Mutexes are used to protect shared data structures being concurrently accessed. If a mutex is destroyed while a thread is blocked waiting for that mutex, critical sections and shared data are no longer protected.
The C Standard, 7.26.4.1, paragraph 2 [ISO/IEC 9899:2011], states
The
mtx_destroyfunction releases any resources used by the mutex pointed to bymtx. No threads can be blocked waiting for the mutex pointed to bymtx.
This statement implies that destroying a mutex while a thread is waiting on it is undefined behavior.
Noncompliant Code Example
This noncompliant code example creates several threads that each invoke the do_work() function, passing a unique number as an ID. The do_work() function initializes the lock mutex if the argument is 0 and destroys the mutex if the argument is max_threads - 1. In all other cases, the do_work() function provides normal processing. Each thread, except the final cleanup thread, increments the atomic completed variable when it is finished.
Unfortunately, this code contains several race conditions, allowing the mutex to be destroyed before it is unlocked. Additionally, there is no guarantee that lock will be initialized before it is passed to mtx_lock(). Each of these behaviors is undefined.
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
mtx_t lock;
/* Atomic so multiple threads can modify safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 };
int do_work(void *arg) {
int *i = (int *)arg;
if (*i == 0) { /* Creation thread */
if (thrd_success != mtx_init(&lock, mtx_plain)) {
/* Handle error */
}
atomic_store(&completed, 1);
} else if (*i < max_threads - 1) { /* Worker thread */
if (thrd_success != mtx_lock(&lock)) {
/* Handle error */
}
/* Access data protected by the lock */
atomic_fetch_add(&completed, 1);
if (thrd_success != mtx_unlock(&lock)) {
/* Handle error */
}
} else { /* Destruction thread */
mtx_destroy(&lock);
}
return 0;
}
int main(void) {
thrd_t threads[max_threads];
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_create(&threads[i], do_work, &i)) {
/* Handle error */
}
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_join(threads[i], 0)) {
/* Handle error */
}
}
return 0;
}
|
Compliant Solution
This compliant solution eliminates the race conditions by initializing the mutex in main() before creating the threads and by destroying the mutex in main() after joining the threads:
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
mtx_t lock;
/* Atomic so multiple threads can increment safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 };
int do_work(void *dummy) {
if (thrd_success != mtx_lock(&lock)) {
/* Handle error */
}
/* Access data protected by the lock */
atomic_fetch_add(&completed, 1);
if (thrd_success != mtx_unlock(&lock)) {
/* Handle error */
}
return 0;
}
int main(void) {
thrd_t threads[max_threads];
if (thrd_success != mtx_init(&lock, mtx_plain)) {
/* Handle error */
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_create(&threads[i], do_work, NULL)) {
/* Handle error */
}
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_join(threads[i], 0)) {
/* Handle error */
}
}
mtx_destroy(&lock);
return 0;
}
|
Risk Assessment
Destroying a mutex while it is locked may result in invalid control flow and data corruption.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON31-C | Medium | Probable | High | P4 | L3 |
Automated Detection
Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Astrée |
| Supported, but no explicit checker | |||||||
| CodeSonar |
| CONCURRENCY.LOCALARG | Local Variable Passed to Thread | ||||||
| Helix QAC |
| DF4961, DF4962 | |||||||
| Parasoft C/C++test |
| CERT_C-CON31-a | Do not destroy another thread's mutex | ||||||
| CERT C: Rule CON31-C | Checks for destruction of locked mutex (rule fully covered) |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
Key here (explains table format and definitions)
Taxonomy | Taxonomy item | Relationship |
|---|---|---|
| CWE 2.11 | CWE-667, Improper Locking | 2017-07-10: CERT: Rule subset of CWE |
CERT-CWE Mapping Notes
Key here for mapping notes
CWE-667 and CON31-C/POS48-C
Intersection( CON31-C, POS48-C) = Ø
CWE-667 = Union, CON31-C, POS48-C, list) where list =
- Locking & Unlocking issues besides unlocking another thread’s C mutex or pthread mutex.
Bibliography
| [ISO/IEC 9899:2011] | 7.26.4.1, "The mtx_destroy Function" |