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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

It is possible to safely use the same handler for multiple signals, but doing so increases the likelihood of a security vulnerability. The delivered signal is masked and is not delivered until the registered signal handler exits. However, if this same handler is registered to handle a different signal, execution of the handler may be interrupted by this new signal. If a signal handler is constructed with the expectation that it cannot be interrupted, a vulnerability might exist. To eliminate this attack vector, each signal handler should be registered to handle only one type of signal.

Non-Compliant Coding Example

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

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

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

void handler(int signum) {
  if (sig1signum == SIGUSR1) {
    sig1 sig2 = 1;
  }
  else if (signum == SIGUSR1sig1) {
    sig1sig2 = 1;
  }
}

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

  while (1sig2 == 0) {
    /* Do nothing ifor (sig2) break;
    sleep(SLEEP_TIME);give up CPU for a while */
  }

  /* ... */

  return 0;
}

The problem with this code is that there is 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. This non-compliant code example also violates SIG31-C. Do not access or modify shared objects in signal handlers.

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.This compliant solution registers two separate signal handlers to process SIGUSR1 and SIGUSR2. The sig1_handler() handler waits for SIGUSER1. After this signal occurs, the sig2_handler() is registered to handle SIGUSER2. This solution is fully compliant and accomplishes the goal of detecting whether one or more SIGUSR1 signals are followed by SIGUSR2.

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

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

void sig1_handler(int signum) {
  if (signum == SIGUSR1) {
    sig1 = 1;
  }

void sig2_handler(int signum  else if (sig1) {
    sig2 = 1;
  }
}

int main(void) {
  struct  signal(SIGUSR1, handler);

  while (1) {
    if (sig1) break;
    sleep(SLEEP_TIME);sigaction act;
  act.sa_handler = &handler;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* Handle error */
  }
  if (sigaddset(&act.sa_mask, SIGUSR1)) {
    /* Handle error */
  }
  if (sigaddset(&act.sa_mask, SIGUSR2)) {
    /* Handle error */
  }

  if signal(SIGUSR2, handler);
  while (1) {
    if (sig2) break;
    sleep(SLEEP_TIME);(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

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]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.

Automated Detection

Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added Image Added 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] \[[Dowd 06 | AA. C References#Dowd 06]\] Chapter 13, "Synchronization and State" (Signal Interruption and Repetition) 12. Signals (SIG)      12. Signals (SIG)       SIG01-A. Understand implementation-specific details regarding signal handler persistence