CodeQL workshop: Syntactical elements of C/C++

Contents

• CodeQL workshop: Syntactical elements of C/C++
  • Contents
  • Setup instructions
  • Workshop
    • Learnings
    • Linux Miscellaneous Driver
  • Exercises
    • Exercise 1
    • Exercise 2
    • Exercise 3
    • Exercise 4
    • Exercise 5
    • Exercise 6
    • Exercise 7
    • Exercise 8
    • Exercise 9
    • Exercise 10
    • Exercise 11

Setup instructions

• Install Visual Studio Code.

• Install the CodeQL extension for Visual Studio Code.

• (Optionally) Install Docker if you want to build your own CodeQL database.

• (Optionally) Install the CodeQL CLI if you want to build your own CodeQL database.

• Clone this repository recursively:

  git clone --recursive https://github.com/rvermeulen/codeql-workshop-elements-of-syntactical-program-analysis-cpp

• Install the CodeQL pack dependencies using the command CodeQL: Install Pack Dependencies and select exercises and solutions.

• Download the prebuilt database or build the database using the predefined Makefile by running make.

• Select the Vulnerable Linux Driver database as the current database by right-clicking on the file vulnerable-linux-driver-db.zip in the File explorer and running the command CodeQL: Set Current Database.

Workshop

Learnings

In this workshop you will learn how to describe syntactical elements of the C/C++ programming language. With the goal of describing the user-mode entry point of the intentionally vulnerable Linux driver you will:

• Discover how QL represents C/C++ program elements.
• Learn to query program elements.
• Learn how to encapsulate descriptions of program elements using QL classes.

This workshop focusses on the syntactical parts. Some parts in this workshop can be generalized using more advanced techniques, such as dataflow analysis, that are covered in other workshops.

Linux Miscellaneous Driver

The intentionally vulnerable Linux driver project implements a Miscellaneous Character Driver to expose multiple vulnerabilities to learn about Kernel driver exploitation.

The miscellaneous character driver was designed for use cases that require a small device driver to support custom hardware or software hacks. To register or unregister a miscellaneous driver, the misc driver exports two functions for user modules, that can be found in the header linux/miscdevice.h, called misc_register and misc_unregister.

With the misc_register function as the starting point we will traverse function calls, expressions, structure definitions, and variable initializations to find the user module entrypoint. This entrypoint will be the start of future workshops that will discuss the vulnerabilities that can be found in this intentionally vulnerable Linux driver.

Exercises

Exercise 1

Find all the function calls in the program by implementing Exercise1.ql.

Hints

• The class FunctionCall can be used to reason about all the function calls in the program.

A solution can be found in the query Exercise1.ql

Exercise 2

Find all the function calls to the function misc_register by implementing Exercise2.ql.

Hints

• The class FunctionCall provides the member predicate getTarget to reason about the called function.
• The class Function provides the member predicate getName to get the name of the function.

A solution can be found in the query Exercise2.ql

Exercise 3

Recall that predicates and classes allow you to encapsulate logical conditions in a reusable format.

Convert your solution to Exercise2.ql into a class in Exercises3.ql by replacing the none formula in the characteristic predicate of the MiscRegisterFunction class.

Besides relying on the name, try to add another property to distinguish the correct function.

Hints

• Each program element represented by the class Element can be related to the primary file the element occurs in using the member predicate getFile.
• Each program element has an absolute path that can be accessed using the member predicate getAbsolutePath on the class File.
• The QL string type provides builtins such as matches and regexpMatch to match patterns in strings. The matches builtin member predicate interprets _ to match any single character and % to match any sequences of characters in the provided pattern.

A solution can be found in the query Exercise3.ql

Exercise 4

The definition of the driver is passed as a parameter to the misc_register function. Obtain the argument to the call, determine the arguments type and primary QL class by implementing Exercise4.ql.

Hints

• The class FunctionCall provides the member predicate getArgument to get a provided argument by index.
• Each expression represented by the class Expr has a type that can be retrieved with the member predicate getType.
• Each program element represented by the class Element has a member predicate getPrimaryQlClass that returns the QL class that is the most precise syntactic category the element belongs to.

A solution can be found in the query Exercise4.ql

Exercise 5

The definition of the miscellaneous driver is defined by the structure miscdevice. Implement the characteristic predicate of the class MiscDeviceStruct in Exercise5.ql so we can reason about its use.

