CERT C: Rule POS35-C

Avoid race conditions while checking for the existence of a symbolic link

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Description

Rule Definition

Avoid race conditions while checking for the existence of a symbolic link.¹

Polyspace Implementation

The rule checker checks for File access between time of check and use (TOCTOU).

Examples

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File access between time of check and use (TOCTOU)

Issue

File access between time of check and use (TOCTOU) detects race condition issues between checking the existence of a file or folder, and using a file or folder.

Risk

An attacker can access and manipulate your file between your check for the file and your use of a file. Symbolic links are particularly risky because an attacker can change where your symbolic link points.

Fix

Before using a file, do not check its status. Instead, use the file and check the results afterward.

Example - Check File Before Using

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

extern void print_tofile(FILE* f);

void toctou(char * log_path) {
    if (access(log_path, W_OK)==0) {
        FILE* f = fopen(log_path, "w"); //Noncompliant
        if (f) {
            print_tofile(f);
            fclose(f);
        }
    }
}

In this example, before opening and using the file, the function checks if the file exists. However, an attacker can change the file between the first and second lines of the function.

Correction — Open Then Check

One possible correction is to open the file, and then check the existence and contents afterward.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

extern void print_tofile(FILE* f);

void toctou(char * log_path) {
    int fd = open(log_path, O_WRONLY);
    if (fd!=-1) {
        FILE *f = fdopen(fd, "w");
        if (f) {
            print_tofile(f);
            fclose(f);
        }
    }
}

Check Information

Group: Rule 50. POSIX (POS)

Version History

Introduced in R2019a

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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