Analyzing data flow in C/C++
============================

Overview
--------

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

For a more general introduction to modeling data flow in QL, see :doc:`Introduction to data flow analysis in QL <../intro-to-data-flow>`.

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. ``Node``\ s 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``:

.. code-block:: ql

   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``:

.. code-block:: ql

   /**
    * 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:

.. code-block:: ql

   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:

.. code-block:: cpp

   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:

.. code-block:: ql

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

Examples
~~~~~~~~

The following query finds the filename passed to ``fopen``.

.. code-block:: ql

   import cpp

   from Function fopen, FunctionCall fc
   where fopen.hasQualifiedName("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:

.. code-block:: ql

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

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

Then we can vary the source, for example an access to a public parameter. The following query finds where a public parameter is used to open a file:

.. code-block:: ql

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

   from Function fopen, FunctionCall fc, Parameter p
   where fopen.hasQualifiedName("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.

.. code-block:: ql

   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 <#exercise-1>`__)

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:

.. code-block:: ql

   import semmle.code.cpp.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 replaced by the name of your class.

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

.. code-block:: ql

   from MyDataFlowConfiguration dataflow, DataFlow::Node source, DataFlow::Node sink
   where dataflow.hasFlow(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 extending the class ``TaintTracking::Configuration`` as follows:

.. code-block:: ql

   import semmle.code.cpp.dataflow.TaintTracking

   class MyTaintTrackingConfiguration extends TaintTracking::Configuration {
     MyTaintTrackingConfiguration() { this = "MyTaintTrackingConfiguration" }

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

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

The following predicates are defined in the configuration:

-  ``isSource``—defines where taint may flow from
-  ``isSink``—defines where taint may flow to
-  ``isSanitizer``—optional, restricts the taint flow
-  ``isAdditionalTaintStep``—optional, adds additional taint steps

Similar to global data flow, the characteristic predicate ``MyTaintTrackingConfiguration()`` defines the unique name of the configuration, so ``"MyTaintTrackingConfiguration"`` should be replaced by the name of your class.

The taint tracking analysis is performed using the predicate ``hasFlow(DataFlow::Node source, DataFlow::Node sink)``.

Examples
~~~~~~~~

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

.. code-block:: ql

   import semmle.code.cpp.dataflow.DataFlow

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

     override predicate isSource(DataFlow::Node source) {
       exists (Function getenv |
         source.asExpr().(FunctionCall).getTarget() = getenv and
         getenv.hasQualifiedName("getenv")
       )
     }

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

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

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-2>`__)

Exercise 3: Write a class that represents flow sources from ``getenv``. (`Answer <#exercise-3>`__)

Exercise 4: Using the answers from 2 and 3, write a query which finds all global data flows from ``getenv`` to ``gethostbyname``. (`Answer <#exercise-4>`__)

What next?
----------

-  Try the worked examples in the following topics: :doc:`Example: Checking that constructors initialize all private fields <private-field-initialization>` and :doc:`Example: Checking for allocations equal to 'strlen(string)' without space for a null terminator <zero-space-terminator>`.
-  Find out more about QL in the `QL language handbook <https://help.semmle.com/QL/ql-handbook/index.html>`__ and `QL language specification <https://help.semmle.com/QL/ql-spec/language.html>`__.
-  Learn more about the query console in `Using the query console <https://lgtm.com/help/lgtm/using-query-console>`__.

Answers
-------

Exercise 1
~~~~~~~~~~

.. code-block:: ql

   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
~~~~~~~~~~

.. code-block:: ql

   import semmle.code.cpp.dataflow.DataFlow

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

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

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

   from StringLiteral sl, FunctionCall fc, LiteralToGethostbynameConfiguration cfg
   where cfg.hasFlow(DataFlow::exprNode(sl), DataFlow::exprNode(fc.getArgument(0)))
   select sl, fc

Exercise 3
~~~~~~~~~~

.. code-block:: ql

   import cpp

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

Exercise 4
~~~~~~~~~~

.. code-block:: ql

   import semmle.code.cpp.dataflow.DataFlow

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

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

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

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

   from DataFlow::Node getenv, FunctionCall fc, GetenvToGethostbynameConfiguration cfg
   where cfg.hasFlow(getenv, DataFlow::exprNode(fc.getArgument(0)))
   select getenv.asExpr(), fc
