vertex for traversing vertices of a graph
• Function fold_edges for traversing edges of a graph

The functions behave similarly to List.fold_left in the OCaml standard library. For their usage, see the example implementations of num_of_assignments and num_of_obvious_bugs in src/CFG.ml.

3. Deriving Inductive Facts

Students will write rules to derive inductive facts. Given a set of constraints, each function derive_* derives new facts following the definition in the lecture slides.

4. Fixpoint computation

The ultimate goal of the analysis is to derive all inductive facts until reaching a fixed point. The fixed point solving can be done by iteratevly applying the analysis rules (i.e., derive_*) to derive analysis facts until there is no more changes. Students will implement this solver in the solve function.

After running the analyzer, the analyzer will report potentially vulnerable points (i.e., Alarm). Also, the final set of derived facts will be shown in result.txt.

5. Format of Input Programs

Input programs in this homework are assumed to have only sub-features of the C language as follows:

• Programs only have a single function definition of main and do not have global variables.
• You should handle assignments, branches, and loops.
• There is no function call except for the ones to source, sanitizer and sink that are declared in this file.
• All the other instructions are considered to be no-op.

References

• OCaml Standard Library

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