Race condition in switch

Overview

If the variable which is switched on is changed while the switch statement is still in progress, undefined activity may occur.

Consequences

• Undefined: This flaw will result in the system state going out of sync.

Exposure period

• Implementation: Variable locking is the purview of implementers.

Platform

• Languages: All that allow for multi-threaded activity

• Operating platforms: All

Required resources

Any

Severity

Medium

Likelihood of exploit

Medium

Avoidance and mitigation

• Implementation: Variables that may be subject to race conditions should be locked for the duration of any switch statements.

Discussion

This issue is particularly important in the case of switch statements that involve fall-through style case statements - i.e., those which do not end with break.

If the variable which we are switching on change in the course of execution, the actions carried out may place the state of the process in a contradictory state or even result in memory corruption.

For this reason, it is important to ensure that all variables involved in switch statements are locked before the statement starts and are unlocked when the statement ends.

Examples

In C/C++:

1. include <sys/types.h>
2. include <sys/stat.h>

int main(argc,argv){

       struct stat *sb;
       time_t timer;

       lstat("bar.sh",sb);

       printf("%d\n",sb->st_ctime);
       switch(sb->st_ctime % 2){
               case 0: printf("One option\n");break;
               case 1: printf("another option\n");break;
               default: printf("huh\n");break;
       }

       return 0;

}

Related problems

• Race condition in signal handler

• Race condition within a thread