CWE-828 非异步安全功能中的信号处理例程

Signal Handler with Functionality that is not Asynchronous-Safe

结构: Simple

Abstraction: Base

状态: Incomplete

被利用可能性: unkown

基本描述

The software 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.

扩展描述

This can lead to an unexpected system state with a variety of potential consequences depending on context, including denial of service and code execution.

Signal handlers are typically intended to interrupt normal functionality of a program, or even other signals, in order to notify the process of an event. When a signal handler uses global or static variables, or invokes functions that ultimately depend on such state or its associated metadata, then it could corrupt system state that is being used by normal functionality. This could subject the program to race conditions or other weaknesses that allow an attacker to cause the program state to be corrupted. While denial of service is frequently the consequence, in some cases this weakness could be leveraged for code execution.

There are several different scenarios that introduce this issue:

Note that in some environments or contexts, it might be possible for the signal handler to be interrupted itself.

If both a signal handler and the normal behavior of the software have to operate on the same set of state variables, and a signal is received in the middle of the normal execution's modifications of those variables, the variables may be in an incorrect or corrupt state during signal handler execution, and possibly still incorrect or corrupt upon return.

常见的影响

范围	影响	注释
['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 software, 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.

可能的缓解方案

['Implementation', 'Architecture and Design']

策略:

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 software 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

策略:

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 software.

示例代码

例

This code registers the same signal handler function with two different signals (CWE-831). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits.

bad C

char logMessage;

void handler (int sigNum) {

syslog(LOG_NOTICE, "%s\n", logMessage);
free(logMessage);
/ artificially increase the size of the timing window to make demonstration of this weakness easier. /

sleep(10);
exit(0);

}

int main (int argc, char argv[]) {

logMessage = strdup(argv[1]);
/ Register signal handlers. /

signal(SIGHUP, handler);
signal(SIGTERM, handler);
/ artificially increase the size of the timing window to make demonstration of this weakness easier. /

sleep(10);

}

The handler function uses global state (globalVar and logMessage), and it can be called by both the SIGHUP and SIGTERM signals. An attack scenario might follow these lines:

None

At this point, the state of the heap is uncertain, because malloc is still modifying the metadata for the heap; the metadata might be in an inconsistent state. The SIGTERM-handler call to free() is assuming that the metadata is inconsistent, possibly causing it to write data to the wrong location while managing the heap. The result is memory corruption, which could lead to a crash or even code execution, depending on the circumstances under which the code is running.

Note that this is an adaptation of a classic example as originally presented by Michal Zalewski [REF-360]; the original example was shown to be exploitable for code execution.

Also note that the strdup(argv[1]) call contains a potential buffer over-read (CWE-126) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array.

例

The following code registers a signal handler with multiple signals in order to log when a specific event occurs and to free associated memory before exiting.

bad C

#include <signal.h>
#include <syslog.h>
#include <string.h>
#include <stdlib.h>

void global1, global2;
char what;
void sh (int dummy) {

syslog(LOG_NOTICE,"%s\n",what);
free(global2);
free(global1);
/ Sleep statements added to expand timing window for race condition /

sleep(10);
exit(0);

}

int main (int argc,char argv[]) {

what=argv[1];
global1=strdup(argv[2]);
global2=malloc(340);
signal(SIGHUP,sh);
signal(SIGTERM,sh);
/ Sleep statements added to expand timing window for race condition /

sleep(10);
exit(0);

}

However, the following sequence of events may result in a double-free (CWE-415):