The signal() function has implementation-defined behavior and behaves differently, for example, on Windows than it does on many Unix systems.

The following code example illustrates this behavior:

#include <stdio.h>
#include <signal.h>

volatile sig_atomic_t e_flag = 0;

void handler(int signum) {
  e_flag = 1;
}

int main(void) {
  signal(SIGINT, handler);
  while (!e_flag) {}
  puts("Escaped from first while ()");
  e_flag = 0;
  while (!e_flag) {}
  puts("Escaped from second while ()");
  return 0;
}

Many Unix (and Unix-like) systems automatically reinstall signal handlers upon handler execution, meaning that the signal handler defined by the user is left in place until it is explicitly removed. For example, when this code is compiled with gcc 3.4.4 and executed under Red Hat Linux, the SIGINT is captured both times by handler.

% ./SIG01-A
^C
Escaped from first while ()
^C
Escaped from second while ()
%

When a signal handler is installed with the signal() function in Windows and some Unix systems, the default action is restored for that signal after the signal is triggered. This means that signal handlers are not automatically reinstalled. For example, when this code is compiled with Microsoft Visual Studio 2005 version 8.0, only the first SIGINT is captured by handler.

> SIG01-A.exe
^C
Escaped from first while ()
^C
>

The second SIGINT executes the default action, which is to terminate program execution.

Different actions must be taken depending on whether or not the application requires signal handlers to be persistent.

Persistent Handlers

Asynchronous signals may originate from potentially hostile sources outside the process. Consequently, vulnerabilities may exist in cases where the signal handler persistence behavior is inconsistent with the developer's expectations, for example, the developer expects the signal handler to persist when it does not.

Non-Compliant Code Example

This non-compliant code example fails to make the signal handler persist on Windows platforms and on those Unix systems where handlers are not persistent by default.

void handler(int signum) {
  /* handling code */
}

Non-Compliant Code Example

A common solution to attempt to create persistent signal handlers is to call signal() from within the handler itself, consequently unresetting the reset signal.

void handler(int signum) {
  signal(signum, handler);
  /* handling code */
}

Unfortunately this solution still contains a race window, starting when the host environment resets the signal and ending when the handler calls signal(). During that time, a second signal sent to the program will trigger the default signal behavior, thereby defeating the persistent behavior.

A secure solution would prevent the environment from resetting the signal in the first place, and thereby guaranteeing persistence. Unfortunately, Windows does not provide a secure solution to this problem.

Compliant Solution (POSIX)

The POSIX sigaction() function assigns handlers to signals in a similar manner to the C99 signal() function, but also allows signal handler persistence to be controlled via the SA_RESETHAND flag. (Leaving the flag clear makes the handler persistent.)

/* Equivalent to signal(SIGUSR1, handler) but make signal persistent */
struct sigaction act;
act.sa_handler = handler;
act.sa_flags = 0;
if (sigemptyset(&act.sa_mask) != 0) {
  /* handle error */
}
if (sigaction(SIGUSR1, &act, NULL) != 0) {
  /* handle error */
}

POSIX recommends sigaction() and deprecates signal(). Unfortunately, sigaction() is not defined in C99 and is consequently not as portable a solution.

Non-Persistent Handlers

Errors may also occur when the developer expects the default action to be restored for a signal, but instead, the signal handler persists.

Non-Compliant Code Example (Unix)

This non-compliant code example fails to reset the signal handler to its default behavior on those Unix systems where handlers are persistent by default.

void handler(int signum) {
  /* handling code */
}

Compliant Solution (Unix & Windows)

A C99-compliant solution to reset the handler on a Unix system is to rebind the signal to the default handler in the first line of the handler itself. Whereas, Windows automatically resets handlers to default.

void handler(int signum) {
#ifndef WINDOWS
  signal(signum, SIG_DFL);
#endif
  /* handling code */
}

With the Compliant Solution for Unix, there is no race condition that can be exploited by an attacker in sending a second signal. And that is because a second signal sent to the handler, before the latter calls signal(signum, SIG_DFL), will merely cause the handler to restart, and call signal() anyway.

Compliant Solution (POSIX)

The POSIX sigaction() function assigns handlers to signals in a similar manner to the C99 signal() function, but also allows signal handler persistence to be controlled via the SA_RESETHAND flag. (Setting the flag makes the handler non-persistent.)

/* Equivalent to signal( SIGUSR1, handler) but make signal non-persistent */
struct sigaction act;
act.sa_handler = handler;
act.sa_flags = SA_RESETHAND;
if (sigemptyset(&act.sa_mask) != 0) {
  /* handle error */
}
if (sigaction(SIGUSR1, &act, NULL) != 0) {
  /* handle error */
}

Risk Assessment

Failure to understand implementation-specific details regarding signal handler persistence can lead to unexpected behavior.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899-1999TR2]] Section 7.14.1.1, "The signal function"

SIG00-A. Mask signals handled by non-interruptible signal handlers      11. Signals (SIG)       SIG02-A. Avoid using signals to implement normal functionality