View Source

Pthread mutual exclusion (mutex) locks are used to avoid simultaneous usage of common resources. Several types of mutex locks are defined by pthreads: NORMAL, ERRORCHECK, RECURSIVE, and DEFAULT.

POSIX describes PTHREAD_MUTEX_NORMAL locks as having the following undefined behavior \[[Open Group 4|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04]\]:

This type of mutex does not provide deadlock detection. A thread attempting to relock this mutex without first unlocking it shall deadlock. An error is not returned to the caller. Attempting to unlock a mutex locked by a different thread results in undefined behavior. Attempting to unlock an unlocked mutex results in undefined behavior.

The DEFAULT mutex pthread is also generally mapped to PTHREAD_MUTEX_NORMAL but is known to vary from platform to platform \[[SOL 10|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view]\].&nbsp; Consequently, NORMAL locks should not be used, and ERRORCHECK or RECURSIVE locks should be defined explicitly when mutex locks are used.

Noncompliant Code Example

This noncompliant code example shows a simple mutex being created using PTHREAD_MUTEX_NORMAL. Note that the caller does not expect a return code when NORMAL mutex locks are used.

 pthread_mutexattr_t attr;
 pthread_mutex_t mutex;
 size_t const shared_var = 0;

 int main(){
     int rc =0;

     rc = pthread_mutexattr_settype(&mutex, PTHREAD_MUTEX_NORMAL);
     if(rc != 0){
       /* Handle Error */
     }

     rc = pthread_mutex_init(&mutex, &attr);
     if(rc != 0){
       /* Handle Error */
     }


     pthread_mutex_lock(&mutex);

     /* Critical Region*/

     pthread_mutex_unlock(&mutex);

     return 0;
 }

Compliant Solution

This compliant solution shows an ERRORCHECK mutex lock being created so that return codes will be available during locking and unlocking.

 pthread_mutexattr_t attr;
 pthread_mutex_t mutex;
 size_t const shared_var = 0;

 int main(){
     int rc =0;

     rc = pthread_mutexattr_settype(&mutex, PTHREAD_MUTEX_ERRORCHECK);
     if(rc != 0){
       /* Handle Error */
     }


     rc = pthread_mutex_init(&mutex, &attr);
     if(rc != 0){
       /* Handle Error */
     }


     rc = pthread_mutex_lock(&mutex);
     if(rc != 0){
       /* Handle Error */
     }

     /* Critical Region*/

     rc = pthread_mutex_unlock(&mutex);
     if(rc != 0){
       /* Handle Error */
     }


     return 0;
 }

Risk Assessment

Using NORMAL mutex locks can lead to deadlocks or abnormal program termination.

Recommendation	Severity	Likelihood	Remediation Cost	Priority	Level
POS04-C	low	unlikely	medium	P1	L3

References

\[[Open Group 4|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04]\]
\[[SOL 10|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view]\]