Attempting to dereference an invalid 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.
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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
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To correct this error, ensure the pointer returned by {{malloc()}} is not null. This also ensures compilance with \[[MEM32-C. Detect and handle critical memory allocation errors]\]. |
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/* ... */
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);
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Risk Assessment
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Dereferencing ana invalidnull 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 \[[van Sprundel 06|AA. C References#van Sprundel 06], [Jack 07|AA. C References#Jack 07]\]. 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. |
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
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EXP34-C | 3 (high) | 3 (likely) | 2 (medium) | P18 | L1 |
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
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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.3.2.3, "Pointers" \[[Jack 07|AA. C References#Jack 07]\] \[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 476|http://cwe.mitre.org/data/definitions/476.html], "NULL Pointer Dereference" \[[van Sprundel 06|AA. C References#van Sprundel 06]\] \[[Viega 05|AA. C References#Viega 05]\] Section 5.2.18, "Null-pointer dereference" |
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