SIG31-C. Do not access or modify shared objects in signal handlers

With one exception accessing or modifying shared objects in signal handlers can lead to race conditions, opening up security holes.

According to the "Signals and Interrupts" section of the C99 Rationale:

The C89 Committee concluded that about the only thing a strictly conforming program can do in a signal handler is to assign a value to a volatile static variable which can be written uninterruptedly and promptly return.

The C99 standard dictates the use of volatile sig_atomic_t. The type of sig_atomic_t is implementation defined, although there are bounding constraints. To be fully compliant, one can only assign values from 0 to 127, inclusive, to a sig_atomic_t.

Non-Compliant Code Example

err_msg is updated to reflect the SIGINT signal that was encountered. Issues will occur if a SIGINT is generated prior to the malloc of err_msg finishing.

#include <signal.h> 
#include <stdlib.h>
#include <string.h>
 
char *err_msg; 
 
void handler() { 
  strcpy(err_msg, "SIGINT encountered.");
} 
 
int main() { 
  signal(SIGINT, handler); 

  err_msg = malloc(24);
  strcpy(err_msg, "No errors yet.");
 
  /* main code loop */

  return 0;
}

Compliant Solution

Signal handlers should only unconditionally set a flag of type volatile sig_atomic_t, and then return.

#include <signal.h> 
#include <stdlib.h>
#include <string.h>
 
char *err_msg; 
volatile sig_atomic_t e_flag = 0;
 
void handler() { 
  e_flag = 1;
} 
 
int main() { 
  signal(SIGINT, handler); 

  err_msg = malloc(24);
  strcpy(err_msg, "No errors yet.");
 
  /* main code loop */
  if(e_flag)
    strcpy(err_msg, "SIGINT received.");


  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 06]].

References

[[Dowd 06]] Chapter 13, Synchronization and State
[[ISO/IEC 03]] "Signals and Interrupts"
[[Open Group 04]] longjmp
[OpenBSD] signal() Man Page
[Zalewski] http://lcamtuf.coredump.cx/signals.txt