SIG30-C. Call only asynchronous-safe functions within signal handlers

Call only asynchronous-safe functions within signal handlers.

According to Section 7.14.1.1 of the C Rationale [[ISO/IEC 03]]:

When a signal occurs, the normal flow of control of a program is interrupted. If a signal occurs that is being trapped by a signal handler, that handler is invoked. When it is finished, execution continues at the point at which the signal occurred. This arrangement could cause problems if the signal handler invokes a library function that was being executed at the time of the signal.

Similarly, Section 7.14.1 paragraph 5 of C99 [[ISO/IEC 9899-1999]] states that if the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if:

...the signal handler calls any function in the standard library other than the abort function, the _Exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler.

Many systems define an implementation-specific list of asynchronous safe functions. In general, I/O functions are not safe to invoke inside signal handlers. Check your system's asynchronous-safe functions before using them in signal handlers.

Non-Compliant Code Example

In this non-compliant code example, the program allocates a string on the heap, and uses it to log messages in a loop. The program also registers the signal handler int_handler() to handle the terminal interrupt signal SIGINT. The int_handler() function logs the last message, calls free(), and exits.

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

enum { MAXLINE = 1024 };
char *info = NULL;

void log_message(void) {
  fprintf(stderr, info);
}

void handler(int signum) {
  log_message();
  free(info);
  info = NULL;
}

int main(void) {
  signal(SIGINT, handler);
  info = (char*)malloc(MAXLINE);

  while (1) {
    /* main loop program code */

    log_message();

    /* more program code */
  }
  return 0;
}

This program has four potential problems. The first is that the log_message() function calls fprintf(), which is an unsafe function to call from within a signal handler, because the handler might have been called when global data (such as stderr) was in an inconsistent state. In general, standard I/O is never safe to invoke within a signal handler.

The second problem is that the free() function is also not [[asynchronous-safe]], and its invocation from within a signal handler is also a violation of this rule. If an interrupt signal is received during the free() call in main(), the heap may be corrupted.

The third problem is if SIGINT occurs after the call to free(), resulting in the memory referenced by info being freed twice. This is a violation of MEM31-C. Free dynamically allocated memory exactly once and SIG31-C. Do not access or modify shared objects in signal handlers.

The fourth and final problem is that the signal handler reads the variable info, which is not declared to be of type volatile sig_atomic_t. This is a violation of SIG31-C. Do not access or modify shared objects in signal handlers.

Implementation Details

POSIX

The following table from the the Open Group Base Specifications [[Open Group 04]] defines a set of functions that are asynchronous-signal-safe. Applications may consequently invoke them, without restriction, from signal-catching functions.

All functions not in this table are considered to be unsafe with respect to signals. In the presence of signals, all functions defined by IEEE standard 1003.1-2001 behave as defined when called from or interrupted by a signal-catching function, with a single exception: when a signal interrupts an unsafe function and the signal-catching function calls an unsafe function, the behavior is undefined.

Note that while raise() is on the list of asynchronous-safe functions, it is specifically covered by SIG33-C. Do not recursively invoke the raise() function.

OpenBSD

The OpenBSD signal() man page identifies functions that are asynchronous-signal safe. Applications may consequently invoke them, without restriction, from signal-catching functions.

The OpenBSD signal() man page list a few additional functions that are asynchronous-safe in OpenBSD but "probably not on other systems" including: snprintf(), vsnprintf(), and syslog_r() (but only when the syslog_data struct is initialized as a local variable).

Compliant Solution

Signal handlers should be as concise as possible, ideally unconditionally setting a flag and returning. They may also call the _Exit() function. Finally, they may call functions listed above as asynchronous-safe.

This example code achieves compliance with this rule by moving the final log message and call to free() outside the signal handler.

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

enum { MAXLINE = 1024 };
volatile sig_atomic_t eflag = 0;
char *info = NULL;

void log_message(void) {
  fprintf(stderr, info);
}

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

int main(void) {
  signal(SIGINT, handler);
  info = (char*)malloc(MAXLINE);

  while (!eflag) {
    /* main loop program code */

    log_message();

    /* more program code */
  }

  log_message();
  free(info);
  info = NULL;

  return 0;
}

Risk Assessment

Invoking functions that are not asynchronous-safe from within a signal handler may result in privilege escalation and other attacks.

Automated Detection

The tool Compass Rose can detect violations of the rule for single-file programs.

Related Vulnerabilities

For an overview of some software vulnerabilities, see Zalewski's paper on understanding, exploiting, and preventing signal-handling related vulnerabilities [[Zalewski 01]]. VU #834865 describes a vulnerability resulting from a violation of this rule.

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

References

[[Dowd 06]] Chapter 13, "Synchronization and State"
[[ISO/IEC 03]] Section 5.2.3, "Signals and interrupts"
[[ISO/IEC 9899-1999]] Section 7.14, "Signal handling <signal.h>"
[[Open Group 04]] longjmp
[[OpenBSD]] signal() Man Page
[[Zalewski 01]]

SIG02-A. Avoid using signals to implement normal functionality      11. Signals (SIG)       SIG31-C. Do not access or modify shared objects in signal handlers