SIG30-C. Only call async-safe functions within signal handlers

Only call asynchronous-safe functions within signal handlers.

According to the "Signals and Interrupts" section 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. Since library functions are not guaranteed to be reentrant, they should not be called from a signal handler that returns.

Similarly, Section 7.14.1 paragraph 5 of C99 [[ISO/IEC 9899-1999:TC2]] 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 refers to any object with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t, or 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.

Non-Compliant Code Example

In this non-compliant code example, main() invokes the malloc() function to allocated space to copy a string. The string literal is copied into the allocated memory, which is then printed and the memory freed. The program also registers the signal handler int_handler() to handle the terminal interrupt signal SIGINT.

Unfortunately, the free() function is not asynchronous-safe and its invocation from within a signal handler is a violation of this rule. If an interrupt signal is received during or after the free() call in main(), the heap may be corrupted.

#include <signal.h>

char *foo;

void int_handler() {
  free(foo);
  _Exit(0);
}

int main(void) {
  foo = malloc(sizeof("Hello World."));
  if (foo == NULL) {
    /* handle error condition */
  }
  signal(SIGINT, int_handler);
  strcpy(foo, "Hello World.");
  puts(foo);
  free(foo);
  return 0;
}

The _Exit() function called from within the int_handler() signal handler causes immediate program termination, and is async-safe, whereas exit() may call cleanup routines first, and is consequently not async-safe.

Implementation Details

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

Compliant Solution

Signal handlers should be as concise as possible, ideally unconditionally setting a flag and returning. They may also call the _Exit() function.

#include <signal.h>

char *foo;

void int_handler() {
  _Exit(0);
}

int main(void) {
  foo = malloc(sizeof("Hello World."));
  if(foo == NULL) {
    /* handle error condition */
  }
  signal(SIGINT, int_handler);
  strcpy(foo, "Hello World.");
  puts(foo);
  free(foo);
  return 0;
}

Risk Assessment

Invoking functions that are not async-safe from within a signal handler may result in privilege escalation and other attacks. 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:TC2]] Section 7.14, "Signal handling <signal.h>"
[[Open Group 04]] longjmp
[OpenBSD] signal() Man Page
[[Zalewski 01]]