Note

The data flow library used in this article has been replaced with an improved library which is available from CodeQL 2.12.5 onwards, see Analyzing data flow in C and C++ (new). The old library has been deprecated in CodeQL 2.14.1 and will be removed in a later release. With the release of CodeQL 2.13.0 both libraries use the new modular API for data flow.

Analyzing data flow in C and C++¶

You can use data flow analysis to track the flow of potentially malicious or insecure data that can cause vulnerabilities in your codebase.

About data flow¶

Data flow analysis computes the possible values that a variable can hold at various points in a program, determining how those values propagate through the program, and where they are used. In CodeQL, you can model both local data flow and global data flow. For a more general introduction to modeling data flow, see “About data flow analysis.”

Local data flow¶

Local data flow is data flow within a single function. 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:

int i = tainted_user_input();
some_big_struct *array = malloc(i * sizeof(some_big_struct));

In this case, the argument to malloc is 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 fopen.

import cpp

from Function fopen, FunctionCall fc
where fopen.hasGlobalName("fopen")
  and fc.getTarget() = fopen
select fc.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 cpp
import semmle.code.cpp.dataflow.DataFlow

from Function fopen, FunctionCall fc, Expr src
where fopen.hasGlobalName("fopen")
  and fc.getTarget() = fopen
  and DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(fc.getArgument(0)))
select src

Then we can vary the source and, for example, use the parameter of a function. The following query finds where a parameter is used when opening a file:

import cpp
import semmle.code.cpp.dataflow.DataFlow

from Function fopen, FunctionCall fc, Parameter p
where fopen.hasGlobalName("fopen")
  and fc.getTarget() = fopen
  and 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 semmle.code.cpp.dataflow.DataFlow
import semmle.code.cpp.commons.Printf

from FormattingFunction format, FunctionCall call, Expr formatString
where call.getTarget() = format
  and call.getArgument(format.getFormatParameterIndex()) = 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 " + format.getQualifiedName() + " isn't hard-coded."

Exercises¶

Exercise 1: Write a query that finds all hard-coded strings used to create a host_ent via gethostbyname, 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¶

The global data flow library is used by implementing the signature DataFlow::ConfigSig and applying the module DataFlow::Global<ConfigSig> as follows:

import semmle.code.cpp.dataflow.DataFlow

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

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

module MyFlow = DataFlow::Global<MyFlowConfiguration>;

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

import semmle.code.cpp.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>.

Examples¶

The following data flow configuration tracks data flow from environment variables to opening files in a Unix-like environment:

import semmle.code.cpp.dataflow.DataFlow

module EnvironmentToFileConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    exists (Function getenv |
      source.asExpr().(FunctionCall).getTarget() = getenv and
      getenv.hasGlobalName("getenv")
    )
  }

  predicate isSink(DataFlow::Node sink) {
    exists (FunctionCall fc |
      sink.asExpr() = fc.getArgument(0) and
      fc.getTarget().hasGlobalName("fopen")
    )
  }
}

module EnvironmentToFileFlow = DataFlow::Global<EnvironmentToFileConfiguration>;

from Expr getenv, Expr fopen
where EnvironmentToFileFlow::flow(DataFlow::exprNode(getenv), DataFlow::exprNode(fopen))
select fopen, "This 'fopen' uses data from $@.",
  getenv, "call to 'getenv'"

The following taint-tracking configuration tracks data from a call to ntohl to an array index operation. It uses the Guards library to recognize expressions that have been bounds-checked, and defines isBarrier to prevent taint from propagating through them. It also uses isAdditionalFlowStep to add flow from loop bounds to loop indexes.

import cpp
import semmle.code.cpp.controlflow.Guards
import semmle.code.cpp.dataflow.TaintTracking

module NetworkToBufferSizeConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node node) {
    node.asExpr().(FunctionCall).getTarget().hasGlobalName("ntohl")
  }

  predicate isSink(DataFlow::Node node) {
    exists(ArrayExpr ae | node.asExpr() = ae.getArrayOffset())
  }

  predicate isAdditionalFlowStep(DataFlow::Node pred, DataFlow::Node succ) {
    exists(Loop loop, LoopCounter lc |
      loop = lc.getALoop() and
      loop.getControllingExpr().(RelationalOperation).getGreaterOperand() = pred.asExpr() |
      succ.asExpr() = lc.getVariableAccessInLoop(loop)
    )
  }

  predicate isBarrier(DataFlow::Node node) {
    exists(GuardCondition gc, Variable v |
      gc.getAChild*() = v.getAnAccess() and
      node.asExpr() = v.getAnAccess() and
      gc.controls(node.asExpr().getBasicBlock(), _)
    )
  }
}

module NetworkToBufferSizeFlow = TaintTracking::Global<NetworkToBufferSizeConfiguration>;

from DataFlow::Node ntohl, DataFlow::Node offset
where NetworkToBufferSizeFlow::flow(ntohl, offset)
select offset, "This array offset may be influenced by $@.", ntohl,
  "converted data from the network"

Exercises¶

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

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

Exercise 4: Using the answers from 2 and 3, write a query which finds all global data flows from getenv to gethostbyname. (Answer)

Answers¶

Exercise 1¶

import semmle.code.cpp.dataflow.DataFlow

from StringLiteral sl, FunctionCall fc
where fc.getTarget().hasName("gethostbyname")
  and DataFlow::localFlow(DataFlow::exprNode(sl), DataFlow::exprNode(fc.getArgument(0)))
select sl, fc

Exercise 2¶

import semmle.code.cpp.dataflow.DataFlow

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

  predicate isSink(DataFlow::Node sink) {
    exists (FunctionCall fc |
      sink.asExpr() = fc.getArgument(0) and
      fc.getTarget().hasName("gethostbyname"))
  }
}

module LiteralToGethostbynameFlow = DataFlow::Global<LiteralToGethostbynameConfiguration>;

from StringLiteral sl, FunctionCall fc
where LiteralToGethostbynameFlow::flow(DataFlow::exprNode(sl), DataFlow::exprNode(fc.getArgument(0)))
select sl, fc

Exercise 3¶

import cpp

class GetenvSource extends FunctionCall {
  GetenvSource() {
    this.getTarget().hasGlobalName("getenv")
  }
}

Exercise 4¶

import semmle.code.cpp.dataflow.DataFlow

class GetenvSource extends DataFlow::Node {
  GetenvSource() {
    this.asExpr().(FunctionCall).getTarget().hasGlobalName("getenv")
  }
}

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

  predicate isSink(DataFlow::Node sink) {
    exists (FunctionCall fc |
      sink.asExpr() = fc.getArgument(0) and
      fc.getTarget().hasName("gethostbyname"))
  }
}

module GetenvToGethostbynameFlow = DataFlow::Global<GetenvToGethostbynameConfiguration>;

from DataFlow::Node getenv, FunctionCall fc
where GetenvToGethostbynameFlow::flow(getenv, DataFlow::exprNode(fc.getArgument(0)))
select getenv.asExpr(), fc

Analyzing data flow in C and C++¶

About data flow¶

Local data flow¶

Using local data flow¶

Using local taint tracking¶

Examples¶

Exercises¶

Global data flow¶

Using global data flow¶

Using global taint tracking¶

Examples¶

Exercises¶

Answers¶

Exercise 1¶

Exercise 2¶

Exercise 3¶

Exercise 4¶

Further reading¶