SIG00-A. Mask signals handled by non-interruptible signal handlers

Signal handlers can be interrupted by their own signals. If a signal is not reset before its handler is called, this means the handler can essentially 'interrupt itself'. A handler that always successfully executes its code despite interrupting itself is said to be re-entrant.

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 un-interruptible, and need not be re-entrant.

Vulnerabilities can arise if a non-re-entrant signal handler is interrupted with its own signal, especially if it manipulates globally-accessible data.

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 if one or more SIGUSR1 signals are followed by SIGUSR2. This code essentially implements 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) {
  signal(SIGUSR1, handler);
  signal(SIGUSR2, handler);

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

  /* ... */

  return 0;
}

The problem with this code is that there is a race condition 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.

Non-Compliant Coding Example (External finite state machine)

This example moves the finite state machine out of the signal handler, making the handler re-entrant.

#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 (signum == SIGUSR2) {
    sig2 = 1;
  }
}

int main(void) {
  int state = 0;
  signal(SIGUSR1, handler);
  signal(SIGUSR2, handler);

  while (state != 2) {
    /* do nothing or give up CPU for a while */
    if (state == 0 && sig1) {
      state = 1;
      sig2 = 0;
    }
    if (state == 1 && sig2) {
      state = 2;
    }
  }

  /* ... */

  return 0;
}

There is still a race condition in this code where a SIGUSR2 sent immediately after a SIGUSR1 gets ignored. This is because the SIGUSR2 gets processed before the while loop sets the state to 1 and sig2 to 0, which erases the evidence of SIGUSR2. To completely eliminate this race condition, the OS must queue subsequent signals while one signal is being handled, and the finite state machine must be handled by the signal handler.

Compliant Solution (POSIX)

POSIX defines the sigaction(2) function, which assigns handlers to signals like signal(2), but also allows one to explicitly set signal masks. One can thus use sigaction(2) and 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)) {
    /* handle error */
  }
  if (sigaddset( &act.sa_mask, SIGUSR2)) {
    /* handle error */
  }

  if (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;
}

In fact, POSIX recommends sigaction(2) and deprecates signal(2). Unfortunately, sigaction(2) is not C99-compliant.

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.

Automated Detection

Related Vulnerabilities

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

References

[[ISO/IEC 03]] Section 5.2.3, "Signals and interrupts"
[[Open Group 04]] longjmp
[OpenBSD] signal() Man Page
[Zalewski] http://lcamtuf.coredump.cx/signals.txt
[[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