Use of Pointer Subtraction to Determine Size

Draft Base

Structure: Simple

Description

The product subtracts one pointer from another in order to determine size, but this calculation can be incorrect if the pointers do not exist in the same memory chunk.

Common Consequences 1

Scope: Access ControlIntegrityConfidentialityAvailability

Impact: Modify MemoryRead MemoryExecute Unauthorized Code or CommandsGain Privileges or Assume Identity

There is the potential for arbitrary code execution with privileges of the vulnerable program.

Detection Methods 2

FuzzingHigh

Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.

Automated Static AnalysisHigh

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Potential Mitigations 1

Phase: Implementation

Save an index variable. This is the recommended solution. Rather than subtract pointers from one another, use an index variable of the same size as the pointers in question. Use this variable to "walk" from one pointer to the other and calculate the difference. Always validate this number.

Demonstrative Examples 1

The following example contains the method size that is used to determine the number of nodes in a linked list. The method is passed a pointer to the head of the linked list.

Code Example:

Bad

// Returns the number of nodes in a linked list from*

However, the method creates a pointer that points to the end of the list and uses pointer subtraction to determine the number of nodes in the list by subtracting the tail pointer from the head pointer. There no guarantee that the pointers exist in the same memory area, therefore using pointer subtraction in this way could return incorrect results and allow other unintended behavior. In this example a counter should be used to determine the number of nodes in the list, as shown in the following code.

Code Example:Good
C
c

References 1

The CLASP Application Security Process

Secure Software, Inc.

2005

https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf(2024-11-17)

ID: REF-18

Likelihood of Exploit

Medium

Applicable Platforms

Languages:

C : UndeterminedC++ : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Incorrect Calculation (CWE-682)

Taxonomy Mapping

CLASP
CERT C Secure Coding
Software Fault Patterns