...
A signal handler may only call signal() if it does not need to be async-safe (in other words, all relevant signals are masked, and it may therefore not be interrupted.)
Non-Compliant Code Example
In this non-compliant code example, the signal handler handler() is bound to signum.
...
If the environment is persistent (that is, it does not reset the handler when the signal is received), the handler's signal() function is redundant.
Compliant Solution
For persistent platforms, the handler's signal() function is unnecessary.
| Code Block | ||
|---|---|---|
| ||
void handler(int signum) {
/* handling code */
}
/* ... */
signal(signum, 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 persistence. One can thus use sigaction(2) and sidestep the race window on non-persistent OS's.
...
In fact, POSIX recommends sigaction(2) and deprecates signal(2). Unfortunately, sigaction(2) is not C99-compliant, and is not supported on some platforms, including Windows.
Exceptions
SIG34-EX1: On a machine with persistent signal handlers, it is safe for a handler to modify the behavior for its own signal. This would include having the signal be ignored, reset to default behavior, or handled by a different handler. A handler assigning itself to its own signal is also safe, as it is a no-op. The handler is impervious to a race condition since multiple invocations of its signal will merely cause it to 'interrupt itself', until it manages to reassign its signal.
...
Not all systems have persistent signal handlers. For more info, see SIG01-A. Understand implementation-specific details regarding signal handler persistence
Risk Assessment
Two signals in quick succession can trigger the race condition on non-persistent platforms, thereby causing the signal's default behavior despite a handler's attempt to override it.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
SIG34-C | 1 (low) | 1 (unlikely) | 3 (low) | P3 | L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
| Wiki Markup |
|---|
\[[ISO/IEC 9899-1999TR2|AA. C References#ISO/IEC 9899-1999]\] Section 7.14.1.1, "The {{signal}} function" |
...