# Analyzing data flow in Java¶

## Overview¶

This topic describes how data flow analysis is implemented in the CodeQL libraries for Java and includes examples to help you write your own data flow queries. The following sections describe how to utilize the libraries for local data flow, global data flow, and taint tracking.

For a more general introduction to modeling data flow, see Introduction to data flow analysis with CodeQL.

## Local data flow¶

Local data flow is data flow within a single method or callable. Local data flow is usually easier, faster, and more precise than global data flow, and is sufficient for many queries.

### Using local data flow¶

The local data flow library is in the module DataFlow, which defines the class Node denoting any element that data can flow through. Nodes are divided into expression nodes (ExprNode) and parameter nodes (ParameterNode). It is possible to map between data flow nodes and expressions/parameters using the member predicates asExpr and asParameter:

class Node {
/** Gets the expression corresponding to this node, if any. */
Expr asExpr() { ... }

/** Gets the parameter corresponding to this node, if any. */
Parameter asParameter() { ... }

...
}


or using the predicates exprNode and parameterNode:

/**
* Gets the node corresponding to expression e.
*/
ExprNode exprNode(Expr e) { ... }

/**
* Gets the node corresponding to the value of parameter p at function entry.
*/
ParameterNode parameterNode(Parameter p) { ... }


The predicate localFlowStep(Node nodeFrom, Node, nodeTo) holds if there is an immediate data flow edge from the node nodeFrom to the node nodeTo. The predicate can be applied recursively (using the + and * operators), or through the predefined recursive predicate localFlow, which is equivalent to localFlowStep*.

For example, finding flow from a parameter source to an expression sink in zero or more local steps can be achieved as follows:

DataFlow::localFlow(DataFlow::parameterNode(source), DataFlow::exprNode(sink))


### Using local taint tracking¶

Local taint tracking extends local data flow by including non-value-preserving flow steps. For example:

String temp = x;
String y = temp + ", " + temp;


If x is a tainted string then y is also tainted.

The local taint tracking library is in the module TaintTracking. Like local data flow, a predicate localTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) holds if there is an immediate taint propagation edge from the node nodeFrom to the node nodeTo. The predicate can be applied recursively (using the + and * operators), or through the predefined recursive predicate localTaint, which is equivalent to localTaintStep*.

For example, finding taint propagation from a parameter source to an expression sink in zero or more local steps can be achieved as follows:

TaintTracking::localTaint(DataFlow::parameterNode(source), DataFlow::exprNode(sink))


### Examples¶

The following query finds the filename passed to new FileReader(..).

import java

where
select call.getArgument(0)


Unfortunately, this will only give the expression in the argument, not the values which could be passed to it. So we use local data flow to find all expressions that flow into the argument:

import java
import semmle.code.java.dataflow.DataFlow

from Constructor fileReader, Call call, Expr src
where
DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(call.getArgument(0)))
select src


Then we can make the source more specific, for example an access to a public parameter. The following query finds where a public parameter is passed to new FileReader(..):

import java
import semmle.code.java.dataflow.DataFlow

from Constructor fileReader, Call call, Parameter p
where
DataFlow::localFlow(DataFlow::parameterNode(p), DataFlow::exprNode(fc.getArgument(0)))
select p


The following example finds calls to formatting functions where the format string is not hard-coded.

import java
import semmle.code.java.dataflow.DataFlow
import semmle.code.java.StringFormat

from StringFormatMethod format, MethodAccess call, Expr formatString
where
call.getMethod() = format and
call.getArgument(format.getFormatStringIndex()) = formatString and
not exists(DataFlow::Node source, DataFlow::Node sink |
DataFlow::localFlow(source, sink) and
source.asExpr() instanceof StringLiteral and
sink.asExpr() = formatString
)
select call, "Argument to String format method isn't hard-coded."


### Exercises¶

Exercise 1: Write a query that finds all hard-coded strings used to create a java.net.URL, using local data flow. (Answer)

## Global data flow¶

Global data flow tracks data flow throughout the entire program, and is therefore more powerful than local data flow. However, global data flow is less precise than local data flow, and the analysis typically requires significantly more time and memory to perform.

### Using global data flow¶

The global data flow library is used by extending the class DataFlow::Configuration as follows:

import semmle.code.java.dataflow.DataFlow

class MyDataFlowConfiguration extends DataFlow::Configuration {
MyDataFlowConfiguration() { this = "MyDataFlowConfiguration" }

override predicate isSource(DataFlow::Node source) {
...
}

override predicate isSink(DataFlow::Node sink) {
...
}
}


The following predicates are defined in the configuration:

• isSource—defines where data may flow from
• isSink—defines where data may flow to
• isBarrier—optional, restricts the data flow
• isAdditionalFlowStep—optional, adds additional flow steps

The characteristic predicate MyDataFlowConfiguration() defines the name of the configuration, so "MyDataFlowConfiguration" should be a unique name, for example, the name of your class.

The data flow analysis is performed using the predicate hasFlow(DataFlow::Node source, DataFlow::Node sink):

from MyDataFlowConfiguration dataflow, DataFlow::Node source, DataFlow::Node sink
where dataflow.hasFlow(source, sink)


### Exercise 3¶

import java

class GetenvSource extends MethodAccess {
GetenvSource() {
exists(Method m | m = this.getMethod() |
m.hasName("getenv") and
m.getDeclaringType() instanceof TypeSystem
)
}
}


### Exercise 4¶

import semmle.code.java.dataflow.DataFlow

class GetenvSource extends DataFlow::ExprNode {
GetenvSource() {
exists(Method m | m = this.asExpr().(MethodAccess).getMethod() |
m.hasName("getenv") and
m.getDeclaringType() instanceof TypeSystem
)
}
}

class GetenvToURLConfiguration extends DataFlow::Configuration {
GetenvToURLConfiguration() {
this = "GetenvToURLConfiguration"
}

override predicate isSource(DataFlow::Node source) {
source instanceof GetenvSource
}

override predicate isSink(DataFlow::Node sink) {
exists(Call call |
sink.asExpr() = call.getArgument(0) and
call.getCallee().(Constructor).getDeclaringType().hasQualifiedName("java.net", "URL")
)
}
}

from DataFlow::Node src, DataFlow::Node sink, GetenvToURLConfiguration config
where config.hasFlow(src, sink)
select src, "This environment variable constructs a URL \$@.", sink, "here"