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Some C standard library functions are not guaranteed to be reentrant with respect to threads. Some functions (such as strtok() and asctime()) return a pointer to the result stored in function-allocated memory on a per-process basis. Other functions (such as rand()) store state information in function-allocated memory on a per-process basis. Multiple threads invoking the same function can cause concurrency problems, which often result in abnormal behavior and can cause more serious vulnerabilities, such as abnormal termination, denial-of-service attack, and data integrity violations.

According to the C Standard, the following library functions may contain data races when invoked by multiple threads:

APIRecommendation
rand(), srand()MSC30-C. Do not use the rand() function for generating pseudorandom numbers
getenv(), getenv_s()ENV00-C. Do not store the pointer to the string returned by getenv()
strtok()strtok_s() in C11 Annex K
strtok_r() in POSIX
strerror()strerror_s() in C11 Annex K
strerror_r() in POSIX
asctime(), ctime(),
localtime(), gmtime()
asctime_s(), ctime_s(), localtime_s(), gmtime_s() in C11 Annex K
setlocale()Protect multithreaded access to locale-specific APIs with a mutex
ATOMIC_VAR_INIT, atomic_init()Do not attempt to initialize an atomic variable from multiple threads
tmpnam()tmpnam_s() in C11 Annex K
tmpnam_r() in POSIX
mbrtoc16(), c16rtomb(),
mbrtoc32(), c32rtomb()
Do not call with a null mbstate_t * argument 

Section 2.9.1 of the System Interfaces volume of Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 (IEEE Std 1003.1, 2013 Edition) [IEEE Std 1003.1:2013] augments the list of functions that are not required to be thread-safe.

Noncompliant Code Example

Consider a multithreaded application that encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number. The C Standard, subclause 7.24.6.2 [ISO/IEC 9899:2011], specifically states that strerror() is not required to avoid data races. Conventionally, it could rely on a static array that maps error numbers to error strings, and that array might be accessible and modifiable by other threads.

#include <errno.h>
#include <stdio.h>
#include <string.h>
 
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char *errmsg = strerror(errno);
    printf("Could not get the file position because of %s\n",
           errmsg);
  }
}

Note that this code first sets errno to 0 to comply with ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure

Compliant Solution (Annex K, strerror_s()

This compliant solution uses the strerror_s() function from Annex K of the C Standard, which has the same functionality as strerror() but guarantees thread-safety.

#define __STDC_WANT_LIB_EXT1__ 1
#include <errno.h>
#include <stdio.h>
#include <string.h>
 
enum { BUFFERSIZE = 64 };
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char errmsg[BUFFERSIZE];
    if (strerror_s(errmsg, BUFFERSIZE, errno) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position because of %s\n",
           errmsg);
  }
}

Note that because Annex K is optional, strerror_s() may not be available in all implementations. 

Compliant Solution (POSIX, strerror_r())

This compliant solution uses the POSIX strerror_r() function, which has the same functionality as strerror() but guarantees thread-safety:

#include <errno.h>
#include <stdio.h>
#include <string.h>
 
enum { BUFFERSIZE = 64 };
 
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char errmsg[BUFFERSIZE];
    if (strerror_r(errno, errmsg, BUFFERSIZE) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position because of %s\n",
           errmsg);
  }
}

Note that Linux provides two versions of strerror_r(), known as the XSI-compliant version and the GNU-specific version. This compliant solution assumes the XSI-compliant version, which you will get if you compile your application as required by POSIX (that is, by defining _POSIX_C_SOURCE or _XOPEN_SOURCE appropriately). Check your strerror_r() manual page to see which version(s) are available on your system.

Risk Assessment

Race conditions caused by multiple threads invoking the same library function can lead to abnormal termination of the application, data integrity violations, or denial-of-service attack.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

CON33-C

Medium

Probable

High

P4

L3

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Automated Detection

Tool

Version

Checker

Description

Compass/ROSE

 

 

A module written in Compass/ROSE can detect violations of this rule

Related Guidelines

Bibliography

[IEEE Std 1003.1:2013]Section 2.9.1, "Thread Safety"
[ISO/IEC 9899:2011]

Subclause 7.24.6.2, "The strerror Function" 

[Open Group 1997b]Section 10.12, "Thread-Safe POSIX.1 and C-Language Functions"

 


 

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