Integer values used as a size argument to malloc(), calloc(), or realloc() must be valid and large enough to contain the objects to be stored. If size arguments are incorrect or can be manipulated by an attacker, then a buffer overflow may occur. Incorrect size arguments, inadequate range checking, integer overflow, or truncation can result in the allocation of an inadequately sized buffer. The programmer must ensure that size arguments to memory allocation functions allocate sufficient memory.
Non-Compliant Code Example (integer overflow)
In this non-compliant code example, cBlocks is multiplied by 16 and the result is stored in the unsigned long long int alloc.
enum { BLOCKSIZE = 16 };
...
void* AllocBlocks(size_t cBlocks) {
if (cBlocks == 0) return NULL;
unsigned long long alloc = cBlocks * BLOCKSIZE ;
return (alloc < UINT_MAX)
? malloc(cBlocks * BLOCKSIZE )
: NULL;
}
For example, if size_t is represented as a 32-bit unsigned value and unsigned long long represented as a 64-bit unsigned value, the result of this multiplication can still overflow because the actual multiplication is a 32-bit operation. As a result, the value stored in alloc will always be less than UINT_MAX.
If both size_t and unsigned long long types are represented as a 64-bit unsigned value, the result of the multiplication operation may not be representable as an unsigned long long value.
See [[INT35-C. Evaluate integer expressions in a larger size before comparing or assigning to that size]] for more information on upcasting.
Compliant Solution (integer overflow)
In this compliant solution, the integer values passed as size arguments to memory allocation functions are of the correct size and have not been altered due to integer overflow [[INT32-C. Ensure that integer operations do not result in an overflow]] or truncation [INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data].
enum { BLOCKSIZE = 16 };
...
void *AllocBlocks(size_t cBlocks) {
size_t alloc;
if (cBlocks == 0 || cBlocks > SIZE_MAX / BLOCKSIZE) return NULL;
return malloc (cBlocks * BLOCKSIZE);
} /* end AllocBlocks */
This example checks the value of cBlocks to make sure the subsequent multiplication operation cannot result in an integer overflow. The code also ensures that cBlocks is not equal to zero (see [[MEM04-A. Do not perform zero length allocations]]).
Non-Compliant Code Example (range checking)
In this non-compliant code example, the string referenced by str and the string length represented by len orginate from untrusted sources. The length is used to perform a memcpy() into the fixed size static array buf. The len variable is guaranteed to be less than BUFF_SIZE. However, because len is declared as an int it could have a negative value that would bypass the check. The memcpy() function implicitly converts len to an unsigned size_t type, and the resulting operation results in a buffer overflow.
int len;
char *str;
char buf[BUFF_SIZE];
/* ... */
if (len < BUFF_SIZE){
memcpy(buf, str, len);
}
/* ... */
Compliant Solution (range checking)
In this compliant solution, len is declared as a size_t to there is no possibility of this variable having a negative value and bypassing the range check.
size_t len;
char *str;
char buf[BUFF_SIZE];
/* ... */
if (len < BUFF_SIZE){
memcpy(buf, str, len);
}
/* ... */
See [[INT01-A. Use rsize_t or size_t for all integer values representing the size of an object]] for more information on representing the size of objects.
Non-Compliant Code Example (incorrectly calculating a size)
In this non-compliant code example, an array of long is allocated and assigned to p. However, sizeof(int) is used to size the allocated memory. If sizeof(long) is larger than sizeof(int) then an insufficient amount of memory is allocated.
void function(size_t len) {
long *p;
if (len == 0 || len > SIZE_MAX / sizeof(long)) {
/* handle overflow */
}
p = (long *)malloc(len * sizeof(int));
if (p == NULL) {
/* handle error */
}
/* ... */
free(p);
}
This example also checks for unsigned integer overflow in compliance with INT32-C. Ensure that integer operations do not result in an overflow.
Compliant Solution (incorrectly calculating a size)
To correct this example, sizeof(long) is used to size the memory allocation.
void function(size_t len) {
long *p;
if (len == 0 || len > SIZE_MAX / sizeof(long)) {
/* handle overflow */
}
p = (long *)malloc(len * sizeof(long));
if (p == NULL) {
/* handle error */
}
/* ... */
free(p);
}
Alternatively, sizeof(*p) can be used to properly size the allocation.
void function(size_t len) {
long *p;
if (len == 0 || len > SIZE_MAX / sizeof(*p)) {
/* handle overflow */
}
p = (long *)malloc(len * sizeof(*p));
if (p == NULL) {
/* handle error */
}
/* ... */
free(p);
}
The code also ensures that len is not equal to zero (see [[MEM04-A. Do not perform zero length allocations]]).
Risk Assessment
Providing invalid size arguments to memory allocation functions can lead to buffer overflows and the execution of arbitrary code with the permissions of the vulnerable process.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
MEM35-C |
3 (high) |
2 (probable) |
1 (high) |
P6 |
L2 |
Automated Detection
Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this rule, except those involving the sizeof operator.
Coverity Prevent. The SIZECHECK checker finds memory allocations that are assigned to a pointer that reference objects larger than the allocated block. 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 7.20.3, "Memory Management Functions"
[[Seacord 05]] Chapter 4, "Dynamic Memory Management," and Chapter 5, "Integer Security"
[[Coverity 07]]