EXP34-C. Ensure a null pointer is not dereferenced

Attempting to dereference a null pointer results in undefined behavior, typically abnormal program termination.

Non-Compliant Code Example

In this example, input_str is copied into dynamically allocated memory referenced by str. If malloc() fails, it returns a null pointer that is assigned to str. When str is dereferenced in strcpy(), the program behaves in an unpredictable manner.

/* ... */
size_t size = strlen(input_str);
if (size == SIZE_MAX) { /* test for limit of size_t */
  /* Handle Error */
}
str = malloc(size+1);
strcpy(str, input_str);
/* ... */
free(str);

In accordance with rule [MEM35-C. Allocate sufficient memory for an object] the argument supplied to malloc() is checked to ensure a numeric overflow does not occur. In most cases it is preferable to check that this value does not exceed some maximum allocation that is typically much smaller than SIZE_MAX.

Compliant Solution

To correct this error, ensure the pointer returned by malloc() is not null. This also ensures compilance with [[MEM32-C. Detect and handle critical memory allocation errors]].

/* ... */
size_t size = strlen(input_str);
if (size == SIZE_MAX) { /* test for limit of size_t */
  /* Handle Error */
}
str = malloc(size+1);
if (str == NULL) {
  /* Handle Allocation Error */
}
strcpy(str, input_str);
/* ... */
free(str);

Risk Assessment

Dereferencing a null pointer results in undefined behavior, typically abnormal program termination. In some situations, however, dereferencing a null pointer can lead to the execution of arbitrary code [[van Sprundel 06], [Jack 07]]. The indicated severity is for this more severe case; on platforms where it is not possible to exploit a null pointer dereference to execute arbitrary code the actual severity is low.

Automated Detection

Coverity Prevent The Coverity Prevent CHECKED_RETURN, NULL_RETURNS, and REVERSE_INULL checkers can all find violations of this rule. The CHECKED_RETURN finds instances where a pointer is checked against NULL, and then later dereferenced. The NULL_RETURNS checker identifies function that can return a null pointer but are not checked. The REVERSE_INULL identifies code that dereferences a pointer and then checks the pointer against NULL. Coverity Prevent cannot discover all violations of this rule so further verification is necessary.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 6.3.2.3, "Pointers"
[[Jack 07]]
[[MITRE 07]] CWE ID 476, "NULL Pointer Dereference"
[[van Sprundel 06]]
[[Viega 05]] Section 5.2.18, "Null-pointer dereference"

EXP33-C. Do not reference uninitialized variables      03. Expressions (EXP)       EXP35-C. Do not access or modify the result of a function call after a subsequent sequence point