CWE-479: Signal Handler Use of a Non-reentrant Function
Signal Handler Use of a Non-reentrant Function
Weakness ID: 479 (Weakness Variant)
Status: Draft
Description
Description Summary
The program defines a signal handler that calls a non-reentrant function.
Extended Description
Non-reentrant functions are functions that cannot safely be called, interrupted, and then recalled before the first call has finished without resulting in memory corruption. This can lead to an unexpected system state an unpredictable results with a variety of potential consequences depending on context, including denial of service and code execution.
Many functions are not reentrant, but some of them can result in the corruption of memory if they are used in a signal handler. The function call syslog() is an example of this. In order to perform its functionality, it allocates a small amount of memory as "scratch space." If syslog() is suspended by a signal call and the signal handler calls syslog(), the memory used by both of these functions enters an undefined, and possibly, exploitable state. Implementations of malloc() and free() manage metadata in global structures in order to track which memory is allocated versus which memory is available, but they are non-reentrant. Simultaneous calls to these functions can cause corruption of the metadata.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
C
C++
Common Consequences
Scope
Effect
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands
It may be possible to execute arbitrary code through the use of a
write-what-where condition.
Integrity
Technical Impact: Modify application
data
Signal race conditions often result in data corruption.
Likelihood of Exploit
Low
Demonstrative Examples
Example 1
In this example, a signal handler uses syslog() to log a
message:
char *message;
void sh(int dummy) {
syslog(LOG_NOTICE,"%s\n",message);
sleep(10);
exit(0);
}
int main(int argc,char* argv[]) {
...
signal(SIGHUP,sh);
signal(SIGTERM,sh);
sleep(10);
exit(0);
}
If the execution of the first call to the signal handler is suspended
after invoking syslog(), and the signal handler is called a second time,
the memory allocated by syslog() enters an undefined, and possibly,
exploitable state.
Require languages or libraries that provide reentrant functionality,
or otherwise make it easier to avoid this weakness.
Phase: Architecture and Design
Design signal handlers to only set flags rather than perform complex
functionality.
Phase: Implementation
Ensure that non-reentrant functions are not found in signal
handlers.
Phase: Implementation
Use sanity checks to reduce the timing window for exploitation of
race conditions. This is only a partial solution, since many attacks
might fail, but other attacks still might work within the narrower
window, even accidentally.
Call only asynchronous-safe functions within signal
handlers
CERT C Secure Coding
SIG32-C
Do not call longjmp() from inside a signal
handler
CERT C Secure Coding
SIG33-C
Do not recursively invoke the raise()
function
CERT C Secure Coding
SIG34-C
Do not call signal() from within interruptible signal
handlers
CERT Java Secure Coding
EXP01-J
Never dereference null pointers
CERT C++ Secure Coding
SIG30-CPP
Call only asynchronous-safe functions within signal
handlers
CERT C++ Secure Coding
SIG33-CPP
Do not recursively invoke the raise()
function
CERT C++ Secure Coding
SIG34-CPP
Do not call signal() from within interruptible signal
handlers
References
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 13, "Signal Vulnerabilities", Page
791.. 1st Edition. Addison Wesley. 2006.
Description
