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A signal is a mechanism for transferring control that is typically used to notify a process that an event has occurred. That process can then respond to the event accordingly. The C Standard provides functions for sending and handling signals within a C program.

Processes handle signals by registering a signal handler using the signal() function, which is specified as

void (*signal(int sig, void (*func)(int)))(int);

This signal handler is conceptually equivalent to

typedef void (*sighandler_t)(int signum);
extern sighandler_t signal(
  int signum, 
  sighandler_t handler
);

Signal handlers can be interrupted by signals, including their own. If a signal is not reset before its handler is called, the handler can interrupt its own execution. A handler that always successfully executes its code despite interrupting itself or being interrupted is async-signal-safe.

Some platforms provide the ability to mask signals while a signal handler is being processed. If a signal is masked while its own handler is processed, the handler is noninterruptible and need not be async-signal-safe. However, even when a signal is masked while its own handler is processed, the handler must still avoid invoking async-signal-safe unsafe functions because their execution may be (or have been) interrupted by another signal.

Vulnerabilities can arise if a signal handler that is not async-signal-safe is interrupted with any unmasked signal, including its own.

Noncompliant Code Example

This noncompliant code example registers a single signal handler to process both SIGUSR1 and SIGUSR2. The variable sig2 should be set to 1 if one or more SIGUSR1 signals are followed by SIGUSR2, essentially implementing a finite state machine within the signal handler.

#include <signal.h>

volatile sig_atomic_t sig1 = 0;
volatile sig_atomic_t sig2 = 0;

void handler(int signum) {
  if (signum == SIGUSR1) {
    sig1 = 1;
  }
  else if (sig1) {
    sig2 = 1;
  }
}

int main(void) {
  if (signal(SIGUSR1, handler) == SIG_ERR) {
    /* Handle error */
  }
  if (signal(SIGUSR2, handler) == SIG_ERR) {
    /* Handler error */
  }

  while (sig2 == 0) {
    /* Do nothing or give up CPU for a while */
  }

  /* ... */

  return 0;
}

Unfortunately, a race condition occurs in the implementation of handler(). If handler() is called to handle SIGUSR1 and is interrupted to handle SIGUSR2, it is possible that sig2 will not be set.

Compliant Solution (POSIX)

The POSIX sigaction() function assigns handlers to signals in a similar manner to the C signal() function, but it also allows signal masks to be set explicitly. Consequently, sigaction() can be used to prevent a signal handler from interrupting itself.

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

volatile sig_atomic_t sig1 = 0;
volatile sig_atomic_t sig2 = 0;

void handler(int signum) {
  if (signum == SIGUSR1) {
    sig1 = 1;
  }
  else if (sig1) {
    sig2 = 1;
  }
}

int main(void) {
  struct sigaction act;
  act.sa_handler = &handler;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* Handle error */
  }
  if (sigaddset(&act.sa_mask, SIGUSR1)

No direct issues come from using the same handler for multiple signals, but it broadens your susceptibility to other vulnerabilities. For instance, be careful that a program doesn't generate two similar signals linked to the same handler, and the code to deal with it get executed twice asynchronously.

Non-Compliant Coding Example

The program is intended to clean up and terminate when it receives either a SIGINT or a SIGTERM. However, if a SIGINT is generated, and then a SIGTERM is generated after the call to free(), but before _Exit() is reached, a double free() will occur.

#include <signal.h> 
 
char *global_ptr; 
 
void handler() { 
  free(global_ptr); 
  _Exit(-1);
} 
 
int main() { 
  global_ptr = malloc(16);
  if (global_ptr == NULL) {
    /* handle error conditionHandle error */
  }
  if (sigaddset(&act.sa_mask, SIGUSR2)) {
    /* Handle error */
  }

  signal(SIGINT, handler); 
  signal(SIGTERM, handler);
 
  /* program codeif (sigaction(SIGUSR1, &act, NULL) != 0) {
    /* Handle error */
  }
  if (sigaction(SIGUSR2, &act, NULL) != 0) {
    /* Handle error */
  }

  while (sig2 == 0) {
    /* Do nothing or give up CPU for a while */
  }

  /* ... */

  return 0;
}

Risk Assessment

Depending on the code, this could lead to any number of attacks, many of which could give root access. For an overview of some software vulnerabilities, see Zalewski's signal article.

POSIX recommends sigaction() and deprecates the use of signal() to register signal handlers. Unfortunately, sigaction() is not defined in the C Standard and is consequently not as portable a solution.

Risk Assessment

Interrupting a noninterruptible signal handler can result in a variety of vulnerabilities [Zalewski 2001].

References

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 03|AA. C References#ISO/IEC 03]\] "Signals and Interrupts" \[[Open Group 04|AA. C References#Open Group 04]\] [longjmp|http://www.opengroup.org/onlinepubs/000095399/functions/longjmp.html] \[OpenBSD\] [{{signal()}} Man Page|http://www.openbsd.org/cgi-bin/man.cgi?query=signal] \[Zalewski\] [http://lcamtuf.coredump.cx/signals.txt]