Hints

• The class Struct inherits the member predicate getName from the class UserType that returns the name of the struct.
• Each program element represented by the class Element can be related to the primary file the element occurs in using the member predicate getFile.
• Each program element has an absolute path that can be accessed using the member predicate getAbsolutePath on the class File.
• The QL string type provides builtins such as matches and regexpMatch to match patterns in strings. The matches builtin member predicate interprets _ to match any single character and % to match any sequences of characters in the provided pattern.

A solution can be found in the query Exercise5.ql

Exercise 6

Now that we can reason about the structure miscdevice we can look for all it instantiations. Implement the characteristic predicate of the class MiscDeviceDefinition in Exercise6.ql so we use it to find all its instances.

Hints

• The class Variable has a member predicate getType that gets the type of this variable.

A solution can be found in the query Exercise6.ql

Exercise 7

The instantiation vuln_device initializes 3 members of the miscdevice structure. Find the type of the third field initialized with &vuln_fops by implementing Exercise7.ql.

Hints

• The class Struct inherits the member predicate getAMember from the class Class that gets the zero-based indexed member declared in the struct.
• The class Field inherits the member predicate getType from the class MemberVariable that returns the type of the field.

A solution can be found in the query Exercise7.ql

Exercise 8

With knowledge of the type of third field om the miscdevice structure we can now identify all file operation definitions such as vuln_fops. Implement the characteristic predicates for the class FileOperationsStruct and FileOperationsDefinition in Exercise8.ql.

Hints

• The class Struct inherits the member predicate getName from the class UserType that returns the name of the struct.
• Each program element represented by the class Element can be related to the primary file the element occurs in using the member predicate getFile.
• Each program element has an absolute path that can be accessed using the member predicate getAbsolutePath on the class File.
• The QL string type provides builtins such as matches and regexpMatch to match patterns in strings. The matches builtin member predicate interprets _ to match any single character and % to match any sequences of characters in the provided pattern.

A solution can be found in the query Exercise8.ql

Exercise 9

The single file operation definition vuln_fops is initialized with, among others, a function pointer for the field unlocked_ioctl. This is the function that is invoked when a user-mode application performs the ioctl system call to communicate with the driver.

Extend the class FileOperationsDefinition with a member predicate getUnlockedIoctl that returns a Function with which the file operations definition is initialized in Exercise9.ql.

Hints

• The class Variable has the member predicate getAnAssignedValue that returns an Expr representing an expression that is assigned to this variable somewhere in the program.
• The class Field inherits the member predicate hasName from the class Declaration that holds if the field has the provided name.
• The class ClassAggregrateLiteral has the member predicate getFieldExpr that returns an Expr that is part of the aggregrate literal that is used to initialize the provided field.

A solution can be found in the query Exercise9.ql

Exercise 10

We have successfully identified the miscellaneous driver definition, the file operations definition, and linked the ioctl handler to the file operations definition. Extend the class MiscDeviceDefinition with the member predicate getFileOperations that returns a FileOperationsDefinition that the miscellaneous driver definition is initialized with in Exercise10.ql.

Hints

• The class Variable has the member predicate getAnAssignedValue that returns an Expr representing an expression that is assigned to this variable somewhere in the program.
• The class Field inherits the member predicate hasName from the class Declaration that holds if the field has the provided name.
• The class ClassAggregrateLiteral has the member predicate getFieldExpr that returns an Expr that is part of the aggregrate literal that is used to initialize the provided field.
• A class can be cast to a subclass using the syntax variable.(Class).predicate(). For example, to cast an expression expr to a AddressOfExpr to get an operand of the expression you can use the syntax expr.(AddressOfExpr).getOperand().
• The class AddressOfExpr that represents the expression taking the address &expr has a member predicate getOperand that returns the expression of which the address is taken.
• The class Variable has a member predicate getAnAccess that returns all the access to this variable.

A solution can be found in the query Exercise10.ql

Exercise 11

In this final exercise we have to relate the call to misc_register to the miscellaneous driver definition so we can find the driver's entrypoint for user-mode. Implement the characteristic predicate for the class MiscDriverUserModeEntry in Exercise11.ql that relates the classes MiscRegisterFunction, MiscDeviceDefinition, and FileOperationsDefinition to obtain the unlocked ioctl handler function.

Hints

• The class Function has a member predicate getACallToThisFunction that returns all the function call to this function.
• The class FunctionCall inherits the member predicate getArgument from the class Call that returns the nth argument for this call.
• A class can be casted to a subclass using the syntax variable.(Class).predicate(). For example, to cast an expression expr to a AddressOfExpr to get an operand of the expression you can use the syntax expr.(AddressOfExpr).getOperand().
• The class Variable has a member predicate getAnAccess that returns all the access to this variable.

A solution can be found in the query Exercise11.ql