CWE-828

VariantIncomplete

Signal Handler with Functionality that is not Asynchronous-Safe

The product defines a signal handler that contains code sequences that are not asynchronous-safe, i.e., the functionality is not reentrant, or it can be interrupted.

1 linked vulnerability

1 related weaknesses

Extended Description

Invocation of non-reentrant functions from within the handler. One example is malloc(), which modifies internal global variables as it manages memory. Very few functions are actually reentrant.
Code sequences (not necessarily function calls) contain non-atomic use of global variables, or associated metadata or structures, that can be accessed by other functionality of the program, including other signal handlers. Frequently, the same function is registered to handle multiple signals.
The signal handler function is intended to run at most one time, but instead it can be invoked multiple times. This could happen by repeated delivery of the same signal, or by delivery of different signals that have the same handler function (CWE-831).

Implementation

Architecture and Design

Eliminate the usage of non-reentrant functionality inside of signal handlers. This includes replacing all non-reentrant library calls with reentrant calls.

Note: This will not always be possible and may require large portions of the product to be rewritten or even redesigned. Sometimes reentrant-safe library alternatives will not be available. Sometimes non-reentrant interaction between the state of the system and the signal handler will be required by design.

Implementation

Where non-reentrant functionality must be leveraged within a signal handler, be sure to block or mask signals appropriately. This includes blocking other signals within the signal handler itself that may also leverage the functionality. It also includes blocking all signals reliant upon the functionality when it is being accessed or modified by the normal behaviors of the product.

Integrity

Confidentiality

Availability

DoS: Crash, Exit, or Restart

Execute Unauthorized Code or Commands

The most common consequence will be a corruption of the state of the product, possibly leading to a crash or exit. However, if the signal handler is operating on state variables for security relevant libraries or protection mechanisms, the consequences can be far more severe, including protection mechanism bypass, privilege escalation, or information exposure.

Automated Static Analysis

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.)

Modes of Introduction

Implementation

Related Weaknesses

CWE-364ChildOf

Example CVEs (6)

CVE-2008-4109

Signal handler uses functions that ultimately call the unsafe syslog/malloc/s*printf, leading to denial of service via multiple login attempts

CVE-2006-5051

Chain: Signal handler contains too much functionality (CWE-828), introducing a race condition (CWE-362) that leads to a double free (CWE-415).

CVE-2001-1349

unsafe calls to library functions from signal handler

CVE-2004-0794

SIGURG can be used to remotely interrupt signal handler; other variants exist.

CVE-2004-2259

SIGCHLD signal to FTP server can cause crash under heavy load while executing non-reentrant functions like malloc/free.

External Links

MITRE CWE Database