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CWE-364: Signal Handler Race Condition
 DescriptionDescription Summary The software uses a signal handler that introduces a race condition.
Extended Description Race conditions frequently occur in signal handlers, since signal handlers support asynchronous actions. These race conditions have a variety of root causes and symptoms. Attackers may be able to exploit a signal handler race condition to cause the software state to be corrupted, possibly leading to a denial of service or even code execution. These issues occur when non-reentrant functions, or state-sensitive actions occur in the signal handler, where they may be called at any time. These behaviors can violate assumptions being made by the "regular" code that is interrupted, or by other signal handlers that may also be invoked. If these functions are called at an inopportune moment - such as while a non-reentrant function is already running - memory corruption could occur that may be exploitable for code execution. Another signal race condition commonly found occurs when free is called within a signal handler, resulting in a double free and therefore a write-what-where condition. Even if a given pointer is set to NULL after it has been freed, a race condition still exists between the time the memory was freed and the pointer was set to NULL. This is especially problematic if the same signal handler has been set for more than one signal -- since it means that the signal handler itself may be reentered. There are several known behaviors related to signal handlers that have received the label of "signal handler race condition":
Signal handler vulnerabilities are often classified based on the absence of a specific protection mechanism, although this style of classification is discouraged in CWE because programmers often have a choice of several different mechanisms for addressing the weakness. Such protection mechanisms may preserve exclusivity of access to the shared resource, and behavioral atomicity for the relevant code:
Time of Introduction
Applicable PlatformsLanguages C: (Sometimes) C++: (Sometimes) Common Consequences
Likelihood of ExploitMedium Demonstrative ExamplesExample 1 This code registers the same signal handler function with two different signals (CWE-831). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits. (Bad Code) char *logMessage; void handler (int sigNum) { syslog(LOG_NOTICE, "%s\n", logMessage);
free(logMessage);
/* artificially increase the size of the timing window to
make demonstration of this weakness easier. */
sleep(10);
exit(0);
} int main (int argc, char* argv[]) { logMessage = strdup(argv[1]);
/* Register signal handlers. */
signal(SIGHUP, handler);
signal(SIGTERM, handler);
/* artificially increase the size of the timing window to
make demonstration of this weakness easier. */
sleep(10);
} The handler function uses global state (globalVar and logMessage), and it can be called by both the SIGHUP and SIGTERM signals. An attack scenario might follow these lines:
At this point, the state of the heap is uncertain, because malloc is still modifying the metadata for the heap; the metadata might be in an inconsistent state. The SIGTERM-handler call to free() is assuming that the metadata is inconsistent, possibly causing it to write data to the wrong location while managing the heap. The result is memory corruption, which could lead to a crash or even code execution, depending on the circumstances under which the code is running. Note that this is an adaptation of a classic example as originally presented by Michal Zalewski (see references); the original example was shown to be exploitable for code execution. Also note that the strdup(argv[1]) call contains a potential buffer over-read (CWE-126) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array. Example 2 The following code registers a signal handler with multiple signals in order to log when a specific event occurs and to free associated memory before exiting. (Bad Code) Example
Language: C #include <signal.h> #include <syslog.h> #include <string.h> #include <stdlib.h> void *global1, *global2; char *what; void sh (int dummy) { syslog(LOG_NOTICE,"%s\n",what);
free(global2);
free(global1);
/* Sleep statements added to expand timing window for race
condition */
sleep(10);
exit(0);
} int main (int argc,char* argv[]) { what=argv[1];
global1=strdup(argv[2]);
global2=malloc(340);
signal(SIGHUP,sh);
signal(SIGTERM,sh);
/* Sleep statements added to expand timing window for race
condition */
sleep(10);
exit(0);
} However, the following sequence of events may result in a double-free (CWE-415):
This is just one possible exploitation of the above code. As another example, the syslog call may use malloc calls which are not async-signal safe. This could cause corruption of the heap management structures. For more details, consult the example within "Delivering Signals for Fun and Profit" (see references). Observed Examples
Potential Mitigations
Relationships
Research Gaps
Affected Resources
Functional Areas
Taxonomy Mappings
References
Content History
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Page Last Updated:
February 20, 2013
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