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Analyzing data flow in Java and Kotlin

You can use CodeQL to track the flow of data through a Java/Kotlin program to its use.

Writing CodeQL queries for Kotlin versus Java analysis

Generally you use the same classes to write queries for Kotlin and for Java. You use the same libraries such as DataFlow, TaintTracking, or SSA, and the same classes such as MethodAccess or Class for both languages. When you want to access Kotlin-specific elements (such as a WhenExpr) you’ll need to use Kotlin-specific CodeQL classes.

There are however some important cases where writing queries for Kotlin can produce surprising results compared to writing queries for Java, as CodeQL works with the JVM bytecode representation of the Kotlin source code.

Be careful when you model code elements that don’t exist in Java, such as NotNullExpr (expr!!), because they could interact in unexpected ways with common predicates. For example, MethodAccess.getQualifier() returns a NotNullExpr instead of a VarAccess in the following Kotlin code:

someVar!!.someMethodCall()

In that specific case, you can use the predicate Expr.getUnderlyingExpr(). This goes directly to the underlying VarAccess to produce a more similar behavior to that in Java.

Nullable elements (?) can also produce unexpected behavior. To avoid a NullPointerException, Kotlin may inline calls like expr.toString() to String.valueOf(expr) when expr is nullable. Make sure that you write CodeQL around the extracted code, which may not exactly match the code as written in the codebase.

Another example is that if-else expressions in Kotlin are translated into WhenExprs in CodeQL, instead of the more typical IfStmt seen in Java.

In general, you can debug these issues with the AST (you can use the CodeQL: View AST command from Visual Studio Code’s CodeQL extension, or run the PrintAst.ql query) and see exactly what CodeQL is extracting from your code.

About this article

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

For a more general introduction to modeling data flow, see “About data flow analysis.”

Note

The modular API for data flow described here is available from CodeQL 2.13.0. The legacy library is deprecated and will be removed in December 2024. For information about how the library has changed and how to migrate any existing queries to the modular API, see New dataflow API for CodeQL query writing.

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

To use the data flow library you need the following import:

import semmle.code.java.dataflow.DataFlow

The DataFlow module defines the class Node denoting any element that data can flow through. Nodes are divided into expression nodes (ExprNode) and parameter nodes (ParameterNode). You can 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. You can apply the predicate recursively by using the + and * operators, or by using the predefined recursive predicate localFlow, which is equivalent to localFlowStep*.

For example, you can find flow from a parameter source to an expression sink in zero or more local steps:

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 y = "Hello " + x;

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

To use the taint tracking library you need the following import:

import semmle.code.java.dataflow.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. You can apply the predicate recursively by using the + and * operators, or by using the predefined recursive predicate localTaint, which is equivalent to localTaintStep*.

For example, you can find taint propagation from a parameter source to an expression sink in zero or more local steps:

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

Examples

This query finds the filename passed to new FileReader(..):

import java

from Constructor fileReader, Call call
where
  fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
  call.getCallee() = fileReader
select call.getArgument(0)

Unfortunately, this only gives 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
  fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
  call.getCallee() = fileReader and
  DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(call.getArgument(0)))
select src

To restrict sources to only an access to a public parameter, rather than arbitrary expressions, we can modify this query as follows:

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

from Constructor fileReader, Call call, Parameter p
where
  fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
  call.getCallee() = fileReader and
  DataFlow::localFlow(DataFlow::parameterNode(p), DataFlow::exprNode(call.getArgument(0)))
select p

The following query 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, MethodCall 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.

Note

You can model data flow paths in CodeQL by creating path queries. To view data flow paths generated by a path query in CodeQL for VS Code, you need to make sure that it has the correct metadata and select clause. For more information, see Creating path queries.

Using global data flow

We can use the global data flow library by implementing the signature DataFlow::ConfigSig and applying the module DataFlow::Global<ConfigSig>:

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

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    ...
  }

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

module MyFlow = DataFlow::Global<MyFlowConfiguration>;

These predicates are defined in the configuration:

• isSource - defines where data may flow from.

• isSink - defines where data may flow to.

• isBarrier - optional, defines where data flow is blocked.

• isAdditionalFlowStep - optional, adds additional flow steps.

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

from DataFlow::Node source, DataFlow::Node sink
where MyFlow::flow(source, sink)
select source, "Data flow to $@.", sink, sink.toString()

Using global taint tracking

Global taint tracking is to global data flow what local taint tracking is to local data flow. That is, global taint tracking extends global data flow with additional non-value-preserving steps. You use the global taint tracking library by applying the module TaintTracking::Global<ConfigSig> to your configuration instead of DataFlow::Global<ConfigSig>:

import java
import semmle.code.java.dataflow.TaintTracking

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    ...
  }

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

module MyFlow = TaintTracking::Global<MyFlowConfiguration>;

The resulting module has an identical signature to the one obtained from DataFlow::Global<ConfigSig>.

Flow sources

The data flow library contains some predefined flow sources. The class RemoteFlowSource (defined in semmle.code.java.dataflow.FlowSources) represents data flow sources that may be controlled by a remote user, which is useful for finding security problems.

Examples

This query shows a taint-tracking configuration that uses remote user input as data sources.

import java
import semmle.code.java.dataflow.FlowSources

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    source instanceof RemoteFlowSource
  }

  ...
}

module MyTaintFlow = TaintTracking::Global<MyFlowConfiguration>;

Exercises

Exercise 2: Write a query that finds all hard-coded strings used to create a java.net.URL, using global data flow. (Answer)

Exercise 3: Write a class that represents flow sources from java.lang.System.getenv(..). (Answer)

Exercise 4: Using the answers from 2 and 3, write a query which finds all global data flow paths from getenv to java.net.URL. (Answer)

Answers

Exercise 1

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

from Constructor url, Call call, StringLiteral src
where
  url.getDeclaringType().hasQualifiedName("java.net", "URL") and
  call.getCallee() = url and
  DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(call.getArgument(0)))
select src

Exercise 2

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

module LiteralToURLConfig implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    source.asExpr() instanceof StringLiteral
  }

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

module LiteralToURLFlow = DataFlow::Global<LiteralToURLConfig>;

from DataFlow::Node src, DataFlow::Node sink
where LiteralToURLFlow::flow(src, sink)
select src, "This string constructs a URL $@.", sink, "here"

Exercise 3

import java

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

Exercise 4

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

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

module GetenvToURLConfig implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    source instanceof GetenvSource
  }

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

module GetenvToURLFlow = DataFlow::Global<GetenvToURLConfig>;

from DataFlow::Node src, DataFlow::Node sink
where GetenvToURLFlow::flow(src, sink)
select src, "This environment variable constructs a URL $@.", sink, "here"

Further reading

• Exploring data flow with path queries in the GitHub documentation.

• CodeQL queries for Java

• Example queries for Java

• CodeQL library reference for Java

• “QL language reference”

• “CodeQL tools”

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